Information

How do nerve impulses travel so quickly?


Nerve impulses must travel incredibly fast to achieve the functions they do. However, I have been taught that sodium ions move down the axons by diffusion (thus causing depolarisation of the next part of the membrane etc, and this is the action potential).

However, I was under the impression that diffusion is a very slow process? How can this be the case?


There is a large gradient created by selective permeability of Na channels and as you're probably aware the speed of diffusion is proportional to the electrochemical gradient.

Very small numbers of ions actually cross the membrane during depolarisation and subsequent AP events.


How do nerve impulses travel so quickly? - Biology

Awesome question. The nervous system is so cool!

Neurons are some speedy guys. That’s why when we pick up a pencil, it seems as if we immediately know what the pencil feels like. What's really happening inside our bodies is a little more complex. The instant we pick up that pencil, a group of neurons in our fingers are activated, and fire, super-fast, through our spinal cord all the way up to our brain. How fast, you ask? Try around 75 meters per second fast! If you were driving in a car, that would be more than 150 miles per hour. Imagine that! But it can get even a little more complex than that.

Only TOUCH neurons will fire around 75 meters every second. Other types of neurons, like pain neurons, travel much slower, around 1 meter per second. That's like a car moving 2 miles per hour. Beep beep! So to sum up, it depends on the type of neuron.

Can't wait for your next question!

Thanks for the great question.
Neurons transmit an electrochemical signal called the action potential. These signals travel down a part of the neuron called the axon, which is like a wire that carries the signal to other nerve cells. On average a nerve cell sends a signal at about 50 meters per second, which is over 100 miles an hour! This means that when you step on something sharp it does take some time for that signal to go from the nerves in your foot to your brain, although not very much time. In fact in taller people it takes longer for a signal to go from one area to another than in shorter people, but the difference is too fast to tell outside of a laboratory.

Depending on a number of factors, signals can be sent even faster. One important factor is how myelinated the axon is. Myelin is a fatty substance that acts as an electrical insulator, increasing the speed at which the signal is sent. A highly myelinated nerve cell can send a signal at up to 120 meters per second, or nearly 270 miles per hour, quite a bit faster than an airplane taking off! These quick speeds are the basis for everything the nervous system does, from making sense of what your eyes see to deciding what you're going to have for lunch.

Neurons transmit their signals from one part of the body to another through long nerve fibers. Depending on the job of the fiber, the speed can change a lot. For instance, some of the nerve fibers that come from your brain and tell your legs to move can travel as fast as 250 miles per hour. For a signal traveling this fast, it takes about 20 milliseconds to travel. However, some signals are much slower like the signals that tell you are being tickled and travel around 1 mile per hour. For this signal it can take a second or more for you to fully feel it.

Different nerve fibers send signals faster or slower based on how thick they are, with nerves that send signals faster being thickened. Also fast nerve fibers also have a protective jacket on them called “myelin” which also makes the signal move faster. Keep in mind that the fastest nerve signals are still about 2.5 million times slower than electricity. So nerve signals have electrical parts to them, but are not purely electrical.

The nervous system is made up of many different types of neurons that all play different roles. You have neurons that transmit commands to your muscles, that respond to touch, pressure, or cold, that respond to pain, and more!

Each neuron has its own speed it transmits impulses at. Muscle command neurons have one of the fastest speeds (80-120 m/s) which makes sense because during running or other physical activities we often need to make quick adjustments to how we are running and what are body is doing. At that speed it would take under 9/1000 (.009 or nine thousandths) of a second for a signal to get from your brain to your hand.

Other neuron speeds vary from .05-2.0 m/s for pain/warmth to 3-30 m/s for touch and pressure. If you have ever grabbed something hot on accident and it has taken a second to realize it, that was caused by the slower neuron speed of pain/warmth neurons.

The speed of the neuron likely depends on the importance of a quick response. If a muscle command neuron worked at the slower speed of a pain neuron (which happens for some medical conditions), it would be very difficult to walk or keep balance. On the other hand, the slow rate of pain neurons doesn’t result in any major loss of function, so those neurons can be slower.

Overall, each neuron type has a different speed they operate at in order for the nervous system to function.


How do nerve impulses travel so quickly? - Biology

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  • Biology Lesson 07 Biology Lesson 15: The Nervous and Endocrine Systems, writing homework help

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Biology Lesson 15: The Nervous and Endocrine Systems

A spider web? Some sort of exotic bacteria? Maybe an illustration of a new species of jellyfish. This is actually a nerve cell, the cell of the nervous system. This cell sends electrical "sparks" that transmit signals throughout your body. In this chapter, you will learn more about nerve cells such as this one and their impressive abilities.

Section 1: The Nervous System

Section Objectives

  • Describe the structure of a neuron, and identify types of neurons.
  • Explain how nerve impulses are transmitted.
  • Identify parts of the central nervous system and their functions.
  • Outline the divisions and subdivisions of the peripheral nervous system.
  • Explain how sensory stimuli are perceived and interpreted.
  • State how drugs affect the nervous system.
  • Identify several nervous system disorders.
  • action potential
  • autonomic nervous system (ANS)
  • axon
  • brain
  • brain stem
  • cell body
  • central nervous system (CNS)
  • cerebellum
  • dendrite
  • drug abuse
  • drug addiction
  • interneuron
  • motor neuron
  • myelin sheath
  • nerve
  • nerve impulse
  • nervous system
  • neuron
  • neurotransmitter
  • peripheral nervous system (PNS)
  • psychoactive drug
  • resting potential
  • sensory neuron
  • sensory receptor
  • somatic nervous system (SNS)
  • spinal cord
  • synapse

Introduction

A small child darts in front of your bike as you race down the street. You see the child and immediately react. You put on the brakes, steer away from the child, and yell out a warning—all in just a split second. How do you respond so quickly? Such rapid responses are controlled by your nervous system. The nervous system is a complex network of nervous tissue that carries electrical messages throughout the body (see Figure below). To understand how nervous messages can travel so quickly, you need to know more about nerve cells.

The human nervous system includes the brain and spinal cord (central nervous system) and nerves that run throughout the body (peripheral nervous system).

Nerve Cells

Although the nervous system is very complex, nervous tissue consists of just two basic types of nerve cells: neurons and glial cells. Neurons are the structural and functional units of the nervous system. They transmit electrical signals, called nerve impulses. Glial cells provide support for neurons. For example, they provide neurons with nutrients and other materials.

Neuron Structure

As shown in Figure below, a neuron consists of three basic parts: the cell body, dendrites, and axon. You can watch an animation of the parts of a neuron at this link: http://www.garyfisk.com/anim/neuronparts.swf.

  • The cell body contains the nucleus and other cell organelles.
  • Dendrites extend from the cell body and receive nerve impulses from other neurons.
  • The axon is a long extension of the cell body that transmits nerve impulses to other cells. The axon branches at the end, forming axon terminals. These are the points where the neuron communicates with other cells.

The structure of a neuron allows it to rapidly transmit nerve impulses to other cells.

The neuron is discussed at:

Myelin Sheath

The axon of many neurons has an outer layer called a myelin sheath (see Figureabove). Myelin is a lipid produced by a type of a glial cell known as a Schwann cell. The myelin sheath acts like a layer of insulation, similar to the plastic that encases an electrical cord. Regularly spaced nodes, or gaps, in the myelin sheath allow nerve impulses to skip along the axon very rapidly.

Types of Neurons

Neurons are classified based on the direction in which they carry nerve impulses.

  • Sensory neurons carry nerve impulses from tissues and organs to the spinal cord and brain.
  • Motor neurons carry nerve impulses from the brain and spinal cord to muscles and glands (see Figurebelow)
  • Interneurons carry nerve impulses back and forth between sensory and motor neurons.

This axon is part of a motor neuron. It transmits nerve impulses to a skeletal muscle, causing the muscle to contract.

Nerve Impulses

Nerve impulses are electrical in nature. They result from a difference in electrical charge across the plasma membrane of a neuron. How does this difference in electrical charge come about? The answer involves ions, which are electrically charged atoms or molecules.

Resting Potential

When a neuron is not actively transmitting a nerve impulse, it is in a resting state, ready to transmit a nerve impulse. During the resting state, the sodium-potassium pump maintains a difference in charge across the cell membrane (see Figure below). It uses energy in ATP to pump positive sodium ions (Na + ) out of the cell and negative potassium ions (K - ) into the cell. As a result, the inside of the neuron is negatively charged, while the extracellular fluid surrounding the neuron is positively charged. This difference in electrical charge is called the resting potential.

The sodium-potassium pump maintains the resting potential of a neuron.

Action Potential

A nerve impulse is a sudden reversal of the electrical charge across the membrane of a resting neuron. The reversal of charge is called an action potential.It begins when the neuron receives a chemical signal from another cell. The signal causes gates in the sodium-potassium pump to open, allowing positive sodium ions to flow back into the cell. As a result, the inside of the cell becomes positively charged and the outside becomes negatively charged. This reversal of charge ripples down the axon very rapidly as an electric current (see Figurebelow).

An action potential speeds along an axon in milliseconds.

In neurons with myelin sheaths, ions flow across the membrane only at the nodes between sections of myelin. As a result, the action potential jumps along the axon membrane from node to node, rather than spreading smoothly along the entire membrane. This increases the speed at which it travels.

The action potential is discussed at:

You may choose to review the sodium-potassium pump prior to watching the action potential videos.

The Synapse

The place where an axon terminal meets another cell is called a synapse. The axon terminal and other cell are separated by a narrow space known as a synaptic cleft (see Figure below). When an action potential reaches the axon terminal, the axon terminal releases molecules of a chemical called a neurotransmitter. The neurotransmitter molecules travel across the synaptic cleft and bind to receptors on the membrane of the other cell. If the other cell is a neuron, this starts an action potential in the other cell. You can view animations of neurotransmission at a synapse at the following links:

At a synapse, neurotransmitters are released by the axon terminal. They bind with receptors on the other cell.

The synapse is further discussed at:

Central Nervous System

The nervous system has two main divisions: the central nervous system and the peripheral nervous system (see Figure below) (image in .pdf file). The central nervous system (CNS) includes the brain and spinal cord (see Figure below) (image in .pdf file)

You can see an overview of the central nervous system at this link:

The two main divisions of the human nervous system are the central nervous system and the peripheral nervous system. The peripheral nervous system has additional divisions.

This diagram shows the components of the central nervous system.

The Brain

The brain is the most complex organ of the human body and the control center of the nervous system. It contains an astonishing 100 billion neurons! The brain controls such mental processes as reasoning, imagination, memory, and language. It also interprets information from the senses. In addition, it controls basic physical processes such as breathing and heartbeat. The brain has three major parts: the cerebrum, cerebellum, and brain stem. These parts are shown in Figure below (image in .pdf file) and described in this section.

You can also take interactive animated tours of the brain at these links:

In this drawing, assume you are looking at the left side of the head. This is how the brain would appear if you could look underneath the skull.

  • The cerebrum is the largest part of the brain. It controls conscious functions such as reasoning, language, sight, touch, and hearing. It is divided into two hemispheres, or halves. The hemispheres are very similar but not identical to one another. They are connected by a thick bundle of axons deep within the brain. Each hemisphere is further divided into the four lobes shown in Figure below.
  • The cerebellum is just below the cerebrum. It coordinates body movements. Many nerve pathways link the cerebellum with motor neurons throughout the body.
  • The brain stem is the lowest part of the brain. It connects the rest of the brain with the spinal cord and passes nerve impulses between the brain and spinal cord. It also controls unconscious functions such as heart rate and breathing.

Each hemisphere of the cerebrum consists of four parts, called lobes. Each lobe is associated with particular brain functions. Just one function of each lobe is listed here (image in .pdf file).

Spinal Cord

The spinal cord is a thin, tubular bundle of nervous tissue that extends from the brainstem and continues down the center of the back to the pelvis. It is protected by the vertebrae, which encase it. The spinal cord serves as an information superhighway, passing messages from the body to the brain and from the brain to the body.

Peripheral Nervous System

The peripheral nervous system (PNS) consists of all the nervous tissue that lies outside the central nervous system. It is shown in blue in Figure below. It is connected to the central nervous system by nerves. A nerve is a cable-like bundle of axons. Some nerves are very long. The longest human nerve is the sciatic nerve. It runs from the spinal cord in the lower back down the left leg all the way to the toes of the left foot. Like the nervous system as a whole, the peripheral nervous system also has two divisions: the sensory division and the motor division.

  • The sensory division of the PNS carries sensory information from the body to the central nervous system. How sensations are detected and perceived is described in a later section of this Section.
  • The motor division of the PNS carries nerve impulses from the central nervous system to muscles and glands throughout the body. The nerve impulses stimulate muscles to contract and glands to secrete hormones. The motor division of the peripheral nervous system is further divided into the somatic and autonomic nervous systems.

The nerves of the peripheral nervous system are shown in yellow in this image. Can you identify the sciatic nerve?

Somatic Nervous System

The somatic nervous system (SNS) controls mainly voluntary activities that are under conscious control. It is made up of nerves that are connected to skeletal muscles. Whenever you perform a conscious movement—from signing your name to riding your bike—your somatic nervous system is responsible. The somatic nervous system also controls some unconscious movements called reflexes. A reflex is a very rapid motor response that is not directed by the brain. In a reflex, nerve impulses travel to and from the spinal cord in a reflex arc, like the one in Figure below (image in .pdf file). In this example, the person jerks his hand away from the flame without any conscious thought. It happens unconsciously because the nerve impulses bypass the brain.

A reflex arc like this one enables involuntary actions. How might reflex responses be beneficial to the organism?

Autonomic Nervous System

All other involuntary activities not under conscious control are the responsibility of the autonomic nervous system (ANS). Nerves of the ANS are connected to glands and internal organs. They control basic physical functions such as heart rate, breathing, digestion, and sweat production. The autonomic nervous system also has two subdivisions: the sympathetic division and the parasympathetic division. You can watch an animation comparing these two subdivisions at this link:

  • The sympathetic division deals with emergency situations. It prepares the body for “fight or flight.” Do you get clammy palms or a racing heart when you have to play a solo or give a speech? Nerves of the sympathetic division control these responses.
  • The parasympathetic division controls involuntary activities that are not emergencies. For example, it controls the organs of your digestive system so they can break down the food you eat.

The Senses

The sensory division of the PNS includes several sense organs—the eyes, ears, mouth, nose, and skin. Each sense organ has special cells, called sensory receptors, that respond to a particular type of stimulus. For example, the nose has sensory receptors that respond to chemicals, which we perceive as odors. Sensory receptors send nerve impulses to sensory nerves, which carry the nerve impulses to the central nervous system. The brain then interprets the nerve impulses to form a response.

Sight

Sight is the ability to sense light, and the eye is the organ that senses light. Light first passes through the cornea of the eye, which is a clear outer layer that protects the eye (see Figure below)(image in .pdf file). Light enters the eye through an opening called the pupil. The light then passes through the lens, which focuses it on the retina at the back of the eye. The retina contains light receptor cells, like those in the photograph on the first page of this chapter. These cells send nerve impulses to the optic nerve, which carries the impulses to the brain. The brain interprets the impulses and “tells” us what we are seeing. To learn more about the eye and the sense of sight, you can go to the link below. Be sure to take the quick quiz at the end of the animation.

The eye is the organ that senses light and allows us to see.

Hearing

Hearing is the ability to sense sound waves, and the ear is the organ that senses sound. Sound waves enter the auditory canal and travel to the eardrum (see Figure below)(image in .pdf file). They strike the eardrum and make it vibrate. The vibrations then travel through several other structures inside the ear and reach the cochlea. The cochlea is a coiled tube filled with liquid. The liquid moves in response to the vibrations, causing tiny hair cells lining the cochlea to bend. In response, the hair cells send nerve impulses to the auditory nerve, which carries the impulses to the brain. The brain interprets the impulses and “tells” us what we are hearing.

The ear is the organ that senses sound waves and allows us to hear. It also senses body position so we can keep our balance.

Balance

The ears are also responsible for the sense of balance. Balance is the ability to sense and maintain body position. The semicircular canals inside the ear (see Figure above)(image in .pdf file) contain fluid that moves when the head changes position. Tiny hairs lining the semicircular canals sense movement of the fluid. In response, they send nerve impulses to the vestibular nerve, which carries the impulses to the brain. The brain interprets the impulses and sends messages to the peripheral nervous system. This system maintains the body’s balance by triggering contractions of skeletal muscles as needed.

Taste and Smell

Taste and smell are both abilities to sense chemicals. Like other sense receptors, both taste and odor receptors send nerve impulses to the brain, and the brain “tells” use what we are tasting or smelling. Taste receptors are found in tiny bumps on the tongue called taste buds (see Figure below)(image in .pdf file). There are separate taste receptors for sweet, salty, sour, bitter, and meaty tastes. The meaty taste is called umami. You can learn more about taste receptors and the sense of taste by watching the animation at the following link:

Taste buds on the tongue contain taste receptor cells.

Odor receptors line the passages of the nose (see Figure below)(image in .pdf file). They sense chemicals in the air. In fact, odor receptors can sense hundreds of different chemicals. Did you ever notice that food seems to have less taste when you have a stuffy nose? This occurs because the sense of smell contributes to the sense of taste, and a stuffy nose interferes with the ability to smell.

Odor receptors. Odor receptors and their associated nerves (in yellow) line the top of the nasal passages.

Touch

Touch is the ability to sense of pressure. Pressure receptors are found mainly in the skin. They are especially concentrated on the tongue, lips, face, palms of the hands, and soles of the feet. Some touch receptors sense differences in temperature or pain. How do pain receptors help maintain homeostasis? (Hint: What might happen if we couldn’t feel pain?)

See the link below for a summary.

Drugs and the Nervous System

A drug is any chemical that affects the body’s structure or function. Many drugs, including both legal and illegal drugs, are psychoactive drugs. This means that they affect the central nervous system, generally by influencing the transmission of nerve impulses. For example, some psychoactive drugs mimic neurotransmitters.

Examples of Psychoactive Drugs

Caffeine is an example of a psychoactive drug. It is found in coffee and many other products (see Table below). Caffeine is a central nervous system stimulant. Like other stimulant drugs, it makes you feel more awake and alert. Other psychoactive drugs include alcohol, nicotine, and marijuana. Each has a different effect on the central nervous system. Alcohol, for example, is a depressant. It has the opposite effects of a stimulant like caffeine.

Caffeine Content of Popular Products

Dark chocolate candy (1.5 oz)

Table 22.2 Many commonly consumed products contain caffeine.

Drug Abuse and Addiction

Psychoactive drugs may bring about changes in mood that users find desirable, so the drugs may be abused. Drug abuse is use of a drug without the advice of a medical professional and for reasons not originally intended. Continued use of a psychoactive drug may lead to drug addiction, in which the drug user is unable to stop using the drug. Over time, a drug user may need more of the drug to get the desired effect. This can lead to drug overdose and death.

Disorders of the Nervous System

There are several different types of problems that can affect the nervous system.

  • Vascular disorders involve problems with blood flow. For example, a stroke occurs when a blood clot blocks blood flow to part of the brain. Brain cells die quickly if their oxygen supply is cut off. This may cause paralysis and loss of other normal functions, depending on the part of the brain that is damaged.
  • Nervous tissue may become infected by microorganisms. Meningitis, for example, is caused by a viral or bacterial infection of the tissues covering the brain. This may cause the brain to swell and lead to brain damage and death.
  • Brain or spinal cord injuries may cause paralysis and other disabilities. Injuries to peripheral nerves can cause localized pain or numbness.
  • Abnormal brain functions can occur for a variety of reasons. Examples include headaches, such as migraine headaches, and epilepsy, in which seizures occur.
  • Nervous tissue may degenerate, or break down. Alzheimer’s disease is an example of this type of disorder, as is Amyotrophic lateral sclerosis, or ALS. ALS is also known as Lou Gehrig's disease. It leads to a gradual loss of higher brain functions.

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Section Summary

  • Neurons are the structural and functional units of the nervous system. They consist of a cell body, dendrites, and axon. Neurons transmit nerve impulses to other cells. Types of neurons include sensory neurons, motor neurons, and interneurons.
  • A nerve impulse begins when a neuron receives a chemical stimulus. The impulse travels down the axon membrane as an electrical action potential to the axon terminal. The axon terminal releases neurotransmitters that carry the nerve impulse to the next cell.
  • The central nervous includes the brain and spinal cord. The brain is the control center of the nervous system. It controls virtually all mental and physical processes. The spinal cord is a long, thin bundle of nervous tissue that passes messages from the body to the brain and from the brain to the body.
  • The peripheral nervous system consists of all the nervous tissue that lies outside the central nervous system. It is connected to the central nervous system by nerves. It has several divisions and subdivisions that transmit nerve impulses between the central nervous system and the rest of the body.
  • Human senses include sight, hearing, balance, taste, smell, and touch. Sensory organs such as the eyes contain cells called sensory receptors that respond to particular sensory stimuli. Sensory nerves carry nerve impulses from sensory receptors to the central nervous system. The brain interprets the nerve impulses to form a response.
  • Drugs are chemicals that affect the body’s structure or function. Psychoactive drugs, such as caffeine and alcohol, affect the central nervous system by influencing the transmission of nerve impulses in the brain. Psychoactive drugs may be abused and lead to drug addiction.
  • Disorders of the nervous system include blood flow problems such as stroke, infections such as meningitis, brain injuries, and degeneration of nervous tissue, as in Alzheimer’s disease.

Extra Practice

1. Tony’s dad was in a car accident in which his neck was broken. He survived the injury but is now paralyzed from the neck down. Explain why.

2. Multiple sclerosis is a disease in which the myelin sheaths of neurons in the central nervous system break down. What symptoms might this cause? Why?

3. Explain how resting potential is maintained and how an action potential occurs.

4. Compare and contrast the somatic and autonomic nervous systems.

Points to Consider

In this Section, you learned that the nervous system enables electrical messages to be sent through the body very rapidly.

  • Often, it’s not necessary for the body to respond so rapidly. Can you think of another way the body could send messages that would travel more slowly? What about a way that makes use of the network of blood vessels throughout the body?
  • Instead of electrical nerve impulses, what other way might messages be transmitted in the body? Do you think chemical molecules could be used to carry messages? How might this work?

Section 2: The Endocrine System

Section Objectives

  • List the glands of the endocrine system and their effects.
  • Explain how hormones work by binding to receptors of target cells.
  • Describe feedback mechanisms that regulate hormone secretion.
  • Identify three endocrine system disorders.

Vocabulary

  • adrenal glands
  • endocrine system
  • gonads
  • hypothalamus
  • pancreas
  • parathyroid glands
  • pineal gland
  • pituitary gland
  • target cell
  • thyroid gland

Introduction

The nervous system isn’t the only message-relaying system of the human body. The endocrine system also carries messages. The endocrine system is a system of glands that release chemical messenger molecules into the bloodstream. The messenger molecules are hormones. Hormones act slowly compared with the rapid transmission of electrical messages by the nervous system. They must travel through the bloodstream to the cells they affect, and this takes time. On the other hand, because endocrine hormones are released into the bloodstream, they travel throughout the body. As a result, endocrine hormones can affect many cells and have body-wide effects.

Glands of the Endocrine System

The major glands of the endocrine system are shown in Figure below (image in .pdf file).

You can access an a similar, animated endocrine system chart at:

The glands of the endocrine system are the same in males and females except for the testes, which are found only in males, and ovaries, which are found only in females.

Hypothalamus

The hypothalamus is actually part of the brain (see Figure below)(image in .pdf file), but it also secretes hormones. Some of its hormones that "tell" the pituitary gland to either secrete or stop secreting its hormones. In this way, the hypothalamus provides a link between the nervous and endocrine systems. The hypothalamus also produces hormones that directly regulate body processes. These hormones travel to the pituitary gland, which stores them until they are needed. The hormones include antidiuretic hormone and oxytocin.

  • Antidiuretic hormone stimulates the kidneys to conserve water by producing more concentrated urine.
  • Oxytocin stimulates the contractions of childbirth, among other functions.

The hypothalamus and pituitary gland are located close together at the base of the brain.

Pituitary Gland

The pea-sized pituitary gland is attached to the hypothalamus by a thin stalk (see Figure above)(image in .pdf file). It consists of two bulb-like lobes. The posterior (back) lobe stores hormones from the hypothalamus. The anterior (front) lobe secretes pituitary hormones. Several pituitary hormones and their effects are listed in Table below. Most pituitary hormones control other endocrine glands. That’s why the pituitary is often called the “master gland” of the endocrine system.

Pituitary Hormones

Adrenocorticotropic hormone (ACTH)

Stimulates the cortex of each adrenal gland to secrete its hormones

Thyroid-stimulating hormone (TSH)

Stimulates the thyroid gland to secrete thyroid hormone

Stimulates body cells to synthesize proteins and grow

Follicle-stimulating hormone (FSH)

Stimulates the ovaries to develop mature eggs stimulates the testes to produce sperm

Stimulates the ovaries and testes to secrete sex hormones stimulates the ovaries to release eggs

Stimulates the mammary glands to produce milk

Table 22.4 Hormones secreted by the pituitary gland control many body processes, often by regulating other endocrine glands.

Other Endocrine Glands

Other glands of the endocrine system are described below. You can refer to Figure above to see where they are located.

  • The thyroid gland is a large gland in the neck. Thyroid hormones increase the rate of metabolism in cells throughout the body. They control how quickly cells use energy and make proteins.
  • The two parathyroid glands are located behind the thyroid gland. Parathyroid hormone helps keep the level of calcium in the blood within a narrow range. It stimulates bone cells to dissolve calcium in bone matrix and release it into the blood.
  • The pineal gland is a tiny gland located at the base of the brain. It secretes the hormone melatonin. This hormone controls sleep-wake cycles and several other processes.
  • The pancreas is located near the stomach. Its hormones include insulin and glucagon. These two hormones work together to control the level of glucose in the blood. Insulin causes excess blood glucose to be taken up by the liver, which stores the glucose as glycogen. Glucagon stimulates the liver to break down glycogen into glucose and release it back into the blood. The pancreas also secretes digestive enzymes into the digestive tract.
  • The two adrenal glands are located above the kidneys. Each gland has an inner and outer part. The outer part, called the cortex, secretes hormones such as cortisol, which helps the body deal with stress, and aldosterone, which helps regulate the balance of minerals in the body. The inner part of each adrenal gland, called the medulla, secretes fight-or-flight hormones such as adrenaline, which prepare the body to respond to emergencies. For example, adrenaline increases the amount of oxygen and glucose going to the muscles.
  • You can see an animation of this response at:
  • The gonads secrete sex hormones. The male gonads are called testes. They secrete the male sex hormone testosterone. The female gonads are called ovaries. They secrete the female sex hormone estrogen. Sex hormones are involved in the changes of puberty. They also control the production of gametes by the gonads.

How Hormones Work

Endocrine hormones travel throughout the body in the blood. However, each hormone affects only certain cells, called target cells. A target cell is the type of cell on which a hormone has an effect. A target cell is affected by a particular hormone because it has receptor proteins that are specific to that hormone. A hormone travels through the bloodstream until it finds a target cell with a matching receptor it can bind to. When the hormone binds to a receptor, it causes a change within the cell. Exactly how this works depends on whether the hormone is a steroid hormone or a non-steroid hormone.

You can watch an animation that shows how both types of hormones work at:

Hormones are discussed at:

Steroid Hormones

Steroid hormones are made of lipids, such as phospholipids and cholesterol. They are fat soluble, so they can diffuse across the plasma membrane of target cells and bind with receptors in the cytoplasm of the cell (see Figure below)(image in .pdf file). The steroid hormone and receptor form a complex that moves into the nucleus and influences the expression of genes. Examples of steroid hormones include cortisol and sex hormones.

A steroid hormone crosses the plasma membrane of a target cell and binds with a receptor inside the cell.

Non-Steroid Hormones

Non-steroid hormones are made of amino acids. They are not fat soluble, so they cannot diffuse across the plasma membrane of target cells. Instead, a non-steroid hormone binds to a receptor on the cell membrane (see Figure below)(image in .pdf file). The binding of the hormone triggers an enzyme inside the cell membrane. The enzyme activates another molecule, called the second messenger, which influences processes inside the cell. Most endocrine hormones are non-steroid hormones, including insulin and thyroid hormones.

A non-steroid hormone binds with a receptor on the plasma membrane of a target cell. Then, a secondary messenger affects cell processes.

Hormone Regulation: Feedback Mechanisms

Hormones control many cell activities, so they are very important for homeostasis. But what controls the hormones themselves? Most hormones are regulated by feedback mechanisms. A feedback mechanism is a loop in which a product feeds back to control its own production. Most hormone feedback mechanisms involve negative feedback loops. Negative feedback keeps the concentration of a hormone within a narrow range.

Negative Feedback

Negative feedback occurs when a product feeds back to decrease its own production. This type of feedback brings things back to normal whenever they start to become too extreme. The thyroid gland is a good example of this type of regulation. It is controlled by the negative feedback loop shown in Figurebelow(image in .pdf file).

You can also watch an animation of this process at:

The thyroid gland is regulated by a negative feedback loop. The loop includes the hypothalamus and pituitary gland in addition to the thyroid.

Here’s how thyroid regulation works. The hypothalamus secretes thyrotropin-releasing hormone, or TRH. TRH stimulates the pituitary gland to produce thyroid-stimulating hormone, or TSH. TSH, in turn, stimulates the thyroid gland to secrete its hormones. When the level of thyroid hormones is high enough, the hormones feed back to stop the hypothalamus from secreting TRH and the pituitary from secreting TSH. Without the stimulation of TSH, the thyroid gland stops secreting its hormones. Soon, the level of thyroid hormone starts to fall too low. What do you think happens next?

This process is discussed at:

Negative feedback also controls insulin secretion by the pancreas. You can interact with a feedback loop of this process at:

Positive feedback

Positive feedback occurs when a product feeds back to increase its own production. This causes conditions to become increasingly extreme. An example of positive feedback is milk production by a mother for her baby. As the baby suckles, nerve messages from the nipple cause the pituitary gland to secrete prolactin. Prolactin, in turn, stimulates the mammary glands to produce milk, so the baby suckles more. This causes more prolactin to be secreted and more milk to be produced. This example is one of the few positive feedback mechanisms in the human body. What do you think would happen if milk production by the mammary glands was controlled by negative feedback instead?

Endocrine System Disorders

Diseases of the endocrine system are relatively common. An endocrine disease usually involves the secretion of too much or not enough hormone. When too much hormone is secreted, it is called hypersecretion. When not enough hormone is secreted, it is called hyposecretion.

Hypersecretion

Hypersecretion by an endocrine gland is often caused by a tumor. For example, a tumor of the pituitary gland can cause hypersecretion of growth hormone. If this occurs in childhood, it results in very long arms and legs and abnormally tall stature by adulthood. The condition is commonly known as gigantism (see Figurebelow) (image in .pdf file).

Hyposecretion

Destruction of hormone-secreting cells of a gland may result in not enough of a hormone being secreted. This occurs in Type 1 diabetes. In this case, the body’s own immune system attacks and destroys cells of the pancreas that secrete insulin. A person with type 1 diabetes must frequently monitor the level of glucose in the blood (see Figure below) (image in .pdf file). If the level of blood glucose is too high, insulin is injected to bring it under control. If it is too low, a small amount of sugar is consumed. To measure the level of glucose in the blood, a drop of blood is placed on a test strip, which is read by a meter.

Hormone Resistance

In some cases, an endocrine gland secretes a normal amount of hormone, but target cells do not respond to the hormone. Often, this is because target cells have because resistant to the hormone. Type 2 diabetes is an example of this type of endocrine disorder. In Type 2 diabetes, body cells do not respond to normal amounts of insulin. As a result, cells do not take up glucose and the amount of glucose in the blood becomes too high. This type of diabetes cannot be treated by insulin injections. Instead, it is usually treated with medication and diet.

Section Summary

  • The endocrine system consists of glands that secrete hormones into the bloodstream. It is regulated by a part of the brain called the hypothalamus, which also secretes hormones. The hypothalamus controls the pituitary gland, which is called the “master gland” of the endocrine system because its hormones regulate other endocrine glands. Other endocrine glands include the thyroid gland and pancreas.
  • Hormones work by binding to protein receptors either inside target cells or on their plasma membranes. The binding of a steroid hormone forms a hormone-receptor complex that affects gene expression in the nucleus of the target cell. The binding of a non-steroid hormone activates a second messenger that affects processes within the target cell.
  • Most hormones are controlled by negative feedback in which the hormone feeds back to decrease its own production. This type of feedback brings things back to normal whenever they start to become too extreme. Positive feedback is much less common because it causes conditions to become increasingly extreme.
  • Endocrine system disorders usually involve the secretion of too much or not enough hormone. For example, a tumor of the adrenal gland may lead to excessive secretion of growth hormone, which causes gigantism. In Type 1 diabetes, the pancreas does not secrete enough insulin, which causes high levels of glucose in the blood.

Points to Consider

In this Section you learned that endocrine hormones can affect cells throughout the body because they travel in the blood through the circulatory system.

  • Do you know what organs make up the circulatory system?
  • Can you explain what causes blood to move through the system?

Opening image copyright by Sebastian Kaulitzki, 2010. Used under license from Shutterstock.com.

Lesson 15 Review Questions

Directions: Answer each of the following questions.

1. List and describe the parts of a neuron.

2. What do motor neurons do?

3. Define resting potential.

4. Name the organs of the central nervous system.

5. Which part of the brain controls conscious functions such as reasoning?

6. What are the roles of the brain stem?

7. Identify the two major divisions of the peripheral nervous system.

9. What is a psychoactive drug? Give two examples.

10. Define drug abuse. When does drug addiction occur?

11. Identify three nervous system disorders.

13. List the major glands of the endocrine system.

Essay submission: Select 1 Biology topic from this lesson, and submit a 3-5 paragraph essay about the topic. Remember to cite your sources!


Summary

  • Neurons are the structural and functional units of the nervous system. They consist of a cell body, dendrites, and axon.
  • Neurons transmit nerve impulses to other cells.
  • Types of neurons include sensory neurons, motor neurons, and interneurons.

[Attributions and Licenses]

This article is licensed under a CC BY-NC-SA 4.0 license.

Note that the video(s) in this lesson are provided under a Standard YouTube License.


Anatomy of the human body notes and summary

Anatomy of the Human Body Notes A

Organization of the Human Body

Many people have compared the human body to a machine. Think about some common machines, such as drills and washing machines. Each machine consists of many parts, and each part does a specific job, yet all the parts work together to perform an overall function. The human body is like a machine in all these ways. In fact, it may be the most fantastic machine on Earth.

Levels of Organization

The human machine is organized at different levels, starting with the cell and ending with the entire organism. At each higher level of organization, there is a greater degree of complexity. The levels are:

Cells

The most basic parts of the human machine are cells—an amazing 100 trillion of them by the time the average person reaches adulthood! Cells are the basic units of structure and function in the human body, as they are in all living things. Each cell carries out basic life processes that allow the body to survive. Many human cells are specialized in form and function. Each type of cell in the figure plays a specific role. For example, nerve cells have long projections that help them carry electrical messages to other cells. Muscle cells have many mitochondria that provide the energy they need to move the body.

Tissues

After the cell, the tissue is the next level of organization in the human body. A tissue is a group of connected cells that have a similar function. There are four basic types of human tissues: epithelial, muscle, nervous, and connective tissues. These four tissue types make up all the organs of the human body.

The human body consists of these four tissue types.

Connective tissue is made up of cells that form the body’s structure. Examples include bone and cartilage.

Epithelial tissue is made up of cells that line inner and outer body surfaces, such as the skin and the lining of the digestive tract. Epithelial tissue protects the body and its internal organs, secretes substances such as hormones, and absorbs substances such as nutrients.

Muscle tissue is made up of cells that have the unique ability to contract, or become shorter. Muscles attached to bones enable the body to move.

Nervous tissue is made up of neurons, or nerve cells, that carry electrical messages. Nervous tissue makes up the brain and the nerves that connect the brain to all parts of the body.

Organs and Organ Systems

After tissues, organs are the next level of organization of the human body. An organ is a structure that consists of two or more types of tissues that work together to do the same job. Examples of human organs include the brain, heart, lungs, skin, and kidneys. Human organs are organized into organ systems. An organ system is a group of organs that work together to carry out a complex overall function. Each organ of the system does part of the larger job.

Vocabulary

cell: Basic unit of structure and function of living things.

connective tissue: Tissue made up of cells that form the body’s structure, such as bone and cartilage.

epithelial tissue: Tissue made up of cells that line inner and outer body surfaces, such as skin.

muscle tissue: Tissue made up of cells that can contract includes smooth, skeletal, and cardiac muscle tissue.

nervous tissue: Tissue made up of neurons, or nerve cells carry electrical messages.

neuron: The structural and functional unit of the nervous system a nerve cell.

organ: Structure composed of more than one type of tissue that performs a particular function.

organ system: Group of organs that work together performing a specific function.

tissue: Group of cells of the same kind that perform a particular function in an organism.

Summary

The human body is organized at different levels, starting with the cell.

Cells are organized into tissues, and tissues form organs.

Organs are organized into organ systems such as the skeletal and muscular systems.

What happens if stability is disrupted?

Remove one stone and the whole arch collapses. The same is true for the human body. All the systems work together to maintain stability or homeostasis. Disrupt one system, and the whole body may be affected.

Homeostasis

All of the organs and organ systems of the human body work together like a well-oiled machine. This is because they are closely regulated by the nervous and endocrine systems. The nervous system controls virtually all body activities, and the endocrine system secretes hormones that regulate these activities. Functioning together, the organ systems supply body cells with all the substances they need and eliminate their wastes. They also keep temperature, pH, and other conditions at just the right levels to support life processes.

Maintaining Homeostasis

The process in which organ systems work to maintain a stable internal environment is called homeostasis. Keeping a stable internal environment requires constant adjustments. Here are just three of the many ways that human organ systems help the body maintain homeostasis:

Respiratory system: A high concentration of carbon dioxide in the blood triggers faster breathing. The lungs exhale more frequently, which removes carbon dioxide from the body more quickly.

Excretory system: A low level of water in the blood triggers retention of water by the kidneys. The kidneys produce more concentrated urine, so less water is lost from the body.

Endocrine system: A high concentration of sugar in the blood triggers secretion of insulin by an endocrine gland called the pancreas. Insulin is a hormone that helps cells absorb sugar from the blood.

Failure of Homeostasis

Many homeostatic mechanisms such as these work continuously to maintain stable conditions in the human body. Sometimes, however, the mechanisms fail. When they do, cells may not get everything they need, or toxic wastes may accumulate in the body. If homeostasis is not restored, the imbalance may lead to disease or even death.

Vocabulary

endocrine system: Organ system of glands that release hormones into the blood.

homeostasis: The process of maintaining a stable environment inside a cell or an entire organism.

hormone: a chemical messenger molecule

nervous system: Organ system that carries electrical messages throughout the body.

Summary

All of the organ systems of the body work together to maintain homeostasis of the organism.

If homeostasis fails, death or disease may result.

Human Skeletal System

The human skeleton is an internal framework that, in adults, consists of 206 bones. You can learn more about bones in the animation “Bones Narrated”: http://medtropolis.com/virtual-body/

In addition to bones, the skeleton also consists of cartilage and ligaments:

Cartilage is a type of dense connective tissue, made of tough protein fibers, that provides a smooth surface for the movement of bones at joints.

A ligament is a band of fibrous connective tissue that holds bones together and keeps them in place.

The skeleton supports the body and gives it shape. It has several other functions as well, including:

protecting internal organs

providing attachment surfaces for muscles

maintaining mineral homeostasis.

Maintaining mineral homeostasis is a very important function of the skeleton, because just the right levels of calcium and other minerals are needed in the blood for normal functioning of the body. When mineral levels in the blood are too high, bones absorb some of the minerals and store them as mineral salts, which is why bones are so hard. When blood levels of minerals are too low, bones release some of the minerals back into the blood, thus restoring homeostasis.

Vocabulary

bone: Hard tissue in most vertebrates that consists of a collagen matrix, or framework, filled in with minerals such a calcium.

cartilage: Dense connective tissue that provides a smooth surface for the movement of bones at joints involved in endochondral ossification.

ligament: Band of fibrous connective tissue that holds bones together.

mineral homeostasis: Having the proper levels of minerals in the blood for normal functioning of the body.

Summary

The adult human skeleton includes 206 bones and other tissues.

The skeleton supports the body, protects internal organs, produces blood cells, and maintains mineral homeostasis.

Structure of Bones

It's common to think of bones as not living. But bones are very much living. In fact, you are constantly making new bone tissue. That means that you are also constantly getting rid of bone. Bone is full of blood and nerves and all sorts of cells and proteins, making it an extremely complex living tissue.

Many people think of bones as being dead, dry, and brittle. These adjectives correctly describe the bones of a preserved skeleton, but the bones in a living human being are very much alive. The basic structure of bones is bone matrix which makes up the underlying rigid framework of bones and is composed of both compact bone and spongy bone. The bone matrix consists of tough protein fibers, mainly collagen, that become hard and rigid due to mineralization with calcium crystals. Bone matrix is crisscrossed by blood vessels and nerves and also contains specialized bone cells that are actively involved in metabolic processes.

Bone Cells

There are three types of specialized cells in human bones: osteoblasts, osteocytes, and osteoclasts. These cells are responsible for bone growth and mineral homeostasis.

Osteoblasts make new bone cells and secrete collagen that mineralizes to become bone matrix. They are responsible for bone growth and the uptake of minerals from the blood.

Osteocytes regulate mineral homeostasis. They direct the uptake of minerals from the blood and the release of minerals back into the blood as needed.

Osteoclasts dissolve minerals in bone matrix and release them back into the blood.

Bones are far from static, or unchanging. Instead, they are dynamic, living tissues that are constantly being reshaped. Under the direction of osteocytes, osteoblasts continuously build up bone, while osteoclasts continuously break it down.

Bone Tissues

Bones consist of different types of tissue, including compact bone, spongy bone, bone marrow, and periosteum.

Compact bone makes up the dense outer layer of bone. Its functional unit is the osteon. Compact bone is very hard and strong.

Spongy bone is found inside bones and is lighter and less dense than compact bone. This is because spongy bone is porous.

Bone marrow is a soft connective tissue that produces blood cells. It is found inside the pores of spongy bone.

Periosteum is a tough, fibrous membrane that covers and protects the outer surfaces of bone.

Vocabulary

bone marrow: Soft connective tissue in spongy bone produces blood cells.

bone matrix: Rigid framework of bone that consists of tough protein fibers and mineral crystals.

collagen: Protein fibers in the extracellular matrix of bone and cartilage.

compact bone: Dense outer layer of bone that is very hard and strong.

osteoblast: Type of bone cell makes new bone cells and secretes collagen.

osteoclast: Type of bone cell dissolves minerals in bone and releases them back into the blood.

osteocyte: Type of bone cell regulates mineral homeostasis by directing the uptake of minerals from the blood and the release of minerals back into the blood as needed.

osteon: The functional unit of compact bone.

periosteum: Tough, fibrous membrane that covers the outer surface of bone.

spongy bone: Light, porous inner layer of bone that contains bone marrow.

Summary

Under the direction of osteocytes, osteoblasts continuously build up bone, while osteoclasts continuously break down bone. These processes help maintain mineral homeostasis.

Bone tissues include compact bone, spongy bone, bone marrow, and periosteum.

Smooth, Skeletal, and Cardiac Muscles

The muscular system consists of all the muscles of the body. Muscles are organs composed mainly of muscle cells, which are also called muscle fibers. Each muscle fiber is a very long, thin cell that can do something no other cell can do. It can contract, or shorten. Muscle contractions are responsible for virtually all the movements of the body, both inside and out. There are three types of muscle tissues in the human body: cardiac, smooth, and skeletal muscle tissues.

Smooth Muscle

Muscle tissue in the walls of internal organs such as the stomach and intestines is smooth muscle. When smooth muscle contracts, it helps the organs carry out their functions. For example, when smooth muscle in the stomach contracts, it squeezes the food inside the stomach, which helps break the food into smaller pieces. Contractions of smooth muscle are involuntary. This means they are not under conscious control.

Skeletal Muscle

Muscle tissue that is attached to bone is skeletal muscle. Whether you are blinking your eyes or running a marathon, you are using skeletal muscle. Contractions of skeletal muscle are voluntary, or under conscious control. Skeletal muscle is the most common type of muscle in the human body.

Cardiac Muscle

Cardiac muscle is found only in the walls of the heart. When cardiac muscle contracts, the heart beats and pumps blood. Cardiac muscle contains a great many mitochondria, which produce ATP for energy. This helps the heart resist fatigue. Contractions of cardiac muscle are involuntary, like those of smooth muscle. Cardiac muscle, like skeletal muscle, is arranged in bundles, so it appears striated, or striped.

Vocabulary

cardiac muscle: Involuntary, striated muscle found only in the walls of the heart.

muscle fiber: Long, thin muscle cell that has the ability to contract, or shorten.

muscular system: Human body system that includes all the muscles of the body.

skeletal muscle: Voluntary, striated muscle that is attached to bones of the skeleton and helps the body move.

smooth muscle: Involuntary, nonstriated muscle that is found in the walls of internal organs such as the stomach.

striated: Striped appearance of cardiac and skeletal muscle.

Summary

There are three types of human muscle tissue: smooth muscle (in internal organs), skeletal muscle, and cardiac muscle (only in the heart).

Skeletal Muscles

Skeletal Muscles

There are well over 600 skeletal muscles in the human body. Skeletal muscles vary considerably in size, from tiny muscles inside the middle ear to very large muscles in the upper leg.

Structure of Skeletal Muscles

Each skeletal muscle consists of hundreds or even thousands of skeletal muscle fibers. The fibers are bundled together and wrapped in connective tissue. The connective tissue supports and protects the delicate muscle cells and allows them to withstand the forces of contraction. It also provides pathways for nerves and blood vessels to reach the muscles. Skeletal muscles work hard to move body parts. They need a rich blood supply to provide them with nutrients and oxygen and to carry away their wastes.

Skeletal Muscles and Bones

Skeletal muscles are attached to the skeleton by tough connective tissues called tendons. Many skeletal muscles are attached to the ends of bones that meet at a joint. The muscles span the joint and connect the bones. When the muscles contract, they pull on the bones, causing them to move.

Muscles can only contract. They cannot actively extend, or lengthen. Therefore, to move bones in opposite directions, pairs of muscles must work in opposition. For example, the biceps and triceps muscles of the upper arm work in opposition to bend and extend the arm at the elbow.

Use It or Lose It

In exercises such as weight lifting, skeletal muscle contracts against a resisting force. Using skeletal muscle in this way increases its size and strength. In exercises such as running, the cardiac muscle contracts faster and the heart pumps more blood. Using cardiac muscle in this way increases its strength and efficiency. Continued exercise is necessary to maintain bigger, stronger muscles. If you don’t use a muscle, it will get smaller and weaker—so use it or lose it.

Vocabulary

joint: Place where two or more bones of the skeleton meet.

muscle fiber: Long, thin muscle cell that has the ability to contract, or shorten.

tendon: Tough connective tissue that attaches skeletal muscle to bones of the skeleton.

Summary

Skeletal muscles are attached to the skeleton and cause bones to move when they contract.

Muscle Contraction

What makes a muscle contract?

It starts with a signal from the nervous system. So it starts with a signal from your brain. The signal goes through your nervous system to your muscle. Your muscle contracts, and your bones move. And all this happens incredibly fast.

Muscle contraction occurs when muscle fibers get shorter. Literally, the muscle fibers get smaller in size. To understand how this happens, you need to know more about the structure of muscle fibers.

Structure of Muscle Fibers

Each muscle fiber contains hundreds of organelles called myofibrils. Each myofibril is made up of two types of protein filaments: actin filaments, which are thinner, and myosin filaments, which are thicker. Actin filaments are anchored to structures called Z lines. The region between two Z lines is called a sarcomere. Within a sarcomere, myosin filaments overlap the actin filaments. The myosin filaments have tiny structures called cross bridges that can attach to actin filaments.

Sliding Filament Theory

The most widely accepted theory explaining how muscle fibers contract is called the sliding filament theory. According to this theory, myosin filaments use energy from ATP to “walk” along the actin filaments with their cross bridges. This pulls the actin filaments closer together. The movement of the actin filaments also pulls the Z lines closer together, thus shortening the sarcomere.

When all of the sarcomeres in a muscle fiber shorten, the fiber contracts. A muscle fiber either contracts fully or it doesn’t contract at all. The number of fibers that contract determines the strength of the muscular force. When more fibers contract at the same time, the force is greater.

Muscle contraction is described by the sliding-filaments model:

ATP binds to a myosin head and is converted to ADP and P i , which remain attached to the myosin head.

Ca 2+ exposes the binding sites on the actin filaments. Ca 2+ binds to the troponin molecule causing tropomyosin to expose positions on the actin filament for the attachment of myosin heads.

Cross bridges between myosin heads and actin filaments form. When attachment sites on the actin are exposed, the myosin heads bind to actin to form cross bridges.

ADP and P i are released, and sliding motion of actin results. The attachment of cross bridges between myosin and actin causes the release of ADP and P i . This, in turn, causes a change in shape of the myosin head, which generates a sliding movement of the actin toward the center of the sacromere. This pulls the two Z discs together, effectively contracting the muscle fiber to produce a power stroke.

ATP causes the cross bridges to unbind. When a new ATP molecule attaches to the myosin head, the cross bridge between the actin and myosin breaks, returning the myosin head to its unattached position.

Without the addition of a new ATP molecule, the cross bridges remain attached to the actin filaments. This is why corpses become stiff with rigor mortis (new ATP molecules are unavailable).

Muscles and Nerves

Muscles cannot contract on their own. They need a stimulus from a nerve cell to “tell” them to contract. Let’s say you decide to raise your hand in class. Your brain sends electrical messages to nerve cells, called motor neurons, in your arm and shoulder. The motor neurons, in turn, stimulate muscle fibers in your arm and shoulder to contract, causing your arm to rise. Involuntary contractions of cardiac and smooth muscles are also controlled by nerves.

Vocabulary

actin: The monomeric subunit of microfilaments, one of the three major components of the cytoskeleton, and thin filaments, part of the myofibril in muscle cells.

cross bridge: Structure of myosin filament that attaches to actin filament.

motor neuron: Type of neuron that carries nerve impulses from the central nervous system to muscles and glands.

myofibril: Organelle of muscle fiber composed of actin and myosin filaments.

myosin: Filamentous protein involved in muscle contraction forms thick filaments of myofibril in muscle cells.

sacromere: Region of myofibril between two Z-lines.

sliding filament theory: Theory that explains muscle contraction by the sliding of myosin filaments over actin filaments within muscle fibers.

Z line: Region of sarcomere where actin filaments are attached.

Summary

According to the sliding filament theory, a muscle fiber contracts when myosin filaments pull actin filaments closer together and thus shorten sarcomeres within a fiber.

When all the sarcomeres in a muscle fiber shorten, the fiber contracts.

What is integumentary? Because the organs of the integumentary system are external to the body, you may think of them as little more than “accessories,” like clothing or jewelry. But the organs of the integumentary system serve important biological functions. They provide a protective covering for the body and help the body maintain homeostasis.

The Skin

The skin is the major organ of the integumentary system, which also includes the nails and hair. In fact, the skin is the body’s largest organ, and a remarkable one at that. Consider these skin facts. The average square inch (6.5 cm2) of skin has 20 blood vessels, 650 sweat glands, and more than a thousand nerve endings. It also has an incredible 60,000 pigment-producing cells. All of these structures are packed into a stack of cells that is just 2 mm thick, or about as thick as the cover of a book.

Although the skin is thin, it consists of two distinct layers, called the epidermis and the dermis.

Epidermis

The epidermis is the outer layer of skin, consisting of epithelial cells and little else. For example, there are no nerve endings or blood vessels in the epidermis. The innermost cells of the epidermis are continuously dividing through mitosis to form new cells. The newly formed cells move up through the epidermis toward the skin surface, while producing a tough, fibrous protein called keratin. The cells become filled with keratin and die by the time they reach the surface, where they form a protective, waterproof layer called the stratum corneum. The dead cells are gradually shed from the surface of the skin and replaced by other cells.

The epidermis also contains melanocytes, which are cells that produce melanin. Melanin is the brownish pigment that gives skin much of its color. Everyone has about the same number of melanocytes, but the melanocytes of people with darker skin produce more melanin. The amount of melanin produced is determined by heredity and exposure to UV light, which increases melanin output. Exposure to UV light also stimulates the skin to produce vitamin D. Because melanin blocks UV light from penetrating the skin, people with darker skin may be at greater risk of vitamin D deficiency.

Dermis

The dermis is the lower layer of the skin, located directly beneath the epidermis. It is made of tough connective tissue and attached to the epidermis by collagen fibers. The dermis contains blood vessels and nerve endings. Because of the nerve endings, skin can feel touch, pressure, heat, cold, and pain. The dermis also contains hair follicles and two types of glands.

Hair follicles are the structures where hairs originate. Hairs grow out of follicles, pass through the epidermis, and exit at the surface of the skin.

Sebaceous glands produce an oily substance called sebum. Sebum is secreted into hair follicles and makes its way to the skin surface. It waterproofs the hair and skin and helps prevent them from drying out. Sebum also has antibacterial properties, so it inhibits the growth of microorganisms on the skin.

Sweat glands produce the salty fluid called sweat, which contains excess water, salts, and other waste products. The glands have ducts that pass through the epidermis and open to the surface through pores in the skin.

Functions of the Skin

The skin has multiple roles in the body. Many of these roles are related to homeostasis. The skin’s main functions are preventing water loss from the body and serving as a barrier to the entry of microorganisms. In addition, melanin in the skin blocks UV light and protects deeper layers from its damaging effects.

The skin also helps regulate body temperature. When the body is too warm, sweat is released by the sweat glands and spreads over the skin surface. As the sweat evaporates, it cools the body. Blood vessels in the skin also dilate, or widen, when the body is too warm. This allows more blood to flow through the skin, bringing body heat to the surface, where it radiates into the environment. When the body is too cool, sweat glands stop producing sweat, and blood vessels in the skin constrict, or narrow, thus conserving body heat.

Skin Problems

In part because it is exposed to the environment, the skin is prone to injury and other problems. Two common problems of the skin are acne and skin cancer.

Acne is a condition in which red bumps called pimples form on the skin due to a bacterial infection. It affects more than 85 percent of teens and may continue into adulthood. The underlying cause of acne is excessive secretion of sebum, which plugs hair follicles and makes them good breeding grounds for bacteria.

“ABCDs of Skin Cancer. A brown spot on the skin is likely to be a harmless mole, but it could be a sign of skin cancer. Unlike moles, skin cancers are generally asymmetrical, have irregular borders, may be very dark in color, and may have a relatively great diameter.”

Vocabulary

acne: Condition in which red bumps called pimples form on the skin due to a bacterial infection.

dermis: Lower layer of the skin made of tough connective tissue contains blood vessels, nerve endings, hair follicles, and glands.

epidermis: Outer layer of skin consists mainly of epithelial cells and lacks nerve endings and blood vessels.

hair follicle: Structure in the dermis of skin where a hair originates.

integumentary system: Human body system that includes the skin, nails, and hair.

keratin: Tough, fibrous protein in skin, nails, and hair.

melanin: Brown pigment produced by melanocytes in the skin gives skin most of its color and prevents UV light from penetrating the skin.

melanocyte: Cell that produces melanin found in the epidermis.

sebaceous gland: Gland in the dermis of skin produces sebum.

sebum: An oily substance produced by sebaceous glands waterproofs the hair and skin.

skin cancer: A disease in which skin cells grow out of control mainly caused by excessive exposure to UV light.

stratum corneum: The outer protective, waterproof layer of the skin.

sweat glands: Gland in the dermis of skin produce the salty fluid called sweat.

vitamin D: Vitamin that is synthesized when sun exposure is adequate needed for bone health.

Summary

The skin consists of two layers: the epidermis, which contains mainly epithelial cells, and the dermis, which contains most of skin’s other structures, including blood vessels, nerve endings, hair follicles, and glands.

Skin protects the body from injury, water loss, and microorganisms. It also plays a major role in maintaining a stable body temperature.

Common skin problems include acne and skin cancer.

Nervous System

A small child darts in front of your bike as you race down the street. You see the child and immediately react. You put on the brakes, steer away from the child, and yell out a warning, all in just a split second. How do you respond so quickly? Such rapid responses are controlled by your nervous system. The nervous system is a complex network of nervous tissue that carries electrical messages throughout the body. It includes the brain and spinal cord, the central nervous system, and nerves that run throughout the body, the peripheral nervous system. To understand how nervous messages can travel so quickly, you need to know more about nerve cells.

Nerve Cells

Although the nervous system is very complex, nervous tissue consists of just two basic types of nerve cells: neurons and glial cells. Neurons are the structural and functional units of the nervous system. They transmit electrical signals, called nerve impulses. Glial cells provide support for neurons. For example, they provide neurons with nutrients and other materials.

Neuron Structure

The cell body contains the nucleus and other cell organelles.

Dendrites extend from the cell body and receive nerve impulses from other neurons.

The axon is a long extension of the cell body that transmits nerve impulses to other cells. The axon branches at the end, forming axon terminals. These are the points where the neuron communicates with other cells.

Myelin Sheath

The axon of many neurons has an outer layer called a myelin sheath. Myelin is a lipid produced by a type of a glial cell known as a Schwann cell. The myelin sheath acts like a layer of insulation, similar to the plastic that encases an electrical cord. Regularly spaced nodes, or gaps, in the myelin sheath allow nerve impulses to skip along the axon very rapidly.

Types of Neurons

Neurons are classified based on the direction in which they carry nerve impulses.

Sensory neurons carry nerve impulses from tissues and organs to the spinal cord and brain.

Motor neurons carry nerve impulses from the brain and spinal cord to muscles and glands.

Interneurons carry nerve impulses back and forth between sensory and motor neurons.

Vocabulary

axon: Long extension of the cell body of a neuron transmits nerve impulses to other cells.

axon terminal: Branches at the end of an axon of a neuron points where the neuron communicates with other cells.

cell body: Central part of a neuron contains the nucleus and other cell organelles.

central nervous system (CNS): One of two main divisions of the nervous system that includes the brain and spinal cord.

dendrite: Extension of the cell body of a neuron receives nerve impulses from other neurons.

glial cell: A cell that provides support for a neuron.

interneuron: Neuron that carries nerve impulses back and forth between sensory and motor neurons.

motor neuron: Neuron that carries nerve impulses from the central nervous system to muscles and glands.

myelin: A lipid produced by a Schwann cell forms the myelin sheath.

myelin sheath: Lipid layer around the axon of a neuron allows nerve impulses to travel more rapidly down the axon.

nerve impulse: Electrical signal transmitted by the nervous system.

nervous system: Body system that carries electrical messages throughout the body.

neuron: Nerve cell structural and functional unit of the nervous system.

peripheral nervous system (PNS): One of two major divisions of the nervous system consists of all the nervous tissue that lies outside the central nervous system.

Schwann cell: A type of glial cell responsible for producing myelin.

sensory neuron: Neuron that carries nerve impulses from tissue and organs to the spinal cord and brain.

Summary

Neurons are the structural and functional units of the nervous system. They consist of a cell body, dendrites, and axon.

Neurons transmit nerve impulses to other cells.

Types of neurons include sensory neurons, motor neurons, and interneurons.

Nerve Impulses

How does a nervous system signal move from one cell to the next?

It literally jumps by way of a chemical transmitter. Notice the two cells are not connected, but separated by a small gap. The synapse. The space between a neuron and the next cell.

Nerve Impulses

Nerve impulses are electrical in nature. They result from a difference in electrical charge across the plasma membrane of a neuron. How does this difference in electrical charge come about? The answer involves ions, which are electrically charged atoms or molecules.

Resting Potential

When a neuron is not actively transmitting a nerve impulse, it is in a resting state, ready to transmit a nerve impulse. During the resting state, the sodium-potassium pump maintains a difference in charge across the cell membrane. It uses energy in ATP to pump positive sodium ions (Na+) out of the cell and potassium ions (K+) into the cell. As a result, the inside of the neuron is negatively charged, compared to the extracellular fluid surrounding the neuron. This is due to many more positivly charged ions outside the cell compared to inside the cell This difference in electrical charge is called the resting potential.

Action Potential

A nerve impulse is a sudden reversal of the electrical charge across the membrane of a resting neuron. The reversal of charge is called an action potential. It begins when the neuron receives a chemical signal from another cell. The signal causes gates in sodium ion channels to open, allowing positive sodium ions to flow back into the cell. As a result, the inside of the cell becomes positively charged compared to the outside of the cell. This reversal of charge ripples down the axon very rapidly as an electric current.

In neurons with myelin sheaths, ions flow across the membrane only at the nodes between sections of myelin. As a result, the action potential jumps along the axon membrane from node to node, rather than spreading smoothly along the entire membrane. This increases the speed at which it travels.

The Synapse

The place where an axon terminal meets another cell is called a synapse. The axon terminal and other cell are separated by a narrow space known as a synaptic cleft. When an action potential reaches the axon terminal, the axon terminal releases molecules of a chemical called a neurotransmitter. The neurotransmitter molecules travel across the synaptic cleft and bind to receptors on the membrane of the other cell. If the other cell is a neuron, this starts an action potential in the other cell.

Vocabulary

action potential: Reversal of electrical charge across the membrane of a resting neuron that travels down the axon of the neuron as a nerve impulse.

ion: Electrically charged atoms or molecules.

nerve impulse: Electrical signal transmitted by the nervous system.

neurotransmitter: Chemical that carries a nerve impulse from one nerve to another at a synapse.

resting potential: Difference in electrical charge across the plasma membrane of a neuron that is not actively transmitting a nerve impulse.

sodium-potassium pump: Active transport protein exchanges sodium ions for potassium ions across the plasma membrane of animal cells.

synapse: Junction where an axon terminal meets another cell.

synaptic cleft: Space between the axon terminals of one cell and the receptors of the next cell.

Summary

A nerve impulse begins when a neuron receives a chemical stimulus.

The nerve impulse travels down the axon membrane as an electrical action potential to the axon terminal.

The axon terminal releases neurotransmitters that carry the nerve impulse to the next cell.

The Heart and Circulatory System

What's the most active muscle in the body?

The human heart. An absolutely remarkable organ. Obviously, its main function is to pump blood throughout the body. And it does this extremely well. On average, this muscular organ will beat about 100,000 times in one day and about 35 million times in a year. During an average lifetime, the human heart will beat more than 2.5 billion times.

The Circulatory System

The circulatory system can be compared to a system of interconnected, one-way roads that range from superhighways to back alleys. Like a network of roads, the job of the circulatory system is to allow the transport of materials from one place to another. The materials carried by the circulatory system include hormones, oxygen, cellular wastes, and nutrients from digested food. Transport of all these materials is necessary to maintain homeostasis of the body. The main components of the circulatory system are the heart, blood vessels, and blood.

The heart is a muscular organ in the chest. It consists mainly of cardiac muscle tissue and pumps blood through blood vessels by repeated, rhythmic contractions. The heart has four chambers: two upper atria (singular, atrium) and two lower ventricles. Valves between chambers keep blood flowing through the heart in just one direction.

Blood Flow Through the Heart

Blood flows through the heart in two separate loops

Blood from the body enters the right atrium of the heart. The right atrium pumps the blood to the right ventricle, which pumps it to the lungs. This loop is represented by the blue arrows in the diagram above.

Blood from the lungs enters the left atrium of the heart. The left atrium pumps the blood to the left ventricle, which pumps it to the body. This loop is represented by the red arrows in the diagram above.

Heartbeat

Unlike skeletal muscle, cardiac muscle contracts without stimulation by the nervous system. Instead, specialized cardiac muscle cells send out electrical impulses that stimulate the contractions. As a result, the atria and ventricles normally contract with just the right timing to keep blood pumping efficiently through the heart. You can watch an animation to see how this happens at this link: http://www.nhlbi.nih.gov/health/dci/Diseases/hhw/hhw_electrical.html.

Vocabulary

atrium (plural, atria): upper chamber of the heart

blood: Fluid connective tissue that circulates throughout the body through blood vessels.

blood vessel: Vessel that transports blood includes the arteries, veins, and capillaries.

circulatory system: Organ system consisting of the heart, blood vessels, and blood that transports materials around the body.

heart: Muscular organ in the chest that that pumps blood through blood vessels when it contracts.

ventricles: lower chambers of the heart

Summary

The heart contracts rhythmically to pump blood to the lungs and the rest of the body.

Specialized cardiac muscle cells trigger the contractions.

Blood Vessels

Blood vessels form a network throughout the body to transport blood to all the body cells. There are three major types of blood vessels: arteries, veins, and capillaries.

Blood vessels include arteries, veins, and capillaries.

Arteries are muscular blood vessels that carry blood away from the heart. They have thick walls that can withstand the pressure of blood being pumped by the heart. Arteries generally carry oxygen-rich blood. The largest artery is the aorta, which receives blood directly from the heart.

Veins are blood vessels that carry blood toward the heart. This blood is no longer under much pressure, so many veins have valves that prevent backflow of blood. Veins generally carry deoxygenated blood. The largest vein is the inferior vena cava, which carries blood from the lower body to the heart. The superior vena cava brings blood back to the heart from the upper body.

Capillaries are the smallest type of blood vessels. They connect very small arteries and veins. The exchange of gases and other substances between cells and the blood takes place across the extremely thin walls of capillaries.

Blood Vessels and Homeostasis

Blood vessels help regulate body processes by either constricting (becoming narrower) or dilating (becoming wider). These actions occur in response to signals from the autonomic nervous system or the endocrine system. Constriction occurs when the muscular walls of blood vessels contract. This reduces the amount of blood that can flow through the vessels. Dilation occurs when the walls relax. This increases blood flows through the vessels.

Constriction and dilation allow the circulatory system to change the amount of blood flowing to different organs. For example, during a fight-or-flight response, dilation and constriction of blood vessels allow more blood to flow to skeletal muscles and less to flow to digestive organs. Dilation of blood vessels in the skin allows more blood to flow to the body surface so the body can lose heat. Constriction of these blood vessels has the opposite effect and helps conserve body heat.

Blood Vessels and Blood Pressure

The force exerted by circulating blood on the walls of blood vessels is called blood pressure. Blood pressure is highest in arteries and lowest in veins. When you have your blood pressure checked, it is the blood pressure in arteries that is measured. High blood pressure, or hypertension, is a serious health risk but can often be controlled with lifestyle changes or medication. You can learn more about hypertension by watching the animation at this link: http://www.healthcentral.com/high-blood-pressure/introduction-47-115.html.

Vocabulary

aorta: The largest artery receives blood directly from the heart.

artery: Type of blood vessel that carries blood away from the heart toward the lungs or body.

blood pressure: Force exerted by circulating blood on the walls of blood vessels.

blood vessel: Vessel that transports blood includes the arteries, veins, and capillaries.

capillary: Smallest type of blood vessel that connects very small arteries and veins.

constriction: Narrowing of the blood vessels occurs when the muscular walls of blood vessels contract.

dilation: Widening of the blood vessels occurs when the walls of blood vessels relax.

hypertension: High blood pressure.

inferior vena cava: The vein that receives blood directly from the heart.

superior vena cava: The vein that brings blood back to the heart from the upper body.

vein: Type of blood vessel that carries blood toward the heart from the lungs or body.

Summary

Arteries carry blood away from the heart, veins carry blood toward the heart, and capillaries connect arteries and veins.

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Travel Through the Ear in Journey of Sound to the Brain

Have you ever wondered how sound waves turn into the familiar sounds we hear and recognize every day? The National Institute on Deafness and Other Communication Disorders (NIDCD), which developed Noisy Planet, has produced a two-and-a-half-minute animated video, Journey of Sound to the Brain, that follows sound waves as they pass through the ear canal and are changed to electrical signals that our brains interpret and understand. The video teaches viewers about the parts of the inner ear and how each part helps us understand and communicate with the world around us.

Perfect for viewing at home or in the classroom, the video can help you begin a conversation with your children or students about how our hearing works and why it’s so important that we protect it. The video is part of the presentation included our Teacher Toolkit, an easy-to-use online resource to help teach children in grades 2 through 6 about the causes and prevention of noise-induced hearing loss. This science-based classroom presentation explains what sound is, how sound travels through the ear, how loud sounds can damage hearing over time, and how to protect your hearing.

The video premiered during a Noisy Planet presentation at Westbrook Elementary School, in Bethesda, Maryland, to a group of engaged fourth graders. After students acted out the sequence of events involved in hearing, they attentively watched the animated version of what they just acted out. The students laughed and danced in their seats to the sounds of the trumpet and other instruments, reporting that they liked the video and learned from it.


Spinal Cord Signals: Reflex

Reflex signals cause involuntary movements. This means that the movement was not conscious. You did not decide to make it. A muscle spasm is a good example of this type of movement. Reflex signals that cause movement do not come from your brain.

A reflex signal comes from the nerves in your body, like sensory signals, but instead of going to your brain, they stop at the spinal cord . Once they reach the spinal cord, they loop through and go straight back to the body part they came from. Reflex signals are designed to protect your body. This is why they happen so quickly. They do not have to wait to reach the brain and then wait for the brain to choose a reaction.

Reflex signals are initiated when the nerves in a muscle are irritated by being stretched or pushed on. This triggers the nerves to send a message to the spinal cord. When the signal reaches the spinal cord, it goes back through at the same level it came in, returning to the muscle that initiated the signal. Once it gets back to the starting point, the signal causes the muscle to react by squeezing or contracting.


If a Ferrari can travel at 110 m/s max and nerve impulses travel at 100 m/s max, does this mean that the driver would be traveling forward mroe quickly than his brain could process the visual information from the road?

If you look at "comparisons as speed" you'll see that it lists the maximum speed of a Ferrari F50 GT1 as 110m/s and the maximum nerve impulse speed as only 100 m/s.

I understand that it's a miniscule difference, but does that mean that the car is traveling forward more quickly than the brain can receive visual information about its location?

The brain has to process information from over 120m ahead, even at only 80mph. So the car has one second to reach the object, while the nerve impulse only needs to travel a couple of meters to the legs. Stopping distance

If the driver noticed a sudden obstacle only 2 meters ahead, then yes the car would get there before the nerve impulse reached the feet!

I always wondered if this also meant you won't hear or feel anything if you shoot yourself in the head. A bullet exits a gun at around 345 m/s which is faster than both the speed of sound and considerably faster than the speed of nerve signals.

Would that mean your brain is mush before you (fully) register?

Additionally, the units in this question, while the same, aren't entirely relevant. A better comparison to answer what I think is your question would be the minimum reaction time of a driver. Usually, it takes about 190ms to detect visual stimulus. The difference here is that your 'reaction' does not have to travel a full meter - just from your eyes to your brain - hence the order of magnitude difference. The physical response will vary from person to person widely (especially in the case of a very highly trained individual, like an F1 driver).

Now, back to the original question. If you have a detection time of .19 seconds, and yourɾ traveling 110 m/s, then you will have already passed the first 21 meters before your brain can process it. What this means is that you will need to be reacting to things further than this distance from you at a minimum (not counting your physical reaction time).


How fast do torpedoes and phasers travel?

So we see space combat in star trek happening virtually instantly, despite some of the distances quoted being vast.

You see distances quoted as being 100,000kms

Which 100,000kms would still take a few seconds easily for objects travelling a fraction of the speed of light to traverse

Now we know torpedoes can be fired at warp, but phasers cannot, this is assuming phasers travel at the speed of light?

Visually, phaser beams are depicted as travelling slow enough to see the leading edge propagate. I think is supposed to be a beam of particles with some mass. It might take more than just few seconds for the shot to impact, which would justify the evasive maneuvers and dodging shown during some space battles. This seems a more reliable way of estimating travel speed rather than using the discrepancy in timing.

Torpedoes are able to sustain the warp field of the vessel that launched it, but are otherwise depicted as being relatively slow (though still faster than ships at impulse).

Even if the beam itself travelled at the speed of light, calculating a firing solution based on the relative velocities of the two ships and then aiming the emitter in accordance with that solution would take at least a little bit of time.

When engaging at the actual theoretical distances of phaser weapons, i.e. thousands of kilometres, that plus the light speed travel time would be significant. Beams would travel "slow" at scale rather than hitting their target instantaneously. There's your chance for dodging.

Either Star Trek ships choose to engage at insanely close range or those sequences on film are "dramatized" for our enjoyment and the real battles happen at much longer ranges.

Torpedoes can be fired at warp, which allows them to temporarily travel at warp speeds thanks to inertia. After a moment, the torpedo would drop to sub-light and continue on under its own propulsion, assuming it hasn’t yet hit anything.

Phasers, up until the TNG era, couldn’t operate while at warp. Being beams of visible light meant that they were limited to use at impulse only. However, during the TNG & DS9 era, the Federation was able to modify phasers to operate at warp by temporarily wrapping the beam in its own extended warp bubble as it was being fired. Due to the limitations of a ship’s ability to project a warp field, this severely limited the range of phaser fire. The ship would have to almost be on top of you to make a shot connect. This physics hack probably also reduced the effective strength of phaser fire. But at the end of the day, weak phasers are better than no phasers.

This update was probably in tandem with the warp field update that allowed ships to turn (make major course corrections) while at warp. Something else that was not possible when TNG started, but became possible as the series went on. An indication that warp theory was constantly being worked on and better understood by Starfleet R&D.


Question: how do we move

There is a special part of the brain that is in charge of planning and controlling the movements we decide to make, the motor cortex, which is located roughly where you would wear a hairband or your headphones.

In order to move, the brain controls our muscles, that contract and relax according to the instructions they receive from the brain. These instructions need to travel from the brain to every muscle in your body, from the head up to your little toes.

They do so using the neural system, which is like a railway connecting brain and body, where the rails are made of nerves (long threads able to transmit electricity) and the trains carrying the instructions are electrical impulses generated in the brain. They travel so fast that we are not able to perceive the delay between the time we decide to move and when we actually move, but it actually takes a (small) fraction of a second.

Vrushali Patil answered on 30 Oct 2020:

What an interesting question! As we are on Earth (which is always moving around the Sun), I suppose even when we sit still we are ‘moving’. And while sitting or standing in one place, our bodies have several things happening inside that keep it ‘moving’ – our hearts are beating constantly, we are breathing in and out thus moving our chest, our nerves are twitching, our eyelids are moving while blinking. In order to reach a different place, I suppose we move by walking, which involves lifiting one foot, changing its location and then letting the other foot follow it. This allows our whole body to ‘move’ from a location to another. When done continuously, depending on the speed, we can either walk or run too!

Rob Mahen answered on 30 Oct 2020:

One way humans move is using muscles – tissues that push and pull against bones and tendons to move all different parts of us. Inside muscles are specialised long proteins that use energy stored in the body to move against each other.

Muscles can change in response to more or less movement, which is one reason why keeping active by moving makes us fit and healthy.

Thomas Williams answered on 3 Nov 2020:

Pairs of muscles pull our bones backwards and forwards when told to by the brain.

The brain can make the muscles move faster and slower, like running and walking, and can be actively controlled by you (like running and walking!), or can just happen in the background (like digesting food).

Guy Yona answered on 3 Nov 2020: last edited 3 Nov 2020 8:06 pm

The decision to move arises somewhere in the brain, maybe in response to an incoming ball entering our field-of-view, or maybe we were just sitting for too long in the same position on our couch. That decision reaches several regions in the brain: one of them, the motor cortex (see Paula’s beautiful explanation for where it is!), is responsible for making out the details of the movement – how exactly are our hands going to reach that incoming ball. Another region, the “basal ganglia” is helping to determine how strong is the movement going to be, or maybe it might decide to drop the idea altogether (is this jump going to bring the winning score, or are you surely loosing and it wouldn’t be worth the fall?). Many other parts of the brain pitch in as well, and eventually a set of motor commands travel down our spine. All that planning takes quite a while, about a tenth of a second, even a bit longer if the ball comes in a funny angle!

Inside our spines, there are groups of brain cells that specialise in making sense of these instructions coming down from the brain. These groups of cells are part of the spinal cord, and each group is responsible for a specific set of muscles. For example, the cells that control our hands are located in the back of our necks, and those that control our legs are in the lower back. It all makes perfect sense. The largest and strongest of cells in these groups have very long “wires” coming out of them that bundle together into nerves. These nerves, which look like thin white wires, exit the spine through special openings, and travel inside the body all the way to the muscle they operate.

Why “the largest and strongest cells”, you might ask? These cells, which we can’t even see without a microscope, need to send electrical signals a long way through their nerves, that branch out to cover a huge muscle (like the thigh muscle), and “tell” it to contract. When the electrical signals reach nerve endings that touch the muscles, they release a chemical called “Acetylcholine”, which is being picked up by the muscle cells. As they sense this acetylcholine, a chemical chain of events occurs that causes the muscle cell to contract. Since many muscle cells are controlled by a single nerve cell, they contract together, like rowers pulling together their oars, and the whole muscle contract. The muscles connect to the bones by flexible rope-like “tendons”, so when they contract – the bone moves.

Pffww… that was.. not all. You wouldn’t think the brain just issues commands without following through? Special sensors in the muscles and tendons monitor every small movement, and send that information back to the spinal cord by a different set of nerves, where it is being used to refine the movements, and back to the brain. The brain compares what we wanted to do with what we actually did, and learns – so next time, it will be better. So, if you missed that ball now, keep practicing!

Anitta Chacko answered on 3 Nov 2020: last edited 3 Nov 2020 4:10 pm

Great question! I’ve too always wondered how I’m actually able to pull of some awesome dance moves or run as fast as I can down the stairs as soon as my pizza delivery has arrived and really the answer lies within us! When you decide its time to dance or run or even to lift a pizza slice into your mouth, your brain acts like a solider, giving commands to different parts of your body such as your muscles.

These commands also known as signals travel through the nervous system. In the case of eating your pizza, the brain will tell your muscles in your hands and wrists to contract and by doing so you’re able to grab that delicious pizza slice. Your brain will then send more signals telling the muscles in your arm to move so that the pizza slice can be moved to your mouth and hey presto your eating your pizza! These signals travel so fast from your brain that it hardly takes any time for you to pick up your pizza slice. As you get much much older, these signals cannot travel as fast from your brain to your muscles leading to slower movement.

So really its the communication between your brain and the signals that reach your muscles that allow your body to move. I hope that helped clear things up a little!

Comments

if you look at a muscle under a microscope (which makes small things look bigger) you’ll see some zig-zag stripes of protein ( the stuff muscle is made of). it looks a bit like hundreds of fingers laced together. There are two main proteins that make up muscles Actin and Myosin, which switch. Actin goes on the outside of the Myosin, and has little loops, while Myosin goes in the middle and looks like a golf club or hook. When our muscles move ( or contract) the little hooks of Myosin hook onto the loops of Actin and pull, then they let go and hook and pull on the next loop of Actin, until thousands of hooks and loops have hooked and unhooked. Each protein moves a teeny, tiny amount- only about 10 nanometers ( or 0.0000000001m) each. To move our leg to kick a ball or our hand so we can type takes a lot of hooking and pulling!

Want to try being Actin and Myosin?
Ok: you left hand is going to be Actin
Your right hand is going to be Myosin

Point both your hands along the desk, so your thumb is up and your fingers are pointing at each other

On your left ( Actin) hand spread your fingers, now slide your right (Myosin) hand in just enough so the tips are touching

Now your right hand ( Myosin) is going to squeeze and pull on your left ( Actin) hand for a little bit, just enough for it to move half a finger tip in length .

Keep your left (Actin) hand still so that when you pull your right myosin hand moves a little bit along your finger, bringing your hands together.

If you keep doing that until your fingers can’t move any further you’ll a have a muscle contraction! A concentric contraction is a scientific name for a muscle getting shorter.Actin and myosin can work the other way too, so it can pull itself longer, called an eccentric contraction. If all the hooks and loops just let go at once that is called relaxing.

In the Actin/ Myosin finger demo you can see that lots of little movements add up to a big movement overall- each pull might move you finger 0.5cm but you hand moved about 5cm by the end. Even move if you have long fingers!