L1 - L5 layers of the brain

I have read in some papers about the layers L1-L5 of the brain (e.g. in this paper). I could not find a definition of these layers. I have found information about the layers V1-V5 in the visual cortex.

Where exactly are L1-L5? And do these definitions vary across mammals?

V1-V5 are different (sub) regions of the visual cortex itself.


The layers L1-L5 (some regions have a layer 6) refer to the different cellular strata in the depth dimension of the cortical mass. This stratification occurs to some extent or another in various areas. Different sections of the cortex, e.g., the primary motor cortex have a predominant level 5 (for the pyramidal tract). Something like the visual cortex has more cells in layers 4 and 6.

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Spinal cord

The spinal cord functions primarily in the transmission of nerve signals from the motor cortex to the body, and from the afferent fibers of the sensory neurons to the sensory cortex. It is also a center for coordinating many reflexes and contains reflex arcs that can independently control reflexes. [1] It is also the location of groups of spinal interneurons that make up the neural circuits known as central pattern generators. These circuits are responsible for controlling motor instructions for rhythmic movements such as walking. [2]


*Number the cranial nerves appropriately.

Color each part according to the key. This picture is not labeled, you may use other resources to help you locate the structures.

Olfactory bulb and tract (purple)
Optic Nerve and Chiasma (dark green)
Oculomotor (dark blue)
Trochlear (gray)
Trigeminal (pink)
Abducens (light green)
Facial (yellow)
Vestibulocochlear / Auditory (red)
Glossopharyngeal (black)
Vagus (brown)
Accessory / Spinal Accessory (dark blue)
Hypoglossal (orange)

Pons (purple)
Cerebellum (light green)
Cerebrum (light blue)
Medulla (yellow)

Data availability

Data (including high-resolution images, segmentation, registration to CCFv3, and automated quantification of injection size, location, and distribution across brain structures) are available through the Allen Mouse Brain Connectivity Atlas portal ( Individual experiment summaries can be viewed using this link:[insert experimental id]. Experimental ids are listed in Supplementary Table 2. In addition to visualization and search tools available at this site, users can download data using the Allen Brain Atlas API ( and the Allen Brain Atlas Software Development Kit (SDK: Through the SDK, structure and voxel-level projection data are available for download. Examples of code for common data requests are provided as part of the Mouse Connectivity Jupyter notebook to help users get started with their own analyses. Source data generated for this study are provided as Supplementary Tables as indicated throughout. Code and data files for hierarchical analyses are available through the Allen SDK and Github (

Why is it Important

L5 is a common site of spondylolisthesis and spondylolysis. Spondylolisthesis is forward displacement of the vertebra when compared to the bone below it. Spondylolysis is a stress fracture or defect in the vertebral arch, which tends to present asymptomatically in most patients. Individuals with fewer or more lumbar vertebrae generally have the last lumbar bone affected with these disorders.

A variety of disorders can affect the lumbar vertebrae, similar to those affect the other vertebrae. Disc herniation is possible, and can put pressure on the spinal cord and cauda equina. Fractures, ligamentous injuries, and muscle strains are also possible with the lower back. Muscle injuries are especially common, given the fact that the lumbar spine supports a great deal of the body’s weight.

In addition, inflammatory diseases including spondylitis, rheumatoid arthritis, and psoriatic arthritis can occur. Tumors or cancer can occur within the lumbar spine, usually secondary to carcinomas. Infections within the bone, spinal cord, or meninges are possible, too.

Muscles and Fasciae [ edit | edit source ]

The spine is unstable without the support of the muscles that power the trunk and position the spinal segments. Back muscles can be divided into four functional groups: flexors, extensors, lateral flexors and rotators [2]

Extensors, arranged in three layers

  1. Most superficial is the strong Erector Spinae or sacrospinalis muscle. Its origin is in the erector spinae aponeurosis, a broad sheet of tendinous fibers attached to the iliac crest, the median and lateral sacral crests and the spinous processes of the sacrum and lumbar spine.
  2. Middle layer is the multifidus. The fibers of the multifidus are centered on each of the lumbar spinous processes. From each spinal process, fibers radiate inferiorly to insert on the lamina, one, two or three levels below. The arrangement of the fibers is such that it pulls downwards on each spinal process, thereby causing the vertebra of origin to extend.
  3. Third layer is made up of small muscles arranged from level to level, which not only have an extension function but are also rotators and lateral flexors.

Lateral flexors and rotators

  • internal and external oblique, the intertransverse and quadratus lumborum muscles.
  • remember that pure lateral flexion is brought about only by the quadratus lumborum.

Spinal Canal [ edit | edit source ]

The spinal canal is made up of the canals of individual vertebrae so that bony segments alternate with intervertebral and articular segments. The shape of the transverse section changes from round at L1 to triangular at L3 and slightly trefoil at L5 (Fig. 1). An anterior wall and a posterior wall, connected through pedicles and intervertebral foramina, form the margins of the canal.

The anterior wall consists of the alternating posterior aspects of the vertebral bodies and the annulus of the intervertebral discs. In the midline these structures are covered by the posterior longitudinal ligament, which widens over each intervertebral disc.

The posterior wall is formed by the uppermost portions of the laminae and the ligamenta flava. Because the superoinferior dimensions of the laminae tend to decrease at the L4 and L5 levels, the ligamenta flava consequently occupy a greater percentage of the posterior wall at these levels. The spinal canal contains the dural tube, the spinal nerves and the epidural tissue.

Dura Mater [ edit | edit source ]

The dura mater is a thick membranous sac, attached cranially around the greater foramen of the occiput, where its fibers blend with the inner periosteum of the skull, and anchored distally to the dorsal surface of the distal sacrum by the filum terminale.

At the lumbar level, the dura contains the distal end of the spinal cord (conus medullaris, ending at L1), the cauda equina and the spinal nerves, all floating and buffered in the cerebrospinal fluid. The lumbar roots have an intra- and extrathecal course. Emerging in pairs from the spinal cord, they pass freely through the subarachnoid space before leaving the dura mater. In their extrathecal course and down to the intervertebral foramen, they remain covered by a dural investment. At the L1 and L2 levels, the nerves exit from the dural sac almost at a right angle and pass across the lower border of the vertebra to reach the intervertebral foramen above the disc. From L2 downwards, the nerves leave the dura slightly more proximally than the foramen through which they will pass, thus having a more and more oblique direction and an increasing length within the spinal canal.

The dura mater has two characteristics that are of cardinal clinical importance: mobility and sensitivity. [2]

Nerve Roots [ edit | edit source ]

The radicular canal contains the intraspinal extrathecal nerve root. The nerve root consists of a sheath (dural sleeve) and fibres. Each structure has a specific behaviour and function, responsible for typical symptoms and clinical signs. This has some clinical consequences: slight pressure and inflammation only involve the sleeve and provoke pain and impaired mobility. More substantial compression of the root will also affect the nerve fibres, which leads to paraesthesia and loss of function. [2]

Nerve Root and Spinal Nerve

The structure of a spinal nerve as it leaves the spinal cord or cauda equina includes:

  • Nerve root: Part of the nerve that branches off the spinal cord or cauda equina. At each level, a pair of nerve roots emerge from the right and left sides of the spinal cord. Each pair consists of a dorsal root at the back and a ventral root in the front.
  • Spinal nerve: A single nerve formed when the dorsal and ventral nerve roots merge, typically inside the intervertebral foramen (bony opening in between adjacent vertebrae). The spinal nerve travels a short distance inside the intervertebral foramen, after which it branches off into several nerves that innervate different parts of the body.

Doctors may sometimes refer to the part of the spinal nerve exiting the intervertebral foramen as the nerve root or use the terms nerve root and spinal nerve interchangeably.

The Vertebral Column

The vertebral column, or spinal column, surrounds and protects the spinal cord, supports the head, and acts as an attachment point for the ribs and muscles of the back and neck. The adult vertebral column is comprised of 26 bones: the 24 vertebrae, the sacrum, and the coccyx bones. In the adult, the sacrum is typically composed of five vertebrae that fuse into one. We begin life with approximately 33 vertebrae, but as we grow, several vertebrae fuse together. The adult vertebrae are further divided into the 7 cervical vertebrae, 12 thoracic vertebrae, and 5 lumbar vertebrae.

Figure (PageIndex<1>): Vertebral column: (a) The vertebral column consists of seven cervical vertebrae (C1&ndash7), twelve thoracic vertebrae (Th1&ndash12), five lumbar vertebrae (L1&ndash5), the sacrum, and the coccyx. (b) Spinal curves increase the strength and flexibility of the spine.

Each vertebral body has a large hole in the center through which the nerves of the spinal cord pass. There is also a notch on each side through which the spinal nerves, which serve the body at that level, can exit from the spinal cord. The names of the spinal curves correspond to the region of the spine in which they occur. The thoracic and sacral curves are concave, while the cervical and lumbar curves are convex. The arched curvature of the vertebral column increases its strength and flexibility, allowing it to absorb shocks like a spring.

Intervertebral discs composed of fibrous cartilage lie between adjacent vertebral bodies from the second cervical vertebra to the sacrum. Each disc is part of a joint that allows for some movement of the spine, acting as a cushion to absorb shocks from movements, such as walking and running. Intervertebral discs also act as ligaments to bind vertebrae together. The inner part of discs, the nucleus pulposus, hardens as people age, becoming less elastic. This loss of elasticity diminishes its ability to absorb shocks.

Spinal Column

The spongy spinal cord is protected by the irregular shaped bones of the spinal column called vertebrae. Spinal vertebrae are components of the axial skeleton and each contain an opening that serves as a channel for the spinal cord to pass through. Between the stacked vertebrae are discs of semi-rigid cartilage, and in the narrow spaces between them are passages through which the spinal nerves exit to the rest of the body. These are places where the spinal cord is vulnerable to direct injury. The vertebrae can be organized into sections, and are named and numbered from top to bottom according to their location along the backbone:

  • Cervical vertebrae (1-7) located in the neck
  • Thoracic vertebrae (1-12) in the upper back (attached to the ribcage)
  • Lumbar vertebrae (1-5) in the lower back
  • Sacral vertebrae (1-5) in the hip area
  • Coccygeal vertebrae (1-4 fused) in the tail-bone

Lumbar Spine

The lumbar spine is the lower back that begins below the last thoracic vertebra (T12) and ends at the top of the sacral spine, or sacrum (S1). Most people have 5 lumbar levels (L1-L5), although it is not unusual to have 6. Each lumbar spinal level is numbered from top to bottom—L1 through L5, or L6.

The low back vertebral bodies are larger, thicker block-like structures of dense bone. From the front (or anterior), the vertebral body appears rounded. However, the posterior bony structure is different—lamina, pedicles and bony processes project off the back of the vertebral body. These processes and vertebral arches create the hollow spinal canal for lumbar nerves structures and the cauda equina.

Detailed views of the lumbar spinal column and bony anatomy. Photo Source:

The lamina, a thin bony plate shields or protects access to the spinal canal. Some people who have lumbar spinal stenosis may undergo a surgical procedure called decompressive laminotomy or laminectomy. The procedure involves removing part of all of the lamina at the affected level and enlarging the space around the compressed nerves.

Lumbar Structures Create Strong Joint Complex

A single intervertebral disc separates 2 vertebral bodies, and together with the facet joints forms a strong joint complex that enables the spine to bend and twist. One pair of facet joints from the top (or superior) vertebral body connects to the lower (or inferior) pair of facet joints. The facet joints are true synovial joints meaning they are lined with cartilage and the joint’s capsule encases synovial fluid that enables joints to glide during movement.

Lumbar intervertebral discs are secured in place by the fibrous endplates of the superior and inferior vertebral bodies. The gel-like center of each disc, called the nucleus pulposus is encased or surrounded by the annulus fibrosus—a tough layer of fibrocartilage that could be likened to a radial tire.

Discs are integral to the joint complex and function to (1) hold the superior and inferior vertebrae together, (2) bear weight, (3) absorb and distribute shock and forces during movement, and (4) create open nerve passageways called foramen or neuroforamen. The neuroforaminal spaces at either side of the disc level allow nerve rootlets to exit the spinal canal and leave the vertebral column.

    is a common cause of lower back pain that can radiate into one or both legs, called lumbar radiculopathy. This condition can develop when lumbar nerves are compressed.

Low Back Supported by Lumbar Ligaments, Tendons and Muscles

Systems of strong fibrous bands of ligaments hold the vertebrae and discs together and stabilize the spine by helping to prevent excessive movements. The 3 major spinal ligaments are the (1) anterior longitudinal ligament, (2) posterior longitudinal ligament and (3) ligamentum flavum. Spinal tendons attach muscles to the vertebrae and together work to limit excessive movement.

Lumbar spinal ligaments support the low back and help limit excessive movement. Photo Source:

Lumbar Spine Nerves

The spinal cord ends between the first and second lumbar vertebrae (L1-L2). Below this level, the remaining nerves form the cauda equina, a bundle of nerves resembling a horse’s tail. These small nerves transmit messages between the brain and structures in the lower body, including the large intestine, bladder, abdominal muscles, perineum, legs and feet.

4 Ways to Protect Your Low Back

Considering upwards of 80% of adults will visit a doctor for lower back pain at some point in their life, it pays to take care of your lumbar spine to help avoid painful, unnecessary wear-and-tear to this vulnerable segment of your spinal column. You can minimize your risk of a lower back problem by:

1. Lose weight. Even a 10-pound loss can help reduce lower back pain.

2. Strengthen and maintain core (abdominal) muscles. The abdominal and lower back muscles work together to form a supportive “girdle” around your waist and lower back. Stronger muscles can help stabilize the lower back and can help reduce injury risk.

3. Stop smoking. Nicotine reduces blood flow to the spinal structures, including the lumbar discs, and can accelerate age-related degenerative changes.

4. Proper posture and body mechanics. Keep your spine erect and lift objects with your legs. Always ask for help to carry heavy objects. Although your lumbar spine is capable of bending and twisting simultaneously, you should avoid doing so.

Watch the video: Πώς μπορούμε να αναπτύξουμε νέους νευρώνες στον εγκέφαλο. TED (January 2022).