The spinal cord: anatomy and physiology

The spinal cord: anatomy and physiology

The spinal cord is the main route of information that connects the brain and peripheral nervous system.


  • 1 Structure and basic concepts
  • 2 spinal nerves
  • 3 nerve roots
  • 4 Dermatomas
  • 5 spinal grooves
  • 6 Description of the bone marrow section
  • 7 The gray substance
  • 8 The white substance
  • 9 Functions of the spinal cord

Structure and basic concepts

It is in the vertebral foramen and it is composed of 31 segments: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal. A pair of spinal nerves leaves each segment of the spinal cord.

The length of the spinal cord is approximately 45 cm in men and 43 cm in women. The spinal cord is shorter than the length of the bone spine; The spinal cord extends down only to the last thoracic vertebra. The nerves that extend from the spinal cord from the lumbar and sacral levels must run in the spinal canal for a distance before they leave the spine. This collection of nerves in the vertebral canal is called cauda equina (which means "horsetail").

It is very delicate and, therefore, has protection systems, among which the spine which is formed by bones called vertebrae. Although the spine is somewhat flexible, some of the vertebrae in the lower parts of the spine fuse.

It is also protected by meninges and cerebrospinal fluid.

The spine is formed by twenty four individual vertebrae which correspond to the cervical (neck), thoracic (chest) and lumbar (lower back) and vertebrae of the sacral and coccygeal (in the pelvic area).

The spinal cord passes through the hole of the vertebrae, from the first cervical vertebra (at the base of the skull) to the upper margin of the second lumbar vertebra and, therefore, is shorter than the vertebral column (represents about 2/3 of the length of the column).

Like the brain, the spinal cord is covered by three layers of tissue (meninges). The spinal cord and meninges are contained in the spinal canal, which passes through the center of the spine.

Just as the skull protects the brain, the vertebrae protect the spinal cord. The vertebrae are separated by discs made of cartilage, which act as cushions, reducing the forces generated by movements such as walking and jumping.

Spinal nerves

The medulla is laterally surrounded by spinal nerves, axons of neurons that enter and leave the spinal cord and communicate with the rest of the body. Thirty-one pairs of nerves enter and leave the medulla, one for each side of the medulla:

  • Cervical spinal nerves (C1 to C8) control the signals to the back of the head, neck and shoulders, arms and hands, and the diaphragm.
  • Thoracic spinal nerves (T1 to T12) control the signals to the chest muscles, some back muscles and parts of the abdomen.
  • The lumbar spinal nerves (L1 to L5) control the signals to the lower parts of the abdomen and back, the buttocks, some parts of the external genital organs and parts of the leg.
  • Sacral Spinal Nerves (S1 to S5) control the signals to the thighs and lower parts of the legs, feet, most of the external genital organs and the area around the anus.
  • The coccygeal nerve It is the only one that carries sensory information of the skin of the lower back.

Each spinal nerve exits through the space between two vertebrae. In adults, the spinal cord is shorter than the spine, therefore, in the lower segments of the medulla the nerves must look for their exit at much lower levels, so that the lumbosacral roots form the horsetail.

The cylinder formed by the medulla does not have the same diameter everywhere, but is thicker at the cervical height (intumescence or cervical dilation, innervation of the arms) and at the lumbosacral height (lumbar intumescence, innervation of the legs). These dilations correspond to regions of the cord that innervate limbs.

Nerve roots

Spinal nerve roots are the bundles of fibers that leave the medulla spinal. For each spinal segment, (which is the area of ​​the spinal cord that corresponds to the level of the vertebra through which the nerves have their exit to all parts of the body), there are four nerve roots: two in the front (ventral) and two behind (dorsals).

Ventral roots

These roots, one right and one left, that is, one on each side of the spine, contain the nerves that control the movement of the bodyor. Nerves and ventral nerve roots are called motor neurons

They are efferent fibers that carry motor information from the cord to the muscles.

Dorsal roots

On the back, the nerves (again one on the right and one on the left) they transport sensory information from the body to the spinal cord or brain and are called sensory neuronss. Once it reaches the spinal cord or brain, sensory information transmitted through sensory neurons is interpreted as sensation.


A dermatome is the area of ​​the skin where it receives the activity of a spinal or cranial nerve.

Dermatomas receive signals from sensory nerves in a spinal nerve root. A spinal nerve root is a mixture of several types of sensory, motor, and autonomic nerves that branch from the spinal cord. The nerve root is in an arc called intervertebral foramen, which is a hole in the side of the spine formed by parts of individual vertebrae as they are stacked on top of each other.

Beyond the foramen, the nerve root begins to branch into individual nerves to reach and innervate all areas of the body.

Therefore, in the medulla we can distinguish different spinal segments, each of which is linked to a spinal nerve. Actually, there is a lot of overlap in the innervation of adjacent areas.

Spinal grooves

Longitudinal grooves divide the spinal cord into the right and left halves. The ventral groove is known as the ventral medial fissure, and the dorsal groove is known as the dorsal medial groove.

  • The median ventral fissure It is a deeper groove that contains the connective tissue of the pia mater and is supplied by the anterior spinal artery.
  • The ventral roots of the spinal nerves are attached to a shallow groove in this region, called the ventrolateral groove (sulcus ventrolateralis).
  • The middle dorsal groove, which is less deep, contains the dorsal septum of the glial elements.
  • The dorsolateral groove (posterolateralis sulcus) is the name given to the position of the dorsal root connections of the spinal nerves.
  • The dorsal intermediate groove (sulcus intermedius posterior) separates the fasciculus gracilis from fasciculus cuneatus.

Description of the bone marrow section

The cross sections of the spinal cord show a clear division between white substance (exterior) and gray substance (interior).

The fundamental structure of the marrow is maintained throughout its entire length, although the proportions vary.

Gray matter

The gray substance is H-shaped or butterfly shaped. In the middle there is a central channel, through which the cerebrospinal fluid circulates. It consists of ventral horns and dorsal horns, and an intermediate zone. On some levels (thoracic and upper lumbar), there is also a small lateral horn (efferent sympathetic neurons.)

White substance

The white substance is formed by nerve fibers, called axons, which extend up and down along the cord. Each group of axons carries a specific type of information that it needs to communicate. The ascending axon tracts communicate with the brain, while the descending ones carry signals from the brain to various muscles and glands throughout the body.

Keeping in mind the macroscopic anatomy and internal organization of the medulla, we can trace the path that the sensory and motor information follows when they enter and leave the medulla.

Functions of the spinal cord

The main functions of the spinal cord include:

Electrical communication Electric currents travel up and down the spinal cord, sending signals that allow different segments of the body to communicate with the brain.

Wandering motion control. During walking, the leg muscle groups contract constantly. The action of moving forward step by step may seem incredibly simple, since we have been doing it a lifetime, but in reality there are many factors that must be properly coordinated to allow this movement to occur. These central pattern generators in the spinal cord are formed by neurons that send signals to the leg muscles, causing them to extend or contract, and produce the alternate movements that occur when a person walks.

Reflexes. These are predictable involuntary responses to stimuli that involve the brain, spinal cord and nerves of the peripheral nervous system (SNP).

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