Spinal cord: Anatomy & Function

Spinal cordAnatomy and function of the spinal cord. Spinal nerves
~ 13 min

In this note, we are going to discuss the anatomy of the spinal cord. We will cover the structure of segments, roots, gray and white matter, and briefly consider the spinal nerves.

What is the spinal cord?

The spinal cord, along with the brain, is a part of the central nervous system. It is located in the vertebral canal from the foramen magnum approximately to the level of L1/L2 and has a segmental organization.

The vertebral canal is formed by vertebral foramina in the center of each vertebra.

Vertebral canal (canalis vertebralis) / Vertebral foramen (foramen vertebrale)

Spinal cord segments

A segment is a part of the spinal cord from which one pair of spinal nerves emerges, meaning one nerve on the left and one on the right.

There are 31 spinal cord segments:

8 cervical

12 thoracic

5 lumbar

5 sacral

1 coccygeal

Segments and spinal nerves are designated with the corresponding letter and an Arabic numeral.

Spinal cord roots

Each spinal nerve is attached to the spinal cord by anterior and posterior roots.

The posterior root contains processes of sensory (or afferent) neurons, carrying information to the CNS.

The bodies of these neurons form an enlargement on the posterior root, known as the spinal ganglion.

The anterior root contains processes of motor (or efferent) neurons, carrying information from the CNS.

Their bodies are located in the anterior horns of the spinal cord’s gray matter. We will discuss this in more detail a bit later.

Spinal nerves exit the spinal cord below the corresponding vertebrae. For example, nerve L5 exits below the L5 vertebra. The exception is related only to cervical nerves, which exit above the corresponding vertebrae, and the C8 nerve – above the first thoracic vertebra.

Enlargements of the Spinal Cord

At the points where spinal nerves, which innervate the limbs, emerge, the spinal cord forms enlargements.

The cervical enlargement, at the level of C5-T1, corresponds to the upper limbs.

The lumbosacral enlargement, at the level of L1-S3, corresponds to the lower limbs.

The roots of the lower spinal nerves are quite long. Due to their external similarity, the bundle of these roots is called the cauda equina (horse’s tail).

Because the spinal cord develops relatively slowly, it does not reach the full length of the vertebral column and ends at the level of the first or second lumbar vertebrae. The lower end of the spinal cord is called the conus medullaris.

It continues into the filum terminale, the upper part of which still contains some elements of nervous tissue, while the lower part does not and consists only of connective tissue elements.

External features

The external surface of the spinal cord has several depressions:

The anterior median fissure runs along the anterior surface.

The posterior median sulcus runs along the posterior surface.

The anterolateral sulcus is located where the anterior roots emerge from the spinal cord.

The posterolateral sulcus is located where the posterior roots enter the spinal cord.

Inside the organ, there is a central canal filled with cerebrospinal fluid.

Superiorly, it communicates with the ventricular system of the brain, and inferiorly, it forms a blind-ending terminal ventricle.

Gray matter

Like other parts of the CNS, the spinal cord is composed of gray and white matter.

The gray matter surrounds the central canal. It consists of neuronal cell bodies and has a «butterfly» or an «H» shape in cross-section.

The gray matter is differentiated into anterior horns and posterior horns. Additionally, from T1 to L2, there is also a lateral horn, which contains the neurons of the sympathetic nervous system.

The gray matter of the left and right sides is connected by the anterior gray commissure and the posterior gray commissure.

Cells with similar structure and functions are grouped into nuclei.

In the anterior horn, there are 5 nuclei, which serve as motor centers
In the posterior horn, there are also 5 nuclei, serving as sensory centers
In the intermediate zone between the horns, there are an additional 2 nuclei

The gray matter can also be divided into 10 layers (the so-called Rexed laminae). Essentially, these are the same as nuclei, just under a different name. Thus, certain elements of the gray matter correspond to specific laminae.

White matter

The gray matter is surrounded by white matter, which forms 3 columns (funiculi):

The anterior funiculus

The posterior funiculus

The lateral funiculus

These funiculi consist of neuronal processes that are grouped into large bundles, known as tracts. These tracts connect different segments of the spinal cord with each other, as well as with the brain.

There are three types of tracts:

Ascending tracts, which go towards the brain, are predominantly found in the posterior funiculus
Descending tracts, which travel from the brain, are predominantly found in the anterior funiculus
Propriospinal (associative) tracts, which go from one spinal segment to another

The lateral funiculus contains both ascending and descending tracts, approximately in equal amounts.

Spinal cord meninges

In the vertebral canal, the spinal cord is covered by three meninges:

The pia mater is the innermost layer.

On the sides, it forms extensions known as the denticulate ligaments,

which hold the spinal cord in the center of the vertebral canal.

The arachnoid mater is separated from the pia mater by the subarachnoid space, which is filled with cerebrospinal fluid.

The dura mater is separated from the bone by the epidural space,

which contains connective and adipose tissue, as well as venous plexuses. The dura mater smoothly continues into the outer covering of the nerves – the epineurium.

Arterial blood supply

Arterial blood supply to the spinal cord is provided by two systems:

These are the longitudinal vessels that ascend and descend along the surface of the spinal cord:

The anterior spinal artery

Two posterior spinal arteries

As well as branches from various arteries, depending on the section of the spinal cord. They enter the vertebral canal through the intervertebral foramina and are called segmental spinal arteries.

In each segment, a segmental artery divides into two branches: an anterior radicular and a posterior radicular artery.

Additionally, segmental medullary arteries branch off from the segmental spinal arteries. They are directly connected with the longitudinal vessels, acting as anastomoses between two systems.

These arteries are not present in every segment; there are about ten in total, the largest of which is called the artery of Adamkiewicz and is located on the left at the level of approximately 9-12 thoracic segments.

Venous Drainage

Venous drainage from the spinal cord is provided by several veins:

The anterior spinal vein

The posterior spinal vein

Two pairs of small veins at the entry points of the anterior and posterior roots

Blood from these veins drains into the vertebral venous plexus, located in the epidural space. From there, it is drained into the large systemic veins of the corresponding body areas.

Dermatomes and Myotomes

Each segment of the spinal cord corresponds to a strictly defined area of the body that receives innervation from that segment. If it is related to the skin area, it is called a dermatome; if it concerns a skeletal muscle or part of it, it is called a myotome. There is a dermatome map used in diagnosing various sensory disorders.

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