Gross Anatomy and Blood Supply of Spinal Cord

Spinal cord is the part of the nervous system houses inside the vertebral column. It serves as a very important conduit for the passage of various ascending and descending nervous pathways to and from the higher centers.

Gross Features:

The spinal cord is roughly cylindrical in shape.

Superiorly, it begins at the foramen magnum in the skull, where it is continuous with the medulla oblongata of the brain.

Inferiorly, it terminates in the adult at the level of the lower border of the first lumbar vertebra. In the young child, it is relatively longer and usually ends at the upper border of the third lumbar vertebra. Here, the spinal cord tapers off into the conus medullaris, from the apex of which a prolongation of the pia mater, the filum terminale, descends to be attached to the posterior surface of the coccyx.

It presents two fusiform enlargements:

Cervical enlargement - in the cervical region, where it gives origin to the brachial plexus, and in the lower thoracic.

Lumbar enlargement – in lumbar regions, where it gives origin to the lumbosacral plexus.

It has two fissures:

a deep longitudinal fissure called the anterior median fissure in the midline anteriorly and a shallow furrow called the posterior median sulcus on the posterior surface.

It is surrounded by the three meninges, the dura mater, the arachnoid mater, and the pia mater.

It gives attachment to 31 pairs of spinal nerves along its entire length by the means of anterior or motor roots and the posterior or sensory roots. Each root is attached to the cord by a series of rootlets, which extend the whole length of the corresponding segment of the cord. Each posterior nerve root possesses a posterior root ganglion or dorsal root ganglion or spinal ganglion, the cells of which give rise to peripheral and central nerve fibers.

Internal structure of spinal cord

The spinal cord is composed of an inner core of gray matter, which is surrounded by an outer covering of white matter.

Grey matter of Spinal Cord:

On cross section, the gray matter is seen as an H-shaped pillar with anterior (ventral) and posterior (dorsal) gray columns, or horns, united by a thin gray commissure containing the small central canal.

A small lateral gray column or horn is present in the thoracic and upper lumbar segments of the cord.

The gray matter of the spinal cord consists of a nerve cells and their processes along with neuroglia, and blood vessels.

Nerve cell groups in the gray columns

Nerve Cell Groups in the Anterior Gray Columns

Most nerve cells are large and multipolar. For practical purposes, the nerve cells of the anterior gray column may be divided into three basic groups or columns:  Medial, Central, and Lateral.

The medial group is present in most segments of the spinal cord and is responsible for innervating the skeletal muscles of the neck and trunk, including the intercostal and abdominal musculature.

The central group is the smallest and is present in some cervical and lumbosacral segments. In the cervical part of the cord, some of these nerve cells (segments C3-5) specifically innervate the diaphragm and are collectively referred to as the phrenic nucleus. 

In the upper five or six cervical segments, some of the nerve cells innervate the sternocleidomastoid and trapezius muscles and are referred to as the accessory nucleus. The axons of these cells form the spinal part of the accessory nerve. 

The lumbosacral nucleus present in the second lumbar down to the first sacral segment of the cord is made up of nerve cells whose axons have an unknown distribution.

The lateral group is present in the cervical and lumbosacral segments of the cord and is responsible for innervating the skeletal muscles of the limbs.

Nerve Cell Groups in the Posterior Gray Columns

There are four nerve cell groups of the posterior gray column: two that extend throughout the length of the cord and two that are restricted to the thoracic and lumbar segments.

The substantia gelatinosa group is situated at the apex of the posterior gray column throughout the length of the spinal cord. It is largely composed of Golgi type II neurons and receives afferent fibers concerned with pain, temperature, and touch from the posterior root.

The nucleus proprius is a group of large nerve cells situated anterior to the substantia gelatinosa throughout the spinal cord. This nucleus receives fibers from the posterior white column that are associated with the senses of position and movement (proprioception), two-point discrimination, and vibration.

The nucleus dorsalis (Clarke's column) is a group of nerve cells situated at the base of the posterior gray column and extending from the eighth cervical segment caudally to the third or fourth lumbar segment. Most of the cells are comparatively large and are associated with proprioceptive endings (neuromuscular spindles and tendon spindles).

The visceral afferent nucleus is a group of nerve cells of medium size situated lateral to the nucleus dorsalis; it extends from the first thoracic to the third lumbar segment of the spinal cord.

Nerve Cell Groups in the Lateral Gray Columns

The intermediolateral group of cells form the small lateral gray column, which extends from the first thoracic to the second or third lumbar segment of the spinal cord. The cells are relatively small and give rise to preganglionic sympathetic fibers.

A similar group of cells found in the second, third, and fourth sacral segments of the spinal cord give rise to preganglionic parasympathetic fibers

White matter of spinal cord:

The white matte may be divided into anterior, lateral, and posterior white funiculi. The anterior funiculus on each side lies between the midline and the point of emergence of the anterior nerve roots; the lateral funiculus lies between the emergence of the anterior nerve roots and the entry of the posterior nerve roots; the posterior funiculus lies between the entry of the posterior nerve roots and the midline.

Structure:

The white matter of the spinal cord consists of a mixture of nerve fibers, neuroglia, and blood vessels. It surrounds the gray matter, and its white color is due to the high proportion of myelinated nerve fibers.

Arrangement of nerve fibers tracts in the white matter of spinal cord:

The relative positions of major ascending and descending tracts within the white matter of spinal cord can be summarized as follow:

Posterior Funiculus 

It is mainly occupied by the ascending tracts called fasciculus gracilis (medially) and fasciculus cuneatus (laterally).

Lateral funiculus

Descending tracts:

Lateral Corticospinal tract

Rubrospinal tract

Lateral reticulospinal tract

Ascending tracts:

Lateral spinothalamic tract

Anterior and posterior spinocerebellar tracts

Anterior funiculus

Descending tracts:

Anterior corticospinal tract

Tectospinal tract

Lateral vestibulospinal tract

Medial vestibulospinal tract

Olivospinal trat

Medial reticulospinal tract

Ascending tract:

                  Anterior spinothalamic tract

Blood supply of the spinal cord:

The spinal cord receives its arterial supply from three small arteries:

the two posterior spinal arteries and the anterior spinal artery

In addition, the supply is reinforced by small segmentally arranged arteries that arise from arteries outside the vertebral column and enter the vertebral canal through the intervertebral foramina. These vessels anastomose on the surface of the cord and send branches into the substance of the white and gray matter.

Posterior Spinal Arteries

The posterior spinal arteries arise either directly from the vertebral arteries inside the skull or indirectly from the posterior inferior cerebellar arteries. Each artery descends on the posterior surface of the spinal cord close to the posterior nerve roots and gives off branches that enter the substance of the cord. The posterior spinal arteries supply the posterior one-third of the spinal cord.

Anterior Spinal Artery

The anterior spinal artery is formed by the union of two arteries, each of which arises from the vertebral artery inside the skull. The anterior spinal artery then descends on the anterior surface of the spinal cord within the anterior median fissure. Branches from the anterior spinal artery enter the substance of the cord and supply the anterior two-thirds of the spinal cord.

Segmental Spinal Arteries

The segmental arteries are branches of arteries outside the vertebral column (deep cervical, intercostal, and lumbar arteries). After entering the vertebral canal, each segmental spinal artery gives rise to anterior and posterior radicular arteries that accompany the anterior and posterior nerve roots to the spinal cord.

Additional feeder arteries enter the vertebral canal and anastomose with the anterior and posterior spinal arteries. One large and important feeder artery, the great anterior medullary artery of Adamkiewicz, arises from the aorta in the lower thoracic or upper lumbar vertebral levels; it is unilateral and, in the majority of persons, enters the spinal cord from the left side. The importance of this artery lies in the fact that it may be the major source of blood to the lower two-thirds of the spinal cord.

Veins of the Spinal Cord

The veins of the spinal cord drain into six tortuous longitudinal channels that communicate superiorly within the skull with the veins of the brain and the venous sinuses. They drain mainly into the internal vertebral venous plexus.

Clinical Notes:

Injury/Lesions of the Anterior and Posterior Nerve Roots

Causes

• Infections (as in syphilitic spinal meningitis or pyogenic meningitis)
• The posterior roots may be involved in tabes dorsalis and herpes zoster.
• Tumors of the vertebral column, a herniated intervertebral disc or a fracture dislocation can press on the spinal nerve roots in the intervertebral foramina
• severe scoliosis (abnormal curvature of vertebral column) may result in compression of the nerve roots.

Manifestation:

A lesion of one posterior spinal nerve root will produce:

     - pain in the area of skin innervated by that root
     - pain in the muscles that receive their sensory nerve supply from that root

A lesion of an anterior root will result in

     - paralysis of any muscle that is supplied exclusively by that root

     - a partial paralysis of any muscle that is supplied partially by that root

In both cases, fasciculation and muscle atrophy occur.

Injury to the Ascending Tracts within the Spinal Cord

In simple term, the sensation carried by the ascending tracts involved will be impaired which is summarized as follow:

Injury to Lateral Spinothalamic Tract

It results into contralateral loss of pain and temperature sensibilities below the level of the lesion The patient will not, therefore, respond to pinprick or recognize hot and cold objects placed in contact with the skin.

Injury to Anterior Spinothalamic Tract

The clinical manifestation in such case is contralateral loss of light touch and pressure sensibilities below the level of the lesion. The patient will not feel the light touch of a piece of cotton placed against the skin or feel pressure from a blunt object placed against the skin.

Injury to Fasciculus Gracilis and Fasciculus Cuneatus

Loss of the supply of information from the muscles and joints to consciousness on the same side; thus, the individual does not know about the position and movements of the ipsilateral limbs below the level of the lesion. With the patient's eyes closed, he or she is unable to tell where the limb or part of the limb is in space.

Loss of vibration sense below the level of the lesion on the same side

Loss of tactile discrimination on the side of the lesion.

REFERENCES:

Gray’s Anatomy, 39th Edition

Snell’s Clinical Neuroanatomy 7th Edition

Lange Clinical Neuroanatomy 25th Edition