Recall that the central nervous system is comprised of the brain and spinal cord. We’ll consider the brain later, but first we will study the spinal cord.

Take time now to review the anatomy of the vertebral column (Unit 9). Recall that there are 7 cervical vertebrae, 12 thoracic vertebrae, 5 lumbar vertebrae, 5 fused sacral vertebrae, and 3-4 fused coccygeal vertebrae. The vertebrae are numbered from top to bottom, with a prefix indicating cervical (C1-C7), thoracic (T1-T12), lumbar (L1-L5), or sacral (S1-S5). These will surround the spinal cord, but neurologists also define spinal cord segments which bear a relationship to the vertebrae nearby. We’ll get into the details of this relationship in Unit 12.

The spinal cord is about the diameter of your little finger at its largest (where it connects to the brain) and tapers to a point at its end (the filum terminale, “terminal thread”).

In cross-section, the spinal cord is oval or round and consists of an H– or butterfly-shaped central core of gray matter surrounded by white matter (i.e., myelinated axons traveling together over a great distance). Posterior (dorsal) roots of spinal nerves bring sensory information into the spinal cord. Ventral (anterior) roots of spinal nerves carry motor information away from the spinal cord.

The H-shaped gray matter is further subdivided into a posterior (dorsal) horn, which processes the sensory information brought in by the posterior root; and an anterior (ventral) horn, which contains cell bodies of α motor neurons (alpha motor neurons) controlling muscles. The axons of the α motor neurons form the anterior root of the spinal nerve, which carries motor commands going out to muscles.

At thoracic and upper lumbar levels, there is also a lateral horn (also called the intermediolateral cell column) which contains the cell bodies of the sympathetic neurons we saw in Objective 2. We’ll study these in detail in Unit 12.

The spinal cord ends in a bundle of nerves called the cauda equina (“horse’s tail”), which floats in a bag of CSF. This allows the clinician to withdraw CSF in a procedure called a lumbar puncture.

The white matter of the spinal cord is on the outside (superficial). It contains the axons of neurons that are either carrying information from the cerebral cortex of the brain to the spinal cord (called corticospinals) and from the brainstem to the spinal cord. Collectively, these are called descending tracts. There are also axons of nerve cells carrying information from the body, or from the spinal cord, to the brain. These are collectively called ascending tracts.

The white matter is further subdivided into three zones that are named pretty logically. The posterior (dorsal) horn gray matter extends to the surface and forms the “walls” of a posterior column. In the lateral white matter is the lateral column. The anterior (ventral) white matter is the anterior column.