The Human Spinal Cord

a. Location and Extent. Referring to figure 4-4, you can see that the typical vertebra has a large opening called the vertebral (or spinal) foramen. Together, these foramina form the vertebral (spinal) canal for the entire vertebral column. The spinal cord, located within the spinal canal, is continuous with the brainstem. The spinal cord travels the length from the foramen magnum at the base of the skull to the junction of the first and second lumbar vertebrae.

(1) Enlargements. The spinal cord has two enlargements. One is the cervical enlargement, associated with nerves for the upper members. The other is the lumbosacral enlargement, associated with nerves for the lower members.

(2) Spinal nerves. A nerve is a bundle of neuron processes which carry impulses to and from the CNS. Those nerves arising from the spinal cord are spinal nerves. There are 31 pairs of spinal nerves.

b. A Cross Section of the Spinal Cord (figure 11-6). The spinal cord is a continuous structure which runs through the vertebral canal down to the lumbar region of the column. It is composed of a mass of central gray matter (cell bodies of neurons) surrounded by peripheral white matter (myelinated processes of neurons). The gray and white matter are thus considered columns of material. However, in a cross section, this effect of columns is lost.

(1) Central canal. A very narrow canal, called the central canal, is located in the center of the spinal cord. The central canal is continuous with the fourth ventricle of the brain.

(2) The gray matter. In the cross section of the spinal cord, one can see a central H-shaped region of gray matter. Each arm of the H is called a horn, resulting in two posterior horns and two anterior horns. The connecting link is called the gray commissure. Since the gray matter extends the full length of the spinal cord, these horns are actually sections of the gray columns.

(3) The white matter. The peripheral portion of the spinal cord cross section consists of white matter. Since a column of white matter is a large bundle of processes, it is called a funiculus. In figure 11 -6, note the anterior, lateral, and posterior funiculi.

Figure 11-6. A cross-section of the spinal cord.
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