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Lesson 4. The Spinal Cord


a. The spinal cord, located within the vertebral canal of the spine, is continuous with the brainstem. (Together, the brain and spinal cord are referred to as the neuraxis.) The spinal cord extends from the foramen magnum of the skull to the level of the first lumbar vertebrae, at which point it tapers to fine threads of tissue.

b. The spinal cord has two enlargements along its length that are due to an increase in the mass of nervous tissue required to serve the limbs.

(1) The cervical enlargement is associated with the nerves of the upper extremities.
(2) The lumbosacral enlargement is associated with the nerves of the lower extremities.

c.  The spinal cord is composed of a central mass of gray matter (cell bodies of neurons) surrounded by white matter (myelinated processes of the neurons). The areas of gray and white matter are referred to as columns.

fig91902_03.jpg (26964 bytes)

Figure 2-3. The spinal cord, cross-section.

d.  In a cross-section of the spinal cord (figure 2-3), the gray matter appears to be H-shaped. Each arm of the H is called a horn, resulting in two anterior horns and two posterior horns. The connecting middle portion is called the gray commissure.

e. A very narrow canal, called the central canal, is located in the center of the spinal cord. This central canal is continuous with the fourth ventricle of the brain and contains CSF.

f. The processes of the neurons that compose the surrounding white matter are grouped into pathways called fiber tracts.

(1) Tracts conducting impulses from the brain are called motor tracts.
(2) Tracts conducting impulses to the brain are called sensory tracts.
(3) At some specific point along the neuraxis, these pathways cross to the opposite side of the cord and continue their path. (Each crossing is called a decussation.) Thus, the right cerebral hemisphere of the brain communicates with the left half of the body, and the left cerebral hemisphere communicates with the right half of the body.

g.     Thirty-one pairs of spinal nerves exit from the spinal cord.

David L. Heiserman, Editor

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Revised: June 06, 2015