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Lesson 7. The Spinal Nerves


a. There are 31 pairs of spinal nerves, identified as follows:

(1) Cervical nerves (8) (C-1 through C-8).
(2) Thoracic nerves (12) (T-1 through T-12).
(3) Lumbar nerves (5) (L-1 through L-5).
(4) Sacral nerves (5) (S-1 through S-5).
(5) Coccygeal nerve (1).

fig91902_04.jpg (64144 bytes)
Figure 2-4. Spinal nerve.

b.  In the human body, every spinal nerve has essentially the same structure and components. By learning the anatomy of one spinal nerve, you can understand the anatomy of all spinal nerves. Like a tree, a typical spinal nerve has roots, a trunk, and branches (rami) (figure 2-4).

(1) Coming off of the posterior and anterior sides of the spinal cord are the posterior (sensory) and anterior (motor) roots of the spinal nerve. An enlargement on the posterior root is the posterior root ganglion. (A ganglion is a collection of neuron cell bodies, together, outside the CNS.)
(2) Laterally, the posterior and anterior roots of the spinal nerve join to form the spinal nerve trunk. The spinal nerve trunk of each spinal nerve is located in the appropriate intervertebral foramen of the vertebral column. (An intervertebral foramen is a passage found on both sides of a vertebrae. It is formed by the columnar alignment of the vertebrae.)
(3) Where the spinal nerve trunk emerges laterally from the intervertebral foramen, the trunk divides into two major branches. These branches are called the anterior (ventral) and posterior (dorsal) primary rami (ramus, singular). The posterior primary rami go to the back. The anterior primary rami go the sides and front of the body and also to the upper and lower extremities.

c. A nerve has been defined as a collection of neuron processes. These processes may belong to different types of neurons--afferent (sensory), efferent (motor), or the visceral motor neurons of the autonomic nervous system. (The ANS will be discussed separately.)

(1) An afferent neuron carries sensory information from the periphery to the CNS.
(2) An efferent neuron carries motor commands from the CNS to the periphery of the body.

David L. Heiserman, Editor

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Revised: June 06, 2015