Spinal cord crosssectional views upper diagram

The upper diagram is a cross-section through the spinal cord at the C8 level, the eighth cervical segmental level of the spinal cord (not the vertebral level, see Figure 1). The gray matter is said to be arranged in the shape of a butterfly (or somewhat like the letter H). The gray matter of the spinal cord contains a variety of cell groups (i.e. nuclei), which subserve different functions. Although it is rather difficult to visualize, these groups are continuous longitudinally throughout the length of the spinal cord.

The dorsal region of the gray matter, called the dorsal or posterior horn, is associated with the incoming (afferent) dorsal root, and is thus related to sensory functions. The cell body of these sensory fibers is located in the dorsal root ganglion (see Figure 1). The dorsal horn is quite prominent in this region because of the very large sensory input to this segment of the cord from the upper limb, particularly from the hand. The situation is similar in the lumbar region (as shown in the middle of the three lower illustrations).

The ventral gray matter, called the ventral or anterior horn, is the motor portion of the gray matter. The ventral horn has the large motor neurons, the anterior horn cells, which are efferent to the muscles (see Figure 44). These neurons, because of their location in the spinal cord, which is "below" the brain, are also known as lower motor neurons. (We will learn that the neurons in the cerebral cortex, at the "higher" level, are called upper motor neurons — discussed with Figure 45.) The ventral horn is again prominent at this level because of the large number of motor neurons supplying the small muscles of the hand. The situation is similar in the lumbar region, with the motor neurons supplying the large muscles of the thigh (as shown in the illustration below).

The area in between is usually called the intermediate gray and has a variety of cell groups with some association-type functions (see Figure 32 and Figure 44).

The autonomic nervous system to the organs of the chest, abdomen, and pelvis is controlled by neurons located in the spinal cord.

• Preganglionic sympathetic neurons form a distinctive protrusion of the gray matter, called the lateral horn, which extends throughout the thoracic region, from spinal cord level T1 to L2 (as shown in the first of the three lower illustrations). The post-ganglionic nerves supply the organs of the thorax, abdomen, and pelvis.

• Parasympathetic preganglionic neurons are located in the sacral area and do not form a separate horn (as shown in the illustration). This region of the spinal cord in the area of the conus medullaris (the last of the three lower illustrations) controls bowel and bladder function, subject to commands from higher centers, including the cerebral cortex.

The parasympathetic control of the organs of the thorax and abdomen comes from the vagus nerve, CN X, a cranial nerve (see Figure 6 and Figure 8A).

The central canal of the spinal cord (see Figure 20A, Figure 20B, and Figure 21) is located in the center of the commissural gray matter. This represents the remnant of the neural tube and is filled with CSF. In adults, the central canal of the spinal cord is probably not patent throughout the whole length of the spinal cord. A histological view of these levels of the spinal cord is shown in Figure 69 in Section C.

Note to the Learner: The white matter, which contains the ascending sensory and descending motor pathways, will be described with the pathways in Section B; a summary diagram with all the tracts is shown in Section C (see Figure 68).

Additional Detail

The parasympathetic supply to the salivary glands travels with cranial nerves (CN) VII and IX (see Figure 8A).

Dorsal root ganglion

Afferent (sensory)

Dorsal horn Intermediate gray Ventral horn

Ventral median fissure

Lumbar

Dorsal root of spinal nerve

Sensory nuclei

Central canal Motor nuclei

Dorsal root of spinal nerve

Sensory nuclei

Central canal Motor nuclei

Dorsal root ganglion

Afferent (sensory)

Dorsal Root

Efferent (motor) neuron

Ventral median fissure

Lumbar

Lateral horn

Sacral

Efferent (motor) neuron

Lateral horn

Sacral

FIGURE 4: Spinal Cord 6 — Cross-Sectional Views

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