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posterior median sulcus. Each half contains white and gray matter, which can be further subdivided. This is illustrated in Figure 21-6.

In the center of the spinal cord is the gray matter. The anterior gray matter, or the anterior horn, is the motor portion of the spinal cord and contains multipolar cells of origin of the anterior roots of the peripheral nerves. The lateral horn (sympathetic preganglionic neurons) is found at the T1 to L2 spinal levels. The posterior gray matter, or the posterior horn, is the receptor portion of the spinal cord.

The white matter of the spinal cord consists of tracts that link segments of the spinal cord and connect it to the brain. There are three main columns (funiculi). Between the anterior median fissure and the anterolateral sulcus is the anterior white column, which contains the descending fibers of the ventral corticospinal tract and the ascending fibers of the ventral spinothalamic tract. The ventral corticospinal tract is involved with voluntary motion, and the ventral spinotha-lamic tract carries impulses related to light touch.

The lateral white column is located between the anterolateral and posterolateral sulci and contains the descending fibers of the lateral corticospinal tract and the ascending spinocerebellar and lateral spinothalamic tracts. The lateral corticospinal tract is responsible for voluntary movement; the spinocerebellar tracts, for reflex proprioception; and the lateral spinothalamic tract, for pain and temperature sensation.

The posterior white column is located between the posterolateral and posterior median sulci. The most important fibers in this column are the ascending fibers of the fasciculus gracilis and fasciculus cuneatus. These tracts are involved with vibration sense, passive motion, joint position, and two-point discrimination.

There are 31 pairs of spinal nerves, each with a ventral (motor) and dorsal (sensory) root. The ventral root consists of efferent nerve fibers, which originate in the anterior and lateral gray matter (T1 to T2 only) and travel to the peripheral nerve and muscle, which constitute the motor root. The dorsal root consists of afferent nerve fibers whose cell bodies are in the dorsal root ganglion, which is the sensory root.

The spinal nerves are grouped into 8 cervical (C1 to C8), 12 thoracic (T1 to T12), 5 lumbar (L1 to L5), 5 sacral (S1 to S5), and 1 coccygeal nerve. These nerves are illustrated in Figure 21-7.

A spinal reflex involves an afferent neuron and an efferent neuron at the same level in the spinal cord. The basis for this reflex arc is an intact sensory limb, functional synapses in the spinal cord, an intact motor limb, and a muscle capable of responding. The afferent and efferent limbs travel together in the same spinal nerve. When a stretched muscle is suddenly stretched further, the afferent sensory limb sends impulses through its spinal nerve that travel to the dorsal root of that nerve. After reaching a synapse in the gray matter of the spinal cord, the impulse is transmitted to the ventral nerve root. These impulses are then conducted through the ventral root to the neuromuscular junction, where a brisk contraction of the muscle completes the reflex arc. Figure 21-8 illustrates a reflex spinal arc.

The afferent sensory limb is important not only in the reflex arc but also in the conscious appreciation of sensation. Nerve fibers carrying pain and temperature sensation enter the spinal cord and cross to the other side within one or two spinal segments. They ascend in the contralateral lateral spinothalamic tract, travel through the brain stem and the thalamus, and end in the postcentral gyrus of the parietal lobe, as illustrated in Figure 21-9A. Fibers carrying proprioceptive sensation from muscles, joints, and tendons enter the dorsal root and participate in the reflex arc. Other fibers carrying proprioceptive sensation pass directly into the posterior columns and ascend in the fasciculi gracilis and cuneatus to their ipsilateral nuclei, cross in the medial lemniscus, reach a synapse in the thalamus, and end in the postcentral gyrus of the parietal lobe. Still other proprioceptive fibers ascend crossed and uncrossed in the spinocerebellar tracts to the cerebellum. These additional pathways are illustrated in Figure 21-9B.

Postcentral gyrus of parietal lobe

Postcentral gyrus of parietal lobe

Postcentral gyrus of parietal lobe

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