Porth's Essentials of Pathophysiology, 4e

839

Organization and Control of Neural Function

C h a p t e r 3 4

vessels. Supporting structures of the spinal cord are dis- cussed further in Chapter 36. Early in fetal life, the spinal cord extends the entire length of the vertebral column and the spinal nerves exit through the intervertebral foramina (openings) near their level of origin. Because the vertebral column and spinal dura grow at a faster rate than the spinal cord, a dispar- ity develops between each succeeding cord segment and the exit of its dorsal and ventral nerve roots through the corresponding intervertebral foramina. In the newborn, the cord terminates at the level of L2 or L3. In the adult, the cord usually terminates in the inferior border of L1, and the arachnoid mater and its enclosed subarachnoid space, which is filled with CSF, do not close down on the filum terminale until they reach the level of S2. This results in the formation of a pocket of CSF, the dural cisterna spinalis , which extends from approximately L2 to S2. Because this area contains an abundant supply of CSF and the spinal cord does not extend this far, the area often is used for sampling the CSF. A procedure called a spinal tap , or puncture, can be done by inserting a special needle into the dural sac at L3 or L4. The spi- nal roots, which are covered with pia mater, are in little danger of trauma from the needle used for this purpose. Spinal Nerves The peripheral nerves that carry information to and from the spinal cord are called spinal nerves . There are 31 left-right pairs of spinal nerves (8 cervical, 12 tho- racic, 5 lumbar, 5 sacral, and 1 coccygeal); each pair is named for the segment of the spinal cord from which it exits. Because the first cervical spinal nerve exits the spinal cord just above the first cervical vertebra (C1), this nerve is given the number of the bony vertebra just below it (see Fig. 34-11A). However, the numbering is changed for all lower levels. An extra cervical nerve, the C8 nerve, exits above the T1 vertebra, and each of the subsequent nerves are numbered for the vertebra just above its point of exit. Each spinal cord segment communicates with its cor- responding body segment through the paired segmental spinal nerves. Each spinal nerve, accompanied by the blood vessels supplying the spinal cord, enters the spinal canal through an intervertebral foramen, where it divides into two branches, or roots. One branch enters the dor- solateral surface of the cord (i.e., dorsal root), carrying the axons of afferent neurons into the CNS. The other branch leaves the ventrolateral surface of the cord (i.e., ventral root), carrying the axons of efferent neurons into the periphery. These two branches or roots fuse at the intervertebral foramen, forming the mixed spinal nerve— “mixed” because it has both afferent and efferent axons. After emerging from the vertebral column, the spi- nal nerve divides into two branches or rami (singular, ramus ): a small dorsal primary ramus and a larger ven- tral primary ramus (Fig. 34-13). The thoracic and upper lumbar spinal nerves also lead to a third branch, the ramus communicans, which contains sympathetic axons supplying the blood vessels, the genitourinary system, and the gastrointestinal system. The dorsal ramus

Pia mater

Dorsal

Subarachnoid space Dorsal root

Arachnoid

Dura mater

Meningeal branch Dorsal primary ramus

Ventral primary ramus

Rami communicans

Ventral root

Third thoracic vertebra

Ventral

Denticulate ligament

FIGURE 34-13. Cross-section of vertebral column at the level of the third thoracic vertebra, showing the meninges, the spinal cord, and the origin of a spinal nerve and its branches or rami.

contains sensory fibers from the skin and motor fibers to muscles of the back. The ventral primary ramus con- tains motor fibers that innervate the skeletal muscles of the anterior body wall and the legs and arms. Spinal nerves do not go directly to skin and muscle fibers; instead, they form complicated nerve networks called plexuses (see Fig. 34-11A). A plexus is a site of intermixing nerve branches. Many spinal nerves enter a plexus and connect with other spinal nerves before exiting from the plexus. Nerves emerging from a plexus form progressively smaller branches that supply the skin and muscles of the various parts of the body. The PNS contains four major plexuses: the cervical, brachial, lumbar, and sacral plexuses. Spinal Reflexes A reflex is a highly predictable relationship between a stimulus and a motor response. A spinal reflex is a response mediated by cells in the spinal cord, bypass- ing any conscious effort from the brain. A reflex may involve neurons at a single spinal cord segment (i.e., seg- mental reflexes), several or many reflexes (i.e., segmental reflexes), or structures in the brain (i.e., suprasegmental reflexes). A spinal reflex involves a sensory receptor, an affer- ent pathway, an integrating center in the spinal cord, an efferent pathway, and an effector muscle or gland. A classic example is the knee-jerk or patellar reflex, in which the leg jerks when the kneecap is briskly tapped with a reflex hammer. The reflex reaction is initiated by sensory receptors at the site of the stimulus, and relayed to the spinal cord via afferent sensory neurons. This happens in a fraction of a second, allowing people to jerk away before the brain is even aware of a prob- lem. The integrating center in the spinal cord, in turn,