Peripheral Nervous System

The PNS includes the nerves and ganglia outside the CNS. Its purpose is to convey sensory infor­mation to the brain and spinal cord and to produce movement of muscle and secretion from glands via its motor nerves.

Spinal Nerves

With the exception of cervical and caudal nerves, a pair of spinal nerves (one right and one left) emerges caudal to the vertebra of the same number and name (Fig. 9-13). For example, the first pair of thoracic nerves emerges through the intervertebral foramina between the first and second thoracic vertebrae; the last pair of thoracic nerves emerges through the intervertebral foramina between the last thoracic and first lumbar vertebrae, and the first pair of lumbar nerves emerges through the foramina between the first and second lumbar vertebrae. Thus there are the same number of pairs of thoracic, lumbar, and sacral nerves as there are similar vertebrae.

The first pair of cervical nerves emerges through the lateral vertebral foramina of the atlas and the second pair between the atlas (C1) and axis (C2). Therefore, there are eight pairs of cervical nerves although only seven cervical vertebrae.

Usually, there are fewer pairs of caudal nerves than caudal vertebrae. Five or six pairs are typically seen in domestic ungulates.

Dorsal and ventral roots arise from the spinal cord and fuse, generally close to the interverte­bral foramen. At this point, the conjoined sensory fibers of the dorsal root and motor fibers of the ventral root become the spinal nerve, characterized as a mixed nerve, since it has both sensory and motor elements.

Almost as soon as the spinal nerve emerges through the intervertebral foramen, it divides into a dorsal branch and a ventral branch. Both of these branches are mixed nerves, because each contains both sensory and motor fibers.

in general, the dorsal branches of spinal nerves innervate structures (muscles and skin) that are dorsal to the transverse processes of the vertebrae.

The spinal nerves tend to innervate the region of the body in the area adjacent to where they emerge from the vertebral column. The limbs, however, are supplied with sensory and motor fibers within tangled arrangements of spinal nerves known as plexuses. The regions of the spinal cord supplying the plexuses are visibly greater in diameter because they have more sensory and motor neurons supplying the mass of the limbs. These enlargements are called intumescences.

Brachial Plexus. Each thoracic limb is sup­plied by a brachial plexus, a network of nerves derived from the last three cervical and first one or two thoracic nerves (Figs. 9-1 and 9-14).

Figure 9-14. A) Nerve supply of thoracic limb of the horse. a, suprascapular n.; b, musculocutaneous n.; c, median n.; d, axillary n.; e, radial n.; f, ulnar n.; g, medial and lateral palmar nn. B) Nerve supply of pelvic limb of the horse. a, femoral n.; b, obturator n.; c, saphenous n.; d, pudendal n.; e, gluteal n.; f, sciatic n.; g, common peroneal n.; g', superficial peroneal n.; g”, deep peroneal n.; h, tibial n.; i, medial and lateral plantar nn.

The spinal cord enlargement associated with the brachial plexus lies primarily in the caudal cervical vertebrae and is consequently described as the cervical intumescence.

The brachial plexus gives rise to specific named nerves that innervate the muscles of the thoracic limb and supply sensation to the same general regions of the skin. Table 9-1 lists the nerves arising from the brachial plexus and the region and muscles supplied by each.

Lumbosacral Plexus. The right and left lum­bosacral plexuses supply nerves to the respec­tive pelvic limbs (Figs. 9-1 and 9-14). The lumbosacral plexuses are made up of the ventral branches of the last few lumbar and first two or three sacral nerves.

Cranial Nerves

Classically, 12 pairs of cranial nerves arising from the brain are described (Fig. 9-15). They

Table 9-1. Nerves of Brachial Plexus

Figure 9-15.

are designated by Roman numerals, numbered from most rostral (I) to most caudal (XII). With the exception of cranial nerves I (olfactory) and II (optic), the cranial nerves arise from the mid­brain, pons, and medulla oblongata and in general resemble ordinary spinal nerves. They do not, however, have discernible dorsal and ventral roots, and some are strictly motor or sensory (as opposed to spinal nerves, which are all mixed nerves).

Cranial nerve II, the optic nerve, only super­ficially resembles an actual nerve of the PNS. Its fibers are actually a tract of the CNS, invested with meninges and with myelin provided by oligodendrocytes. Features of the 12 cranial nerves are outlined in Table 9-3.