Page 220 - Anatomy and Physiology of Farm Animals, 8th Edition

Anatomy of the Nervous System / 205

entire cortex and initiate aversive behav- Peripheral Nervous System
iors. Still others are relayed directly to the
VetBooks.ir parts of the cortex that create conscious The PNS includes the nerves and ganglia

awareness of the painful stimulus.
Like pain, touch and temperature sense outside the CNS. Its purpose is to convey
sensory information to the brain and spi-
information is carried in a variety of nal cord and to produce movement of
ascending tracts. Some of these are found muscle and secretion from glands via its
in the dorsal columns and others in the motor nerves.
anterolateral system.

Motor Tracts. Motor systems can be Spinal Nerves
functionally grouped into two main
categories: a ventromedial motor system, As indicated previously, spinal nerves
largely located in the ventral funiculus, and emerge from the vertebral canal at the
a dorsolateral motor system, found in the intervertebral foramina. The first pair of
dorsal part of the lateral funiculus. cervical nerves emerges through the lateral
Movement driven by the brain (as opposed vertebral foramina of the atlas and the sec-
to local reflex arcs) may be both voluntary ond pair between the atlas (C1) and axis
and therefore under control of the cerebral (C2). Therefore, there are eight pairs of
cortex (usually indirectly; see Fig. 10‐14), cervical nerves although only seven cervi-
or it may be involuntary as in the case of cal vertebrae. For the thoracic, lumbar, and
large postural adjustments arising reflexly sacral regions, there are the same number
from the brainstem. of spinal cord segments (and pairs of spinal
The ventromedial motor system pri- nerves) as vertebrae in each region. Usually,
marily is responsible for activity in the there are fewer pairs of caudal nerves than
axial and proximal limb muscles, espe- caudal vertebrae. Five or six pairs are typi-
cially extensors and antigravity muscles. cally seen in domestic ungulates.
Activity in the tracts of this system assists The spinal nerve is created when dorsal
with the support phase of gait, when limbs and ventral roots arising from the spinal
are in weight‐bearing position with joints cord fuse. At this point, the conjoined
extended. A particularly noteworthy tract sensory fibers of the dorsal root and motor
of the ventromedial motor system is the fibers of the ventral root create a mixed
lateral vestibulospinal tract, which nerve, since it has both sensory and motor
originates in the region of the pons and elements (Fig. 10‐12).
medulla. Almost as soon as the spinal nerve
The dorsolateral motor system is in emerges through the intervertebral fora-
many ways complementary to the ventro- men into the periphery, it divides into a
medial system. Dorsolateral tracts tend to dorsal branch and a ventral branch. Both
control the muscles of the distal limb, of these branches are mixed nerves,
especially the flexors. Activity here is because each contains both sensory and
most important in the flexion or swing motor fibers.
phase of gait, when limbs are lifted from In general, the dorsal branches of spinal
the ground and advanced while flexed. In nerves innervate structures (muscles and
quadrupeds, the most prominent tract in skin) that are dorsal to the transverse pro-
the dorsolateral motor system is the cesses of the vertebrae. The ventral
rubrospinal tract, which arises from branches supply structures ventral to the
the midbrain. In primates, and most espe- transverse processes and most of the
cially humans, the corticospinal tracts thoracic and pelvic limbs.
arising directly from the motor cortex The spinal nerves tend to innervate the
of the cerebrum are especially well region of the body in the area adjacent
developed. to where they emerge from the vertebral