Autonomic Nervous system
Contrast the structure of the somatic and autonomic motor neurons.
The somatic and autonomic nervous systems differ in the number of
fibers and in their diameter and presence of myelin. In both systems, the cell bodies of the
somatic motor neuron and the autonomic preganglionic neuron are located within the CNS. The
axons of these neurons are myelinated. Those of the somatic division are thicker in diameter. The
preganglionic motor neurons of the ANS synapse with postganglionic neurons in a ganglion
external to the CNS. The postganglionic fibers are thin and unmyelinated.
Identify three anatomical differences that distinguish the
sympathetic division from
The two subdivisions of the ANS can be distinguished by (1) the
location where the
fibers leave the CNS, (2) the length of the postganglionic fibers and location of the ganglia, and
(3) the degree of branching of the fibers. First, the sympathetic division arises from the
thoracolumbar region of the spinal cord, whereas the parasympathetic arises from the cranial and
sacral regions. Second, the postganglionic sympathetic fibers are longer than those of the
parasympathetic division. The sympathetic fibers run from the sympathetic ganglia near the
vertebral column to distant target tissues. The parasympathetic ganglia are often within the walls
of the target tissue, giving rise to very short postganglionic fibers. Third, the fibers of the
sympathetic division branch, allowing for innervation of multiple target tissues and simultaneous
activation by the same preganglionic fiber.
The sympathetic and parasympathetic divisions differ markedly in the
branching of the
postganglionic fibers. Explain the anatomical difference and its significance.
Answer: The postganglionic fibers of the parasympathetic division are short
The postganglionic fibers of the parasympathetic division are short
and often enclosed
within the tissues they innervate. In contrast, the postganglionic sympathetic fibers are highly
branched and innervate multiple tissues; nerve impulses from one fiber will simultaneously
activate multiple target tissues. The simultaneous activation of wide-ranging tissues and organs
is necessary for a fight-or-flight response.
Explain the relationship between the vagus and pelvic splanchnic
nerves, the cardiac and
pulmonary plexus, celiac plexus, and hypogastric plexus.
Some parasympathetic preganglionic fibers exit the CNS through the
vagus nerve (CN
X). Additional fibers exit the spinal cord as the pelvic splanchnic nerves at the level of the
sacrum. These preganglionic fibers branch and interconnect to form plexi. The cardiac,
pulmonary, and celiac plexi are derived from these pre-ganglionic vagus nerve fibers. The
hypogastric plexus arises from thoracic, lumbar, pelvic and sacral splanchnic nerves. These plexi
contain both sympathetic and parasympathetic fibers.
Describe the correct pathway that a nerve impulse to blood vessels
and hair follicles (glands
and arrector pili muscles) in the dermis must travel using the following structures: ventral ramus,
dorsal root, ventral root, sympathetic trunk, gray ramus communicans, white ramus
communicans, and spinal nerve.
Answer: Efferent impulses leave
Efferent impulses leave the spinal cord through the ventral root to
merge with a spinal
nerve. The spinal nerve branches to form a dorsal and ventral ramus. The ventral ramus of the
spinal nerve connects to the sympathetic trunk via a white ramus communicans. Sympathetic
impulses move through preganglionic fibers within the sympathetic trunk. These fibers may
either project inferiorly or superiorly through the chain ganglia. Within the ganglia, these
preganglionic fibers synapse with postganglionic fibers that exits via gray rami communicantes.
These postganglionic fibers will then proceed through the dorsal rami of spinal nerves to the skin
on the posterior torso or through the ventral rami to the skin covering the rest of the body.