Parasympathetic Nervous System
The parasympathetic nervous system arises from cranial nerves
III, VII, IX, and X, as well as from sacral segments. Unlike in the sympathetic
nervous system, the ganglia of the parasympathetic nervous system are proximal to
or within the innervated organ. This location of ganglia makes the parasympathetic
nervous system more targeted and less robust than the sympathetic nervous system.
Preganglionic fibers of the parasympathetic nervous system originate
in three areas of the CNS: the midbrain, the medulla oblongata, and the sacral part
of the spinal cord. Fibers arising in the Edinger-Westphal nucleus of the oculomotor
nerve course in the midbrain to synapse in the ciliary ganglion. This pathway innervates
the
Figure 16-6
Schematic representation of the autonomic nervous system
depicting the functional innervation of peripheral effector organs and the anatomic
origin of peripheral autonomic nerves from the spinal cord. Although both paravertebral
sympathetic ganglia chains are presented, the sympathetic innervation to the peripheral
effector organs is shown only on the right part of the figure, whereas the parasympathetic
innervation of peripheral effector organs is depicted on the left. The roman numerals
on nerves originating in the tectal region of the brainstem refer to the cranial
nerves that provide parasympathetic outflow to the effector organs of the head, neck,
and trunk. (From Ruffolo R: Physiology and biochemistry of the peripheral
autonomic nervous system. In Wingard L, Brody T,
Larner J, et al [eds]: Human Pharmacology: Molecular to Clinical. St. Louis, Mosby-Year
Book, 1991, p 77.)
smooth muscle of the iris and the ciliary muscle. In the medulla oblongata lie parasympathetic
components of the facial (lacrimatory nucleus), glossopharyngeal, and vagus (dorsal
nucleus) nerves. The facial nerve gives off parasympathetic fibers to the chorda
tympani and greater superficial petrosal nerve, which subsequently synapse in the
ganglia of the submaxillary or sublingual glands and the sphenopalatine ganglion,
respectively. The glossopharyngeal nerve synapses in the otic ganglion. These postganglionic
fibers innervate the mucous, salivary, and lacrimal glands; they also carry vasodilator
fibers.
The vagus is unquestionably the most important of the parasympathetic
nerves. The vagus transmits fully three fourths of the traffic of the parasympathetic
nervous system. It supplies the heart, tracheobronchial tree, liver, spleen, kidney,
and the entire gastrointestinal tract except the distal colon. The preganglionic
fibers of the vagus are long, whereas the postganglionic fibers are short. Most
vagal fibers do not synapse until they arrive at small ganglia on and about the thoracic
and abdominal viscera. Although the parasympathetic nerves may synapse with a 1:1
ratio of nerve to effector cells, the vagal innervation of the Auerbach plexus may
connect one nerve fiber to 8000 cells.
The second through fourth sacral segments contribute the nervi
erigentes, or pelvic splanchnic nerves. They synapse in terminal ganglia associated
with the rectum and genitourinary organs.
