Chapter 165. The Autonomic Nervous System: The Parasympathetic System - Review of Medical Embryology Book

The parasympathetic system is less extensive than the sympathetic system. Preganglionic fibers arise only in certain centers of the cerebral trunk and in the sacral portion of the spinal cord. Thus it is called the craniosacral portion of the autonomic nervous system

THE PREGANGLIONIC FIBERS of the parasympathetic system follow the path of specific cranial nerves, namely, the oculomotor (III), the facial (VII), the glossopharyngeal (IX), and the vagus (X). In addition, they follow the sacral spinal nerves arising from segments S2, S3, and S4 of the cord
LIKE THE SYMPATHETIC NEUROBLASTS, their ganglion cells come from the neural crest and neural tube, but only at the level of the preganglionic fibers. Their long migrating preganglionic fibers take them to the viscera; and the short postganglionic fibers pass to the branchial arches and the cardiac, pulmonary, and intestinal plexuses
ALL THE PARASYMPATHETIC GANGLIA are preaortic or visceral and do not appear in the chain ganglia
SOME SPECIALIZED PARASYMPATHETIC RECEPTORS

The carotid and aortic bodies are mesenchymal chemoreceptors, innervated by the glossopharyngeal (IX) cranial nerv The neurosensory cells which make up these structures are of parasympathetic origin, and they migrate along nerve IX from the neural crest or neural tube

THE POSTGANGLIONIC NEURONS of the sympathetic nervous system are adrenergic, while those of the parasympathetic nervous system are predominately cholinergic

Antagonism of the sympathetic and parasympathetic systems helps to maintain equilibrium of involuntary functions, although in some instances the 2 systems may work together
The entire autonomic nervous system is under the control of the hypothalamus, which coordinates information relating to involuntary body functions

ABNORMAL ORGANOGENESIS of the autonomic nervous system is responsible for certain problems, especially of the digestive system, such as is seen in Hirschsprung's disease or megacolon, which results in congenital dilatation of the colon with anomalies of Meissner's and Auerbach's plexuses