The body of the parasympathetic preganglionic neurons is localized at the level of the brain stem and at the sacral segments of the spinal cord. For this reason, the parasympathetic system is divided into cranial and sacral parasympathetic regions. The body of the postganglionic neurons form ganglia included in the organ wall (intramural ganglia) or in its vicinity.
The fibers of the cranial parasympathetic region belong to the following cranial nerves: oculomotor (III), facial (VII), glossopharyngeal (IX) and vagus (X). The parasympathetic pathways in the cranial nerves are described in the "Cranial nervous system chapter".
The bodies of the sacral preganglionic neurons form the sacral parasympathetic nucleus located in a mediodorsal position in the intermediate gray matter of the sacral spinal cord segments. Their site corresponds to the lateral horn of the thoracolumbar spinal segments. The dorsal portion of the nucleus is related to the control of the intestine and, the ventral portion, to the contraction of the bladder (Page 810 of "Miller's anatomy of the dog" by Evans, H. 3 rd ed.). The axons of the preganglionic neurons leave the spinal cord as part of the sacral ventral roots, to form the pelvic nerve or nerves. The pelvic nerve receives postganglionar sympathetic nerve fibers (from the hypogastric nerve) to form the pelvic plexus on the lateral aspect of the rectum. Some parsympathetic ganglia are distributed in the plexus and others are located on the walls of the pelvic viscera (Page 59 of "The cardiorrespiratory system" by King, A.S., Blackwell Science. 1999). Some parasympathetic fibers from the pelvic nerve join the hypogastric nerve to travel cranially, pass the caudal mesenteric ganglion without synapsing and innervate the descending colon.
The postganglionic parasympathetic neurons have a vasodilatory action on erectile tissues of the penis and clitoris. They are excitatory on smooth muscle and glands of the rectum and descending colon, and inhibitory on the internal anal sphincter. Ejaculation is facilitated by reflex contractions of the urethral striated muscle. In turn, the postganglionic sympathetic fibers increase the tone of the neck of the urinary bladder (preventing the backflow of semen during ejaculation) and induce the emission of semen.