The pelvic plexus: innervation of pelvic and extrapelvic visceral tissues

The pelvic plexus is an association of neurons that govern visceral tissues involved in eliminative and reproductive functions. It is the singular site in the autonomic nervous system where sympathetic and parasympathetic neurons occur in the same ganglia. Within the plexus, ganglia are not randomly positioned; sympathetic neurons tend to occur more ventrally while parasympathetic neurons are located more dorsally, both in accordance with the location of their target tissues and the entry point of their corresponding preganglionic nerve tracts. For example, the vas deferens and seminal vesicle are ventral in position and thus are innervated by more ventrally located pelvic neurons. Neurochemical studies of pelvic ganglia indicate that there are some characteristic associations of putative neurotransmitters which are based on target organ distribution and in part, dictated by the variety of target tissues within each organ. Penile neurons comprise a uniform population in that they are cholinergic and also may release vasoactive intestinal polypeptide (VIP) and nitric oxide. In contrast, target tissues of the internal genitalia are more diverse, requiring adrenergic and nonadrenergic innervation and a complementary neuropeptide. Preganglionic innervation may also be coded and although sympathetic and parasympathetic fibers are cholinergic, they may differ in respect to neuropeptides and nitric oxide. Sensory neuron collaterals may also influence principal neurons as do intrinsic neurons such as small intensely fluorescent cells. Transmission through pelvic ganglia may be simple as is apparent in penile innervation, or shows a greater integrative capacity, as exemplified by the innervation of the urinary bladder. The extent of interaction of sympathetic and parasympathetic pathways at the level of the pelvic plexus remains largely unknown.