Ultrastructure of vesicourethral innervation. II. Postganglionic axoaxonal synapses in intrinsic innervation of the vesicourethral lissosphincter: a new structural and functional concept in micturition.

The interrelationships of postganglionic axons in intrinsic neuroplexuses of the vesicourethral lissosphincter ('internal sphincter') were studied ultrastructurally in the cat and rat of both sexes. Direct axoaxonal contracts between cholinergic and adrenergic axons were common in preterminal axon bundles and at neuroeffector junctions. Similar contacts between 2 cholinergic or 2 adrenergic axons were less frequent. Most contacts were ultrastructurally simple, but some had focal areas of very close axolemmal apposition, and some were focally specialized as bona fide chemical synaptic complexes. The axoaxonal contacts are introduced as a complex interaxonal synaptic system that chemically, and possibly also electrically, modulates neurotransmission in postganglionic axons innervating the lissosphincter, presynaptically (prejunctionally) at the infraganglionic level, especially by reciprocal cholinergic/adrenergic axonal interaction. This synaptic system is introduced as a potential, hitherto unrecognized, site of action of autonomic drugs, and as a key factor in the neural mechanism controlling continent closure of the lissosphincter during bladder filling, its opening to initiate and maintain voiding, and its closure to terminate the micturition cycle.