Ultrastructure of vesicourethral innervation. III. Axoaxonal synapses between postganglionic cholinergic axons and probably SIF-cell derived processes in the feline lissosphincter.

Ultrastructurally nonspecialized axoaxonal-type synapses between postganglionic cholinergic axons innervating the lissosphincter and apposed probably SIF-cell derived neuronal processes are described. These processes were considered as the peripheral extraganglionic counterparts of those belonging to type I SIF cell interneurons, described in many mammalian peripheral autonomic ganglia. On this basis, the herein reported cholinergic/probably SIF-cell derived axoaxonal-type synapses are proposed as an auxiliary mechanism of prejunctional catecholaminergic inhibition of the normally excitatory cholinergic (postganglionic parasympathetic) neuroeffector transmission in the feline lissosphincter.