Different types of synaptic vesicles in axons of the retractor penis muscle of the bull.

Three types of axon profiles were observed in the smooth muscle of the retractor penis and the penile artery of the bull: 1. profiles containing small granular vesicles, presumably representing adrenergic axons; 2. profiles containing small agranular vesicles, presumably representing cholinergic axons; 3. profiles containing numerous large and small granular vesicles. The third type of profile was not found in the vas deferens or the metatarsal artery. It is therefore possible that this type of profile represents the non-adrenergic, non-cholinergic inhibitory nerves, the presence of which has previously been pharmacologically indicated in these tissues.