Innervation of the muscle of the bladder in the rat.

A quantitative ultrastructural analysis of the organisation and terminal density of the nerves associated with the muscle of the body of the bladder was undertaken in the rat. The results obtained were suggestive of the organisation of the intramuscular plexuses into 2 separate and possibly functionally distinct networks. The nerves ramifying within the muscle fascicles mostly contained between 1 and 3 axons and on the basis of counts made in 12 animals, appeared to provide the muscle with only a moderate level of innervation. Although adrenergic terminals were frequently seen in the perivascular plexuses, they were rarely encountered in the nerves within the muscle fascicles. The majority of the terminals in the intrafascicular nerves were similar to those normally classified as cholinergic and, in many, the exposed parts of the axonal membrane were closely related to that of adjacent muscle cells. In addition to clear vesicles and large dense-cored vesicles, many of these terminals contained scattered small dense-cored vesicles. It was suggested that the involvement of the material present in the small dense-cored vesicles with acetylcholine in impulse transmission might account for the atypical features of such transmission in the bladder and in particular its resistance to blockade by atropine. The nerves forming the interfascicular networks contained few axons with cholinergic terminals, and were characterised by much larger numbers of a morphologically distinctive type of terminal. Some of these were indistinguishable from those recently defined as the terminals of purinergic axons but their distribution in the nerves suggests that they represent the terminals of afferent axons and is consistent with the concept that, together with those observed in other tissues, such axons are involved in pain perception.