The structure and innervation of smooth muscle in the wall of the bladder neck and proximal urethra.

Neurohistochemical and electron microscopic techniques have been employed to compare the structure and autonomic innervation of the bladder detrusor with that of the bladder neck and urethra in male and female cats, rats and guinea-pigs. In all specimens the structure and arrangement of smooth muscle in the detrusor was different from that in the bladder neck and proximal urethra. In male specimens this circular smooth muscle extended into the proximal urethra as far as the entrance of the ejaculatory ducts. A similar muscle coat was less well developed in the female but could be identified along the length of the urethra. Differences in the distribution of autonomic nerves have also been demonstrated. Presumptive cholinergic nerves were frequently encountered amongst the smooth muscle cells of the detrusor whilst noradrenergic nerves were relatively sparse. In contrast, muscle in the bladder neck and urethra was richly innervated by both types of autonomic nerve. In the male these findings are considered to provide support for the concept of a sphincteric mechanism which prevents retrograde ejaculation. In both sexes the possible functional role of the autonomic innervation of the bladder neck and proximal urethra during micturition has also been discussed.