Ultrastructure of vesicourethral innervation. I. Neuroeffector and cell junctions in the male internal sphincter

The ultrastructure of neuroeffector and cell junctions in smooth muscle of the internal sphincter was studied in the male cat and rat. Muscle cells of the sphincter have the same features of smooth muscle elsewhere, with frequent adherens-type junctions. Neuroeffector junctions are established with sphincter muscle cells by cholinergic and/or adrenergic axons, which probably are not distributed on a 1:1 nerve:muscle basis. The neuroeffector junctions are classified according to their cleft width, their overall morphology, the functional class of their axonal component, and the number of axonal and muscle cell elements involved. These observations unequivocally confirm the principle of dual cholinergic-adrenergic control of the male internal sphincter, and indicate that its mechanism of neuromuscular transmission is both direct via neuroeffector and indirect via muscle cell junctions.