The continence mechanisms: the effect of bladder filling on the urethra.

We developed a surgical model in the cat, in which the bladder was completely separated from the urethra, in order to examine the changes in urethral resistance during bladder filling. An increase in urethral resistance occurred in two phases: an "initial" increase attributable to the activity of both the striated sphincter and the sympathetically innervated smooth muscle, and a "late" increase that is alpha-sympathetically mediated. The latter was secondary to trigonal distension. We concluded that the increased urethral activity during bladder filling is reflexly mediated and that the trigone plays a role in its control.