Continence

As long as the pressure in the bladder (intravesical pressure) is lower than the urethral pressure, urine will be stored, and continence maintained. During micturition, the situation must be reversed. Synchronous contraction of the bladder smooth muscle must occur, with a drop in the urethral pressure, and the whole process is under voluntary control, making micturition an interesting example of the voluntary control of smooth muscles. The detrusor is a very compliant structure, and during filling with urine, the bladder can accommodate large volumes without a significant rise in intraluminal pressure. This is largely due to the intrinsic properties of the bladder wall, although there is some evidence that the activity in the parasympathetic ganglia is kept low during filling by activation of a reflex inhibitory pathway, possibly involving their sympathetic innervation. During the filling phase of the micturition cycle high pressure can be recorded in the urethra. There is evidence that the urethral smooth muscles can develop myogenic tone, and it is probable in the human that the autonomic nerves are also involved in maintaining the urethral pressure, although this has not been conclusively established. At present, many urologists believe that the high intraurethral pressure is due largely to activity of the striated muscle of the rhabdosphincter, although others believe that these muscles are mainly important in a reflex response designed to prevent leakage of urine when there is an increase in intra-abdominal pressure (and

Fig. 10.14 Pressure recording in the female bladder and urethra. Upper trace, pressure recorded in the urethra, middle trace, pressure recorded in the bladder, bottom trace subtracted pressure, showing the element generated by the urethra and its surrounding structures. A, B and C are urethral pressure profiles, recorded by pulling the recording catheter through the length of the urethra from the bladder (Lt) to the external meatus (right). In C the subject was asked to cough during the recording. Coughing increases abdominal pressure, which is reflected equally in the bladder and proximal urethra, as can be seen on the left of the subtracted trace where the pressure rises are cancelled out. In the centre of the urethra, the subtracted trace shows a positive response to coughing, but as the catheter is pulled further, the negative going deflections indicate that the bladder pressure is not passively transferred to the distal urethra. In D, the catheter is kept in the maximum pressure region of the urethra, and again the patient asked to give a single, then a series of, coughs. In this region the subtracted trace shows the active response of the urethra] wall. This reflex helps prevent leakage when the abdominal pressure rises.

chapter 10

Renal, Urinary and Reproductive Tract Systems

Fig. 10.14 Pressure recording in the female bladder and urethra. Upper trace, pressure recorded in the urethra, middle trace, pressure recorded in the bladder, bottom trace subtracted pressure, showing the element generated by the urethra and its surrounding structures. A, B and C are urethral pressure profiles, recorded by pulling the recording catheter through the length of the urethra from the bladder (Lt) to the external meatus (right). In C the subject was asked to cough during the recording. Coughing increases abdominal pressure, which is reflected equally in the bladder and proximal urethra, as can be seen on the left of the subtracted trace where the pressure rises are cancelled out. In the centre of the urethra, the subtracted trace shows a positive response to coughing, but as the catheter is pulled further, the negative going deflections indicate that the bladder pressure is not passively transferred to the distal urethra. In D, the catheter is kept in the maximum pressure region of the urethra, and again the patient asked to give a single, then a series of, coughs. In this region the subtracted trace shows the active response of the urethra] wall. This reflex helps prevent leakage when the abdominal pressure rises.

hence the pressure in the bladder), as occurs with such events as coughing, sneezing or straining (Fig. 10.14). This striated muscle is supplied by motor nerves with their cell bodies in S2-S4, the fibres running mostly in the pudendal nerve and pelvic nerves. The striated muscle fibres are heterogeneous (particularly in the male), being a mixture of the slow-twitch and fast-twitch types (Fig. 10.15), the slow-twitch fibres being capable of sustained contraction over relatively long periods of time. The striated muscle of the pelvic floor also contains fast-twitch and slow-twitch fibres, and may be important in maintaining continence, particularly for holding the urethra in the correct position. It might also supply an additional occlusive force on the urethra.

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