The integrative physiology of the bladder

Abstract

Normal bladder function is complex, resulting from the co-operative interaction of numerous regulatory cell types, of which the interstitial cells and the peripheral neurones are particularly interesting. Collectively, these comprise the myovesical plexus, which appears to confer structural and functional characteristics on the bladder loosely akin to those of the gut. These include functional modularity, which gives rise to the potential for localised and propagating peristalsis-like movements in the bladder wall according to the prevailing physiological conditions. Localised modular activity during filling may contribute to normal generation of sensation and exaggerated modular activity may give rise to urinary urgency. Enhanced co-ordination of modular activity occurs in various models of detrusor overactivity; it leads to surges of contraction over a large part of the bladder wall, generating phasic changes in intravesical pressure. During voiding, the myovesical plexus sustains detrusor contraction at the behest of the brainstem, monitoring state of bladder fullness as it does so, as a guide to the required duration for which it has to keep up the effort. Accordingly, the bladder wall itself may house structures which render the bladder the effector level in a hierarchy of lower urinary tract regulation. Dysfunction in these vital regulatory structures is an underestimated factor in the pathophysiology of clinical bladder problems.

Figure 1

Apparatus for simultaneous measurement of pressure and movement in an isolated rodent bladder. The bladder is held under physiological conditions of oxygenation, pH and temperature by maintaining appropriate conditions in the organ bath.

Figure 2

Localised movement in an isolated mouse bladder. Measurements of separation between two pairs of points; in one pair (above) the points periodically got closer together, in the other, separation increased at the same time as the shortenings in the neighbouring pair.

Figure 3

Contrasting response to increased bladder filling in bladder isolated from a normal rat (above) and a rat bladder rendered overactive by a period of partial bladder outlet obstruction (below). Measurements of propagating movements in the bladder wall, made longitudinal (long) or transverse (trans) to the main axis of the bladder. In the normal animal, distension caused smaller contractions of higher frequency. In the obstructed animal, movements became larger and occurred at lower frequency.

Figure 4

Above: the servo-assistance model. The relationship between the pontine micturition centre (PMC) and the components of the myovesical plexus (see text). Below: evidence that bladder volume affects the response to cholinergic stimulation. Pressure recording during applications of a cholinergic agonist at low volume (left) and high volume (right). Both applications caused a shift in pressure, but only at high volume was there significant phasic activity