Bladder Filling
During filling the detrusor muscle is stretched and the sensation of bladder fullness is transmitted to the sacral micturition centre (S2-S4). A simple reflex response is transmitted to the bladder causing the detrusor muscle to relax, so that pressure does not build up within the bladder during filling. This reflex response is also transmitted to the external urethral sphincter, promoting contraction of this sphincter muscle so that continence is maintained at the bladder neck during filling. As this phase is controlled by sacral spinal reflexes, an individual is unaware of bladder filling at this stage.
However, during filling, messages are also sent up to the brain via the spino-thalamic tract and pontine micturition centre to the thalamus. The thalamus acts as a relay station for incoming sensory information and it is not until the bladder is approximately 2/3 full that these messages are transmitted onwards from the thalamus to the frontal lobes. It is not until this message reaches the frontal micturition centre that perception of the sensation of bladder fullness takes place.
The bladder is constantly either filling and storing urine, or emptying (micturition). The pontine micturition centre switches between these two states, under the influence of the frontal micturition centre (Fowler, 1999).
