The muscles of the bladder, urethra and pelvic floor are under the control of the nervous system. A simplified picture would be that as the bladder fills, sensory receptors in the bladder wall trigger the micturition reflex - a simultaneous contraction of the detrusor and relaxation of the urethral and periurethral muscles. The neurological signals for these actions are co-ordinated by a micturition centre. Two locations are currently postulated for the site of the micturition reflex:
The traditional view had been that the site was located entirely in the sacral part of the cord,
A current hypothesis proposed by De Groat, Blaivas and others suggests that the reflex is not located solely in the sacral spine but rather in the pontine part of the brain stem much higher up. To support this hypothesis he quotes results from patients with spinal lesions. If the micturition reflex is purely sacral then suprasacral lesions of the spine should result in physiological voiding. However in Blavais' series of 155 patient with definite suprasacral lesions, only 41 % showed normal voiding with detrusor - external sphincter dyssynegia in 34% and detrusor areflexia in 35%.
the pontine centre
De Groat et al, Blavais
|During bladder filling distension
results in afferent pelvic discharge;
after synapse in pudendal nucleus efferent pudendal impulses produce contraction of external sphincter
helping to maintain continence. Other efferent sympathetic fibres synapse in post ganglionic parasympathetic neuron, resulting in inhibition of detrusor contraction.
During voiding the pelvic nerve is the motor nerve of detrusor contraction.
|Afferent pelvic nerve discharges
ascend in spinal cord, synapse in pontine micturition
centre Descending efferent pathways cause
Inhibition of pudendal firing - relaxation of sphincter
Inhibition of sypathetic firing - opens bladder neck
pelvic parasypathetic firing - detrusor contraction
If the micturition reflex had total control over voiding, micturition would occur automatically at certain bladder volumes and this is the case with babies and depending on the level of the lesion, some spinally injured patients. However as the diagram shows, the brain has an interaction with the pontine micturition centre and voiding can be inhibited. When voiding is desired the voluntary control of the reflex is removed and voiding should take place in a co-ordinated fashion. Thus an intact spinal cord is required for voluntary control of micturition.
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