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Micturition - Anatomy & Physiology (view source)

Revision as of 14:03, 1 September 2008

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==The Reflex==
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=='''The Reflex'''==
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===Storage phase - Sympathetic system dominant===
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* This involves contraction of the urethral sphincters to prevent leakage of urine
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* Alpha-adenergic stimulation causes constriction of the bladder neck
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* Beta-adenergic stimulation causes relaxation of the detrusor muscle and so promotes low pressure filling of the bladder
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* Direct inhibition of the pelvic nerves is provided by the hypogastric nerve
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* The main barrier to urine outflow though is the constriction of the '''urethralis''' muscle being provided with constant tone by the pudendal nerve.
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===The Process===
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The bladder when empty is a small organ with folded epithelium.  It is able to stretch to an organ which is much larger than when in its empty state.  It has many stretch sensitive nerve endings within its wall which monitor how full it is.  As the bladder is filling sympathetic supply dominates and keeps the bladder wall relaxed and the internal urethral sphincter closed.  This stops urine from escaping the bladder.  As the bladder fills it stretches more and pressure is placed on itself and the sphincters keeping the urethra closed.  As the tone within the stretch sensitive fibres increases due to filling then we see a switch to the parasympathetic system.  Above a certain level this triggers a contraction within the detrusor muscle which in turn further increases the pressure and thus further increases the tone in the sensory fibres.  This is an example of a positive feedback loop.  The parasympathetic tone also causes the internal sphincter to relax leaving only the external urethral sphincter to stop to urine.  In untrained animals the pressure exerted on the external sphincter overcomes it by activating reflexes which over come its contraction.  However in trained animals there is some level of control from the cerebral cortex and pons which reduces the autonomic reflex and maintains the contraction of the external sphincter which allows the passing of urine to be consciously delayed.  This is not indefinite however and it will be overcome eventually.
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Urination normally occurs until the bladder is empty thanks to positive feedback mechanisms in the urethra which increase parasympathetic tone in response to urine being present.
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Once bladder capacity is reached the stretch receptors are stimulated which send impulses via the pelvic nerve to the sacral spinal cord and finally to the pons. Here integration occurs and there is concious acknowledgement of the need to urinate.
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===Emptying phase - Parasympathetic system dominant===
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The efferent impulses from the pons travel to the sacral region and simultaneously:
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* Inhibition of the pudendal nerve, thus causing relaxation of the urethralis muscle.
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* Stimulation of the parasymphathetic neurons, inhibiting adrenergic tone to the neck thus leading to relaxation of the internal urethral sphincter. This increased input also leads to contraction of the detrusor muscle.
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This co-ordinated detrusor contraction and sphincter relaxation leads to complete voiding of the bladder and at the same time prevents pressure build up with in the bladder.
Jtulloch
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