Difference between revisions of "Micturition - Anatomy & Physiology"

Revision as of 14:03, 1 September 2008

Micturition is the normal process of the passive storage and active voiding of urine.

Introduction

After entering the renal pelvis the modification of the urine is over in all domestic species other than the horse where mucin is added. The urine passes along the ureters and enters the bladder. It is here that the urine is stored until it is to be voided. Urine is not constantly excreted and it is only when there is a significant amount present in the bladder that the process of voiding occurs. Both the Bladder and the Urethra have smooth muscle and thus receive autonomic influence with regard to their activity. However the urethra also has an element of skeletal muscle giving the animal some degree of conscious control over the voiding of urine.

Sensory Innervation

The bladder has stretch receptors (alpha receptors in the neck and trigone, and beta receptors in the body) which detect fullness of the bladder. These impulse to the pelvic nerves and so to the sacral spinal cord. The urethra has afferents detecting flow, distension and pain that go via the pudendal nerve to the sacral cord.

Central Intregration

The co-ordination of the urethralis and detrusor muscles from information recieved from the stretch receptors occurs in the pons. This micturition centre also branches to the thalamus, cerebellum and cerebral cortex. It is the cerebral cortex that is responsible for voluntary control of micturition.

The Muscles

The motor components of the reflex are the Detrusor Muscle, Internal Urethral Sphincter and the External Urethral Sphincter. They former two are supplied by the autonomic nervous system with the latter being of somatic innervation.

The Reflex

Storage phase - Sympathetic system dominant

This involves contraction of the urethral sphincters to prevent leakage of urine
Alpha-adenergic stimulation causes constriction of the bladder neck
Beta-adenergic stimulation causes relaxation of the detrusor muscle and so promotes low pressure filling of the bladder
Direct inhibition of the pelvic nerves is provided by the hypogastric nerve
The main barrier to urine outflow though is the constriction of the urethralis muscle being provided with constant tone by the pudendal nerve.

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.

Emptying phase - Parasympathetic system dominant

The efferent impulses from the pons travel to the sacral region and simultaneously:

Inhibition of the pudendal nerve, thus causing relaxation of the urethralis muscle.
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.

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.

@@ Line 26: / Line 26: @@
-==The Reflex==
+=='''The Reflex'''==
+===Storage phase - Sympathetic system dominant===
+* This involves contraction of the urethral sphincters to prevent leakage of urine
+* Alpha-adenergic stimulation causes constriction of the bladder neck
+* Beta-adenergic stimulation causes relaxation of the detrusor muscle and so promotes low pressure filling of the bladder
+* Direct inhibition of the pelvic nerves is provided by the hypogastric nerve
+* The main barrier to urine outflow though is the constriction of the '''urethralis''' muscle being provided with constant tone by the pudendal nerve.
-===The Process===
-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.
-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.
+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.
+===Emptying phase - Parasympathetic system dominant===
+The efferent impulses from the pons travel to the sacral region and simultaneously:
+* Inhibition of the pudendal nerve, thus causing relaxation of the urethralis muscle.
+* 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.
+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.