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Micturition - Physiology
Overview:
Parasympathetic nervous system ACTIVATES urination.
Sympathetic nervous system INHIBITS it.
Somatomotor system (volitional control) INHIBITS it.
The major functional structures of the lower urinary system:
The urinary bladder.
Urethra.
The detrusor muscle (the bladder wall muscle).
The internal urethral sphincter.
The external urethral sphincter.
During the filling phase,
Detrusor relaxes (stretches).
Internal sphincter contracts (closes).
External sphincter contracts (closes).
During the micturition (voiding) phase,
Detrusor contracts.
Internal sphincter relaxes (opens).
External sphincter relaxes (opens).
The sympathetic preganglionic origins are in the intermediolateral cell column from T10 – L2).
The sympathetic fiber system acts on the detrusor muscle and internal urethral sphincter to promote bladder filling: this is an unconscious action.
Onuf 's nucleus (aka nucleus of Onufrowicz) comprises the S2 – S4 motor neurons in the anterior horn of the spinal cord; these motor neurons provide volitional innervation to the pelvis.
The somatomotor system acts on the detrusor muscle and internal urethral sphincter to promote bladder filling: this is under volitional control – so you can "hold" your urine.
Stretch receptors, which are mechanoreceptors in the bladder walls excite the micturition response when there is sufficient bladder wall distention, typically at 400ml of urine.
The parasympathetic preganglionic originates in the intermediolateral cell column of S2 – S4.
It acts on the detrusor muscle and internal urethral sphincter to promote emptying: this is an unconscious action.
Sympathetic fibers inhibit bladder wall contraction and excite internal urethral sphincter constriction, which inhibits urination.
Parasympathetic fibers excite bladder wall contraction and inhibit internal urethral sphincter constriction, which activates urination.
Somatomotor efferents provide tonic activation of the external urethral sphincter, which inhibits urination.
The pontine micturition center lies in the medial (M) region of the dorsolateral pontine tegmentum and the pontine continence center lies ventro-lateral to it in the lateral (L) region. Barrington first described the pontine micturition center, so it is often referred to as the Barrington nucleus.
These regions receive innervation from the brain, including the periaqueductal gray area, frontal lobes, hypothalamus, limbic system, and others; their action on micturition is understood from their general roles in the nervous system.
Two pathologic conditions:
Urinary retenion (overdistension of the bladder) occurs from mechanical causes (enlarged prostate) or failure of bladder wall contraction (spinal shock).
Stress incontinence involves sudden, unnanticipated micturition, which can occur from a weak external urinary sphincter or an overexcitable bladder (such as from chronic spinal cord injury).