last updated 21-12-2024

Micturition reflex

Micturition reflex

The functional control unit of urine storage and emptying consists of the bladder, smooth muscle tissue, sphincters and a complex neuronal control network. The detrusor muscle is able to contract the bladder in case of emptying. This detrusor is innervated by parasympathetic, cholinergic neurons (b) with acetylcholine as main neurotransmitter, which stimulates muscarinic acetylcholine receptors on the smooth muscle tissue. The outflow region receives sympathetic, adrenergic innervation (c, d). The distal urethra and the pelvic floor are innervated by the nervus pdendus (e); a somatic innervation.

 

The micturition reflex starts when the detrusor is distended. The moment of distension depends on the urine volume, age, sex and eventual bladder disorders. This signal is transported through the sacral micturition centre to the pontine micturition centre via the afferent branch (a). The micturition centre in the pons is under voluntary control and can react by sending action potentials down to the sacral micturition centre in the spinal cord. The outflow region relaxes and the detrusor contracts because the efferent branches (c, d and e) are inhibited. At the same moment, the pelvic nerve (b) is activated and causes the bladder to contract further. After micturition, the sympathetic nerves are stimulated and thus the sphincters contract again and the detrusor relaxes.

 

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