Mechanism of Micturition

MICTURITION REFLEX

Physiologically, micturition involves coordination between the central, autonomic, and somatic nervous systems.   Central nervous system: Prefrontal cortex, precentral gyrus, postcentral gyrus, hypothalamus, amydaloid nucleus, periaqueductal gray, pontine micturition center Autonomic nervous system:  Sympathetic: Lateral horn cells of T10-L2 segments of spinal cord (Preganglionic motor fibres)→Synapse with superior hypogastric plexus→ Hypogastric nerve (Postganglionic motor fibres) → UB  Parasympathetic: Lateral horn cells of S2-S4 segments of spinal cord (Preganglionic motor fibres) → Form pelvic splanchnic nerve→ Synapse with the cell bodies of postganglionic motor fibres situated close to or within the bladder wall→ UB Somatic nervous systems: Anterior horn cells (Onuf’s nucleus) of S2-S4 segments of spinal cord→ Pudendal nerve

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Storage phase:
Low-frequency (as stretch is low) afferent impulse from the stress receptor of UB

Pelvic nerves (splanchnic, hypogastric)

Spinal cord

Inhibition of sacral parasympathetic Stimulation lumbar sympathetic preganglionic neurons preganglionic neurons

Stimulation of Onuf's nucleus

Relaxation of detrusor & contraction relaxation of detrusor & contraction contraction of the sphincter of sphincter vesicae of sphincter vesicae urethrae

Voiding phase: Distention of urinary bladder

Sensory stretch receptors in the bladder wall become stimulated

Afferent impulse from these receptors goes to the spinal cord mainly S2-S4 segments

Afferent impulse ascend through ipsilateral tract of Gall (F. gracilis)

Internal arcuate fibers

Medial lamniscus

Thalamus

Internal capsule & corona radiate

Post-central gyrus of cerebral cortex

Prefrontal cortex Hypothalamus

Insular cortex Basal ganglia

Thalamus Cerebellum

Voluntary signal is sent to begin voiding if socially acceptable

Periaqueductal gray matter ( In the midbrain)

Pontine micturition center (+)