Neural Control of Micturition

a | Urine storage reflexes. During the storage of urine, distention of the bladder produces low-level vesical afferent firing. This in turn stimulates the sympathetic outflow in the hypogastric nerve to the bladder outlet (the bladder base and the urethra) and the pudendal outflow to the external urethral sphincter. These responses occur by spinal reflex pathways and represent guarding reflexes, which promote continence. Sympathetic firing also inhibits contraction of the detrusor muscle and modulates neurotransmission in bladder ganglia. A region in the rostral pons (the pontine storage centre) might increase striated urethral sphincter activity. b | Voiding reflexes. During the elimination of urine, intense bladder-afferent firing in the pelvic nerve activates spinobulbospinal reflex pathways (shown in blue) that pass through the pontine micturition centre. This stimulates the parasympathetic outflow to the bladder and to the urethral smooth muscle (shown in green) and inhibits the sympathetic and pudendal outflow to the urethral outlet (shown in red). Ascending afferent input from the spinal cord might pass through relay neurons in the periaqueductal grey (PAG) before reaching the pontine micturition centre. Note that these diagrams do not address the generation of conscious bladder sensations, nor the mechanisms that underlie the switch from storage to voiding, both of which presumably involve cerebral circuits above the PAG. R represents receptors on afferent nerve terminals.

• The external urethral sphincter is under voluntary (somatic) control, and muscle fiber contraction causes closure of the sphincter.
• Activation of S2-S4 α-motor neurons causing contraction of the external urethral sphincter striated muscle fibers

• A full bladder sends afferent signals causing inhibition of sympathetic tone and an increase in parasympathetic activity to the bladder through the pelvic splanchnic nerves.
• The parasympathetic activity relaxes the internal urethral sphincter and induces bladder contraction.