When the bladder fills with urine stretch receptors, the resceptors get stimulated.
They send nerve impulses to the spinal cord in the sacral area , which then sends a reflex nerve impulse back to the internal sphincter valve at the neck of the bladder that causes it to relax and allow the flow of urine into the urethra.
The internal urethral sphincter is involuntary and controlled by the autonomic nerves.
The external sphincter is made up of striated muscles so it is controled by somatic nerves. it only opens when impulse from somatic nerve trvels to external sphincter causing it to relax.
RegardsArun (askIITians forum expert)
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