How do the internal and external urinary sphincters control urination?

The internal and external urinary sphincters work in coordination to regulate the storage and release of urine from the bladder.

• Internal Urinary Sphincter: Located at the bladder neck, this sphincter is composed of smooth muscle and is under involuntary control. It is stimulated to contract by the sympathetic nervous system, which keeps the urethral opening closed to retain urine. During micturition, the inhibition of sympathetic outflow allows this sphincter to relax.12
• External Urinary Sphincter: This structure consists of striated (skeletal) muscle and is under voluntary control. It receives tonic excitation from the pudendal nerve, which helps maintain continence. To initiate urination, higher brain centers send signals to inhibit this motor outflow, causing the sphincter to relax.12

The process is a complex reflex involving the autonomic nervous system and the spinal cord. While the external sphincter can maintain continence even if the internal sphincter is compromised, effective emptying requires the relaxation of both sphincters alongside the contraction of the bladder's detrusor muscle.12