A sphincter is a muscle that surrounds a body opening (the urethra or the anus, for example) and which should unconsciously remained tightly closed. During urination, sphincters relax and open when messages are relayed from your nerves and your brain to the pelvic floor muscles. Internal and external sphincter muscles control the storage and voiding of urine. The internal sphincter instinctively squeezes the urethra closed.
The external sphincter is more under your control for short periods of time and can be squeezed to prevent the leakage of urine when you sneeze, cough, laugh, or do anything that puts additional pressure on your bladder. The internal and external sphincter muscles work in tandem, expanding and contracting as the bladder fills and empties. Nerves automatically relay messages to your muscles ensuring that the sphincters remain closed and that you don’t leak.
When enough urine has accumulated in the bladder and it has filled to the point that its internal pressure has increased and its bladder wall has stretched, your nervous system sends a message to your brain, which in turn sends a message to your bladder’s detrusor muscle, telling it to relax the internal sphincter muscle. When that happens, the external sphincter muscle becomes tight, and you have a distinct urge to urinate. When you are ready to urinate, you relax your external sphincter, and urine flows.
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Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.