noun, plural: urethral sphincters
A sphincter is a ring-like band of muscle fibres that constricts a passage or closes a natural orifice. One of these is the urethral sphincter. As the name implies, the urethral sphincter is a sphincter that is associated with the urethra. In humans, there are two urethral sphincters: (1) external urethral sphincter and (2) internal urethral sphincter.
The external urethral sphincter is a urethral sphincter muscle found at the distal inferior end of the urinary bladder in females and inferior to the prostate in males. It is made up of skeletal muscle and therefore is associated with the voluntary control. It controls the flow of urine through the urethra. In males, it can also control the passage of semen trough the urethra.
While the external urethral sphincter is made up of skeletal muscle, the internal urethral sphincter is made up of smooth muscle. Thus, the internal urethral sphincter is associated with involuntary movement and autonomic control. The internal urethral sphincter is a sphincter muscle that is found at the inferior end of the urinary bladder and the proximal end of the urethra. It is the primary muscle that prohibits the release of urine. In males, the internal urethral sphincter may exist to prevent reflux of semen into bladder. It is kept contracted tonically by the lumbar plexus and relaxes during micturition via the branches from the inferior hypogastric plexus of the parasympathetic nervous system.