The internal sphincter muscle of urethra is a urethral sphincter muscle which constricts the internal urethral orifice. It is the junction of the urethra with the urinary bladder. The muscle is made of smooth muscle, so therefore it is under involuntary control. It is kept tonically contracted by Lumbar plexus of the sympathetic nervous system.
During micturition it is relaxed via branches from the pelvic plexus (S2-S4) parasympathetic nervous system. This is the primary muscle for maintenance of continence of urine. Its two major roles are:
- Prevent urine leakage as the muscle is contracted via sympathetic stimulation (via the Inferior hypogastric plexus).
- During ejaculation, the contraction of the muscle prevents reflux of semen in the urinary bladder.