The external sphincter is supplied by the pudendel nerve S 2,3 and 4. The pudenal nerve leaves the pelvis via the greater sciatic framen. It runs under the lower border of pyriformis, corsses the ischial spine and sacro-spinous ligament to enter Alcock's canal in the lateral aspect of the ischio-rectal fossa. The first major branch of the pudendal nerve is the inferior rectal nerve, which gives motor supply to the external sphincter and sensation to the perianal skin. After giveing off the inferior rectal nerve the pudenal nerve continues as the perineal nerve. The perineal nerve gives motor supply to the anterior pernineal muscles and the sphincter urtehtra and continues then as the dorsal nerve of the penis/clitoris.
The puborectalis muscle gets some motor supply from the pudendal nerve but most of the motor supply comes from a direct branc of S4.