Innervation of the external urethral sphincter, which shares the same sacral roots as the anal sphincter, can be tested by assessing: Perineal sensation Volitional anal sphincter contraction (S2 to S4) The anal wink reflex (S4 to S5), which is anal sphincter contraction triggered by lightly stroking perianal skin The bulbocavernosus reflex (S2 to S4), which is anal sphincter contraction triggered by pressure on the glans penis or clitoris However, the absence of these reflexes is not necessarily pathologic.