The inferior rectal (inferior anal) nerve is the first branch of the pudendal nerve in the pudendal canal. It crosses the fat of the ischioanal fossa toward the anus and supplies the external anal sphincter. It also carries sensation from the skin around the anus and the lining of the anal canal below the pectinate line.
| Origin | Pudendal nerve (S2-S4) at the posterior end of the pudendal canal |
|---|---|
| Course | Leaves the pudendal canal on the lateral wall of the ischioanal fossa and crosses the fossa medially to reach the external anal sphincter and perianal skin |
| Motor | |
| Sensory |
|
Because the inferior rectal nerve supplies the lower anal canal with somatic pain fibres, conditions below the pectinate line, such as fissures, thrombosed external haemorrhoids and perianal abscesses, are very painful. Stretching of the nerve in prolonged or instrumental childbirth contributes to weakness of the external sphincter and faecal incontinence. It is anaesthetised in perianal blocks and must be considered when draining ischioanal abscesses.
Stretching of the pudendal nerve and its inferior rectal branch during a prolonged second stage of labour or forceps delivery weakens the external anal sphincter, contributing to faecal incontinence later in life.
A tear in the anal lining below the pectinate line stimulates the somatic pain fibres of the inferior rectal nerve, producing sharp pain on defecation and spasm of the internal sphincter.
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