Perineal nerve

The perineal nerve, also known as the perineal branch of the pudendal nerve, is the largest terminal branch of the pudendal nerve which is derived from S2, S3 and S4 nerve roots of the sacral plexus. The perineal nerve gives muscular branches to the superficial and deep perineal muscles as well as the external urethral sphincter.

Gross anatomy

Origin

The perineal nerve is the largest and most inferior of the three branches of the pudendal nerve (the other branches being the inferior rectal nerve and the dorsal nerve of the penis or clitoris). The nerve originates in the pudendal canal, which is formed by the fascia of the obturator internus muscle.  The pudendal canal also known as Alcock canal (after eminent Irish anatomist Benjamin Alcock) also houses the internal pudendal artery and veins.

After passing into the perineum, the pudendal nerve bifurcates into two terminal branches: the perineal nerve which travels superficial to the urogenital diaphragm, and the dorsal nerve which passes deep to it.

Course

After bifurcating from the dorsal nerve, the perineal nerve passes into the urogenital triangle superficial to the urogenital diaphragm. The nerve then travels anteriorly over the superficial transverse perineal muscles and ramifies into terminal cutaneous and motor branches.

Branches

As the perineal nerve travels anteriorly on the superficial surface of the urogenital diaphragm, it branches into superficial and deep branches.

• the superficial branch of the perineal nerve pierces the fascia of the urogenital diaphragm and accompanies the posterior scrotal or posterior labial branches of the internal pudendal artery; this this nerve runs forward and superficially to supply skin over the scrotum or labia majora.

• the deep or muscular branches of the perineal nerve are distributed to the muscles of the perineum including the ischiocavernosus, bulbospongiosus, deep transverse perinei, superficial transverse perinei, levator ani and sphincter urethrae; the branch to bulbospongiosus also supplies sensory fibres to the mucous membrane of the urethra.

Relations

The perineal branch of the pudendal nerve is closely associated with the internal pudendal artery and its branches. Within the pudendal canal the terminal branches of the pudendal nerve run with the internal pudendal artery. The dorsal nerve runs superior to the internal pudendal artery and the perineal nerve travels inferior to it. After exiting the pudendal canal, the posterior scrotal or labial branch of the internal pudendal artery is closely related to the superficial branch of the perineal nerve, and the muscular branches of the perineal nerve are related to the superficial and transverse perineal arteries.

Related pathology

Regional anaesthetic blocks targeting the pudendal and perineal nerves have been used in vaginal deliveries and for minor surgeries of the vagina and perineum, as well as in chronic perineal pain ^ref.