Deep branch of perineal nerve

The deep branch of perineal nerve (a.k.a. muscular branch of perineal nerve) innervates various muscles in the perineal region, including the levator ani muscle and muscles in the superficial perineal pouch (i.e. bulbospongiosus and bulb of penis, ischiocavernosus and superficial transverse perineal muscles). Through contraction of these muscles, the deep branch helps in bringing about erection (in both sexes) and ejaculation in males.

The deep branch of perineal nerve also innervates muscles inside the deep perineal pouch (i.e. deep transverse perineal muscle and external urethral sphincter, compressor urethrae and sphincter urethrovaginalis). Contraction of these muscles especially external urethral sphincter, aids in voluntary control of micturition. Hence, dysfunction due to damage to deep branch of perineal nerve could lead to urinary incontinence and sexual dysfunction.

The deep branch is one of the terminal offshoots of the perineal nerve. The perineal nerve, as you may know, is a branch of the pudendal nerve. The latter winds around the ischial spine and sacrospinous ligament to enter the perineal region, where it divides into three branches, including the perineal nerve, dorsal nerve of penis/clitoris, and inferior rectal nerve. The perineal nerve further ends up dividing into superficial and deep branches.

Please note that, unlike the superficial branch, the deep branch of perineal nerve does not provide cutaneous innervation to the perineal areas.