Onuf's nucleus is a distinct group of neurons located in the ventral part (lamina IX) of the anterior horn of the sacral region of the human spinal cord involved in the maintenance of micturition and defecatory continence, as well as muscular contraction during orgasm.

[2] Onuf's nucleus is a distinct group of neurons located in the ventral part (lamina IX) of the anterior horn of the sacral region of the human spinal cord involved in the maintenance of micturition and defecatory continence, as well as muscular contraction during orgasm.

This innervation, or nerve supply, is arranged in a neuropil and averages approximately 625 neurons in both the left and right ventral horns,[3] and measures about 4–6 mm on each side.

Use of the Klüver-Barrera staining method showed that Onuf's nucleus appears clear due to the presence of many vertically arranged unmyelinated fibers.

The neurons of Onuf's nucleus are responsible for controlling external sphincter muscles of the anus and urethra in humans.

Onufrowicz also proposed that Onuf's nucleus controlled the ischiocavernosus and bulbocavernosus muscles which function in penile erection and ejaculation in males.

When the 5-HT and NE receptors are stimulated, the guarding reflex occurs to prevent voiding of the bladder caused by unexpected abdominal pressure.

It has also been shown that the sex differences in Onuf's nucleus can be reduced (or in some cases eliminated) by exposing a prenatal female to high levels of androgen.

[8][9][10] Stress urinary incontinence (SUI) is a common disease in women caused by pelvic floor muscle weakness.

Finally the somatic nerve supply arises from the urethral sphincter motor neurons in the ventral horn of the sacral spinal cord; better known as Onuf's nucleus.

[7] Onuf's nucleus controls rhabdosphincter motor neurons and has been shown to contain a dense array of 5-HT (serotonin) and NE terminals.

In amyotrophic lateral sclerosis, Onuf's nucleus is preserved but the other anterior horn cell groups atrophy.

This discovery reinforced the notion that Onuf's nucleus controlled the muscles related to sphincter function in the anus and urethra.

When the sacral sections of the spinal cord were studied in patients with Shy–Drager syndrome, it was revealed that cell death was confined to the area of Onuf's nucleus.

As in humans, prenatal androgen plays an important role in establishing the sex differences in Onuf's nucleus of these species.

If a female is exposed to excess androgen during the prenatal period, the sexual dimorphism does not occur in Onuf's nucleus.