Onuf's Nucleus

Onuf's Nucleus

Onuf’s nucleus is a distinct group of neurons located in the ventral part (laminae 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. It contains motor neurons, and is the origin of the pudendal nerve. The sacral region of the spinal cord is fourth segment (cervical, thoracic, and lumbar being the first three) of vertebrae in the spinal cord which consists of the vertebrae 26-30. While working in New York City in 1899, Bronislaw Onuf-Onufrowicz discovered this group of unique cells and originally identified it as “Group X.” “Group X” was considered distinct by Onufrowicz because the cells were different in size from the surrounding neurons in the anterolateral group, suggesting that they were independent.

This small group of neural cells is located between S1 and S2 or S2 and S3 and although Onuf’s nucleus is located primarily in S2, it can extend to the caudal end of the first sacral segment or to the middle part of the third sacral segment. Also, Onuf’s nucleus is found almost symmetrically on both sides of the ventral horn. This innervation, or nerve supply, is arranged in a neuropil and averages approximately 300-500 in both the left and right ventral horns in animals. Humans average 625 neurons total across both sides of the spine which measures about 4–6 mm on each side.

Many staining techniques have been used to study the anatomy of Onuf’s nucleus. The Nissl method was commonly used as well as myelin sheath stains and silver stains. Use of the K-B staining method showed that Onuf’s nucleus appears clear due to the presence of many vertically arranged unmyelinated fibers. The sizes of the cells in Onuf’s nucleus are small in comparison to other lateral group cells. The neurons in Onuf’s nucleus are motoneurons, and like most motoneurons they are characterized by their mulipolarity and large Nissl bodies.