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.