Onuf's nucleus is a small group of motor neurons located in the anterior horn of the S2 segment of the spinal cord. It is named after Bronislaw Onufrowicz, a neurologist, who first discovered the structure in 1899. It is the site of origin of the pudendal nerve.
Through the pudendal nerve, Onuf's nucleus innervates the external anal and urethral sphincters, as well as two perineal muscles: ischiocavernosus and bulbospongiosus. Hence, it maintains bowel and bladder continence through the voluntary control of both external sphincters and enables sexual functions like penile erection and ejaculation through perineal muscle contractions.
In addition to its motor functions, Onuf's nucleus also performs autonomic functions by association with parasympathetic neurons in the sacral segment of the spinal cord.
Onuf's nucleus, interestingly, is the only motor neuron preserved in Amyotrophic Lateral Sclerosis. Additionally, it is also spared in Spinal Muscular Atrophy, Duchenne Muscular Dystrophy, and Poliomyelitis. In stark contrast, it is one of the most affected structures in Multiple System Atrophy.
Therefore, the presence or absence of incontinence and/or impotence allows clinicians to rule in or out the conditions mentioned above, from their differential diagnosis, subsequently saving time during a clinical examination.
- Watson C, Paxinos G, Kayalioglu G. The Organization of the Spinal Cord. In: Watson C, Kayalioglu G, editors.The Spinal Cord: A Christopher and Dana Reeve Foundation Text and Atlas. London: Academic Press, 2008. p5.
- Mannen T. Neuropathology of Onuf's nucleus. Rinsho Shinkeigaku 1991; 31(Suppl 12):1281-5.
- Mannen T. Neuropathological findings of Onuf's nucleus and its significance. Neuropathology 2000; 20:30-3.
- Mannen T, Iwata M, Toyokura Y, Nagashima K. Preservation of a certain motoneurone group of the sacral cord in amyotrophic lateral sclerosis: its clinical significance. J Neurol Neurosurg Psychiatry 1977; 40:464–469.