Onuf's nucleus
Description[edit | edit source]
Onuf's nucleus is a small group of motor neurons located in the anterior horn of the S2 segment of the spinal cord, specifically in Lamina IX. It is named after Bronislaw Onuf-Onufrowicz, a neurologist, who first discovered the structure in 1899. It is the site of origin of the pudendal nerve.
Functions[edit | edit source]
Through the pudendal nerve, Onuf's nucleus innervates both the external anal and urethral sphincters, as well as two perineal muscles; ischiocavernosus and bulbospongiosus. Therefore, the Onuf's nucleus maintains bowel and bladder continence through the voluntary control of both external sphincters and enables sexual functions like penile erection and ejaculation through the contraction of perineal muscles.
In addition to its motor functions, it also performs an autonomic function by association with the parasympathetic neurons in the sacral segment of the spinal cord.
Significance[edit | edit source]
Interestingly, Onuf's nucleus is the only motor neuron preserved in Amyotrophic Lateral Sclerosis. Additionally, it is also spared in Type I Spinal Muscle Atrophy. In stark contrast, it is the one of the most affected structures in Multiple System Atrophy. Therefore, the presence or absence of incontinence and/or impotence can allow the clinician to either rule in or out the possibility of the above conditions from their differential diagnosis and save a lot of time during clinical examination.
