Onuf's nucleus

Introduction

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

Through the pudendal nerve, Onuf's nucleus innervates 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 perineal muscle contractions.

In addition to its motor functions, it also performs autonomic functions by association with the parasympathetic neurons in the sacral segment of the spinal cord.

Significance

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 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, subsequently saving a lot of time during clinical examination.