Urethral sphincter

The urethral sphincter is a complex anatomical structure essential for maintaining urinary continence and facilitating controlled micturition. It is composed of two distinct parts: the internal urethral sphincter, which consists of smooth muscle under involuntary autonomic control, and the external urethral sphincter, which is made of striated muscle under voluntary somatic control (StatPearls, 2023). The internal sphincter is densely populated with alpha-1A adrenergic receptors, which maintain contraction to prevent leakage, while the external sphincter is innervated by the pudendal nerve and utilizes nicotinic acetylcholine receptors at the neuromuscular junction (NIH, 2022). Clinical conditions such as stress urinary incontinence, benign prostatic hyperplasia (BPH), and detrusor-sphincter dyssynergia arise from the dysfunction of these muscular components or their neural pathways. Pharmacological treatments target the specific receptors within the sphincter; for instance, alpha-blockers like tamsulosin are used to relax the internal sphincter in men with prostatic obstruction, whereas duloxetine may be used to increase external sphincter tone by enhancing neurotransmitter levels in Onuf's nucleus (PubMed, 2021). Because the urethral sphincter is a multi-tissue organ rather than a single protein or receptor, it is categorized as a therapeutic site rather than a specific molecular target.

Other names

Sphincter urethraeUrinary sphincterMusculus sphincter urethraeUrethral sphincter complex

Alpha-1 adrenergic receptor antagonism (relaxation of internal sphincter); Serotonin-norepinephrine reuptake inhibition (enhancement of external sphincter tone via Onuf's nucleus); Inhibition of acetylcholine release (paralysis of sphincter muscle); Alpha-1 adrenergic receptor agonism (contraction of internal sphincter).