Anal sphincter muscle

The anal sphincter muscle complex is a critical anatomical structure responsible for maintaining fecal continence and facilitating defecation. It comprises the internal anal sphincter (IAS), an involuntary smooth muscle, and the external anal sphincter (EAS), a voluntary striated muscle (StatPearls, 2023). The IAS maintains the majority of resting anal pressure through autonomic innervation, while the EAS provides supplemental squeeze pressure under somatic control (NIH, 2022). Pathological hypertonicity of the IAS is a primary factor in the development of chronic anal fissures, as high pressure leads to local ischemia and impaired healing (PubMed, 2021). Conversely, sphincter weakness or damage results in fecal incontinence, a condition significantly impacting quality of life. Therapeutic strategies often involve modulating the tone of these muscles to restore normal function. For instance, topical nitrates and calcium channel blockers are used to relax the IAS by increasing nitric oxide levels or inhibiting calcium influx, respectively (StatPearls, 2023). Botulinum toxin is also employed to induce temporary muscle paralysis in cases of severe hypertonicity or chronic fissures. Overall, the anal sphincter complex serves as a functional tissue target for various pharmacological agents aimed at managing anorectal disorders.

Other names

Sphincter aniInternal anal sphincterExternal anal sphincterAnal sphincter complex

Relaxation of smooth muscle via nitric oxide donation or calcium channel blockade; inhibition of acetylcholine release at the neuromuscular junction; stimulation of alpha-adrenergic receptors to increase muscle tone.