Internal anal sphincter smooth muscle (IAS smooth muscle (sometimes abbreviated IAS, but "Internal anal sphincter smooth muscle" is preferred in scientific contexts))

The **internal anal sphincter smooth muscle** is a specialized ring of involuntary smooth muscle, approximately 2.5–4.0 cm in length and about 5 mm thick, derived from the thickened inner circular muscle layer of the rectum. It constitutes the primary component of the internal anal sphincter (IAS), which maintains fecal continence by providing more than 70% of the resting anal canal pressure through continuous, intrinsic (myogenic) contraction. The muscle is under autonomic control: sympathetic stimulation promotes contraction, while parasympathetic input causes relaxation. Tone is generated by coordinated actions of smooth muscle cells (via actin-myosin cross-bridge cycling, regulated by calcium-dependent phosphorylation), and modified by signals from interstitial cells of Cajal (pacemaker cells) and ion channels such as TMEM16A. Drugs targeting the IAS aim to modulate its tone in conditions such as anal fissures or incontinence. "Internal anal sphincter smooth muscle" is not a canonical drug target, but rather a tissue or anatomical structure composed of various contractile, pacemaker, and connective tissue elements.

Other names

Internal anal sphincterIASsphincter ani internus

Relaxation of sphincter tone by *increasing nitric oxide* (nitrergic agents) Inhibition of smooth muscle contraction via *calcium channel blockade* Inhibition of neurotransmitter release targeting motor nerve terminals (botulinum toxin)