Internal Anal Sphincter
Pelvic Floor
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Involuntary smooth muscle that keeps the anal canal closed at rest — working 24/7 without conscious effort. Accounts for about 85% of resting anal tone.
Function
Maintains involuntary anal tone
Origin
Thickened circular smooth muscle of rectal wall
Insertion
Encircles upper anal canal
Innervation
Autonomic: sympathetic maintains tone; parasympathetic relaxes