Anatomy study note · Perineum
External anal sphincter
The voluntary skeletal-muscle sphincter of the anal canal. It surrounds the involuntary internal sphincter (thickened circular smooth muscle at the upper end of the canal).
Key points
- Subcutaneous, superficial, deep parts.
- Inferior rectal nerve (pudendal) + S4.
- Anorectal ring = internal sphincter + deep external sphincter + puborectalis.
- Above the pectinate line: visceral, painless, portal, IMA nodes. Below: somatic, painful, systemic, inguinal nodes.
- Striated muscle: myotomes + local mesenchyme.
- Nerve supply indicates origin.
- External anal sphincter: skeletal, voluntary.
- Skeletal muscle: multinucleated, peripheral nuclei, striated.
The full note in the atlas also covers
Features · Origin · Insertion · Action · Nerve supply · Histology · Development · Clinical notes
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Written from
- Snell’s Clinical Anatomy by Regions, 10th ed., ch. 10, PDF pp. 1167–1177, 1181–1186
- Inderbir Singh’s Human Embryology, ch. 7, PDF pp. 92, 108–109
- BRS Cell Biology & Histology, 7th ed., ch. 8, PDF pp. 135–144; ch. 16, PDF p. 298
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