Pelvic floor muscles, ischioanal fossa, perineal region

Summary

The pelvic floor is a complex structure of muscles and connective tissue that supports the pelvic organs and assists in urinary and fecal continence. Closely associated are the ischioanal fossae, which allow for expansion of the anal canal during defecation, and the perineal region, which includes key structures of the urogenital and anal triangles. A solid understanding of this region is crucial for clinical practice and anatomy examinations.

Pelvic floor muscles

Overview

The pelvic floor, or pelvic diaphragm, consists of a funnel-shaped muscular layer that separates the pelvic cavity from the perineum. It provides critical support for pelvic viscera (bladder, rectum, uterus) and resists intra-abdominal pressure.

Components

The pelvic diaphragm is primarily composed of:

Levator ani muscle group

• Puborectalis: Forms a sling around the anorectal junction; critical for maintaining fecal continence.
• Pubococcygeus: Runs from the pubis to the coccyx; supports pelvic organs.
• Iliococcygeus: From tendinous arch of obturator internus to the coccyx; helps lift the pelvic floor.

Coccygeus (ischiococcygeus)

• Extends from the ischial spine to the sacrum and coccyx; supports pelvic viscera posteriorly.

Innervation

• Levator ani: Branches of S3–S4, and inferior rectal nerve (branch of pudendal nerve).
• Coccygeus: Direct branches from S4–S5.

Function

• Support of pelvic organs
• Fecal and urinary continence
• Resistance to intra-abdominal pressure (e.g., during coughing, lifting)
• Important in childbirth and sexual function

Ischioanal fossa

The ischioanal fossa is a wedge-shaped fat-filled space located lateral to the anal canal and inferior to the pelvic diaphragm. It allows for the expansion of the anal canal during defecation.

Boundaries

• Lateral: Obturator internus muscle and ischial tuberosity
• Medial: External anal sphincter and levator ani
• Posterior: Sacrotuberous ligament and gluteus maximus
• Anterior: Body of pubis
• Roof (superior): Levator ani
• Floor (inferior): Skin of perineum

Contents

• Fat (allows for anal canal expansion)
• Pudendal canal (Alcock's canal): Contains the pudendal nerve, internal pudendal artery and vein
• Inferior rectal nerves and vessels: Branches of pudendal nerve/artery; supply external anal sphincter and lower anal canal
• Perineal branches of S4

Clinical Note

• Common site of ischioanal abscesses, which may spread across the midline via the deep postanal space.
• Pudendal nerve block is performed here for obstetric anesthesia.

Perineal region

The perineum is the region below the pelvic diaphragm, between the thighs. It is divided into:

• Urogenital triangle (anterior): Contains external genitalia and openings of the urogenital system.
• Anal triangle (posterior): Contains anal canal and ischioanal fossae.

Boundaries

• Anterior: Pubic symphysis
• Posterior: Coccyx
• Lateral: Ischial tuberosities
• Forms a diamond shape when viewed from below.

Perineal Pouches

Superficial Perineal Pouch

• Located between the perineal (Colles') fascia and the perineal membrane.
• Contents:
  • Erectile tissues (corpora cavernosa, corpus spongiosum)
  • Superficial perineal muscles: ischiocavernosus, bulbospongiosus, superficial transverse perineal muscle
  • Branches of pudendal nerve and vessels

Deep Perineal Pouch

• Located between the perineal membrane and the pelvic diaphragm.
• Contents:
  • External urethral sphincter
  • Deep transverse perineal muscle
  • Bulbourethral glands (in males)
  • Dorsal neurovascular structures of the penis/clitoris

Innervation and Blood Supply

• Pudendal nerve (S2–S4): Motor and sensory innervation
• Internal pudendal artery and vein: Branch of internal iliac artery

References

• Moore KL, Dalley AF, Agur AMR. Clinically Oriented Anatomy. 8th ed.
• Netter FH. Atlas of Human Anatomy.
• Gray’s Anatomy for Students.