Makindo Medical Notes

Anal Sphincter, External

Origin: Perineal body or central tendinous point of the perineum

Insertion: Encircles the anal canal; superficial fibres attach to the coccyx

Action: Constricts the anal canal

Innervation: Inferior rectal nerves (from the pudendal nerve)

Artery: Inferior rectal a.

Notes: Skeletal (voluntary) muscle, as contrasted with sphincter ani internus, which is smooth (involuntary) muscle; the external anal sphincter is considered part of the pelvic diaphragm.

Anal Sphincter, Internal

Origin: Encircles the anal canal

Insertion: Encircles the anal canal

Action: Constricts the anal canal

Innervation: Parasympathetic fibers from S4

Artery: Middle rectal a.

Notes: Smooth muscle (involuntary), as contrasted with sphincter ani externus, which is skeletal muscle (voluntary).

Bulbospongiosus, in Female

Origin: Perineal body and fascia of the bulb of the vestibule

Insertion: Perineal membrane and corpus cavernosum of the clitoris

Action: Compresses the vestibular bulb and constricts the vaginal orifice

Innervation: Deep branch of the perineal nerve (from pudendal nerve)

Artery: Perineal a.

Notes: Skeletal muscle.

Bulbospongiosus, in Male

Origin: Central tendinous point and the midline raphe on the bulb of the penis

Insertion: Perineal membrane, dorsal surface of the corpus spongiosum, deep penile fascia

Action: Compresses the bulb of the penis, compresses the spongy urethra

Innervation: Deep branch of the perineal nerve (from pudendal nerve)

Artery: Perineal a.

Notes: Expels the last drops of urine from urethra; expels semen during ejaculation.

Coccygeus

Origin: Ischial spine

Insertion: Side of the coccyx and lower sacrum

Action: Elevates the pelvic floor

Innervation: Branches of the ventral primary rami of spinal nerves S3-S4

Artery: Inferior gluteal a.

Notes: Coccygeus and levator ani combined form the pelvic diaphragm.

Deep Transverse Perineus

Origin: Medial surface of the ischial ramus

Insertion: Contralateral muscle and perineal body/central tendinous point

Action: Fixes and stabilizes the perineal body/central tendinous point

Innervation: Deep branch of perineal nerve from pudendal nerve

Artery: Internal pudendal a.

Notes: Superficial and deep transverse perineus muscles are separated by the perineal membrane.

Detrusor of Bladder

Origin: Smooth muscle in the wall of the urinary bladder

Insertion: Fascicles are arranged roughly in three layers

Action: Compresses the urinary bladder

Innervation: Parasympathetic nerve fibers from the pelvic splanchnic nerves (S2-S4 spinal cord levels)

Artery: Superior and inferior vesical aa.

Notes: Muscles in the neck of bladder must contract and cause the internal urethral orifice to open before the detrusor muscle can void the bladder (Latin, detrusor = to push down).

Iliococcygeus

Origin: Arcus tendineus levator ani and the ischial spine

Insertion: Anococcygeal raphe and the coccyx

Action: Elevates the pelvic floor

Innervation: Branches of the ventral primary rami of spinal nerves S3-S4

Artery: Inferior gluteal a.

Notes: The combination of puborectalis, pubococcygeus, and iliococcygeus is called the levator ani m.

Ischiocavernosus

Origin: Medial surface of the ischial tuberosity and the ischiopubic ramus

Insertion: Corpus cavernosum and crus of the penis/clitoris

Action: Compresses the corpus cavernosum

Innervation: Deep branch of the perineal nerve (from pudendal nerve)

Artery: Perineal a.

Notes: Ischiocavernosus m. is closely applied to the surface of the crus penis/clitoris.