Perineum - LO 10

10. Understand basics of the autonomic supply of the pelvis.

There is sympathetic, parasympathetic, and visceral afferent innervation of the pelvis.

Parasympathetic fibers’ main roles:

• • Contraction of smooth muscle of bladder and rectum/anal canal

  • Relaxation of smooth muscle that allows for increased blood flow into erectile tissues, leading to erection

  • Initiates glandular secretions

Sympathetic fibers’ main roles:

• • Vasoconstriction

  • Inhibition of rectal/anal smooth muscle contraction

  • Initiating emission (seminal fluid into urethra)

  • Contraction of smooth muscle of genital organs leading to ejaculation (in conjunction with dominant somatic innervation - pudendal n.)

Visceral afferent fibers

• • Reflexive sensations all travel with pelvic splanchnic nn. (parasympathetic)

  • Pain sensation is dependent on relationship with ‘pelvic pain line’

    • • In the pelvis, the pelvic pain line is synonymous with the inferior-most extent of peritoneum 

      • Superior to pelvic pain line: travel with sympathetic fibers

      • Inferior to pelvic pain line: travel with parasympathetic fibers

There are three autonomic plexuses/nerves associated with the pelvis: superior hypogastric plexus, hypogastric nn., and inferior hypogastric plexus. It is important to note that these are continuous with one another and with abdominal autonomic plexuses. The inferior hypogastric plexuses are considered the main sources of autonomics to the pelvis.

The superior hypogastric plexus lies within extraperitoneal connective tissue in the midline, anterior to the abdominal aortic bifurcation. It is derived from three sources: lumbar splanchnic nn. (sympathetic), pelvic splanchnic nn. (parasympathetic), and the aortic plexus (sympathetic and parasympathetic). The organization of this plexus is widely variable, with possible presentations ranging from a web-like appearance to distinct nerve trunks.

The typically paired hypogastric nn. are located between the superior & inferior hypogastric plexuses, medial to internal iliac vasculature and lateral to the anterior sacral foramina. While these are referred to as nerves, they typically present as condensations of nerve fibers. These nerves consist of both sympathetic and parasympathetic fibers.

The inferior hypogastric plexuses are located on either side of the rectum. The main sources of autonomics for this plexus are derived from the pelvic splanchnic nn. (parasympathetic) and sacral splanchnic nn. (sympathetic). Sacral splanchnic nn. are typically derived from the 1st and 2nd sympathetic sacral ganglia. Pelvic splanchnic nn. consist of parasympathetic fibers derived from the ventral rami of S2-S4. The main destination of pelvic splanchnic nn. is the inferior hypogastric plexus, but some fibers travel to the hypogastric nn. or directly to the descending and sigmoid colon.

It is important to note that the inferior hypogastric plexus sends sub-plexuses to most internal pelvic viscera, including uterus, vagina, urinary bladder, seminal vesicles, and prostate.