42 Innervation

STUDYAIMS

At the end of your study, you should be able to:

Describe the formation of the sacral plexus

Know the main branches of this plexus and what theysupply

Describe the parasympathetic innervation of the pelvis

Describe the sympathetic innervation of the pelvis

Describe the innervation of the perineum

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GUIDE

Pelvis and Perineum: Innervation

[Plate 390, Nerves of Pelvic Viscera: Male]

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[Plate 392, Nerves of Pelvic Viscera: Female]

Innervation of the Pelvis

Pelvic innervation is both somatic and autonomic

Manynerves pass through the pelvis without innervating anystructures

Sacral plexus (somatic contribution)

Located on posterior wall of true pelvis on anterior surface of piriformis muscle

Formed byventral rami of S1-S4 spinal nerves and the lumbosacral trunk, formed from fibers from the ventral rami of L4-L5 spinal nerves Branches supply: lower limb; pelvic floor and wall and perineum; most branches leave the pelvis through greater sciatic foramina

Two major branches are the sciatic nerve and pudendal nerve

Branches of the Sacral Plexus

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Autonomic nerves

Sympathetic innervation

Sympathetic trunks

Contain fibers from lower thoracic and upper lumbar spinal segments Descend on posterior abdominal wall on either side of the vertebral bodies Descend behind common iliac artery, in front of sacrum and piriformis muscle Fuse on anterior surface of coccyxas ganglion impar

Are source of sacral splanchnic nerves that join inferior hypogastric (pelvic) plexus in pelvis

Contain either postsynaptic fibers from synapses in ganglia superiorlyor presynaptic fibers that will synapse in one of four sympathetic ganglia in pelvis

Primaryfunction is to send postsynaptic fibers to sacral spinal nerves via greyrami communicantes

No white rami communicantes associated with sacral sympathetic trunks because sympathetic outflow is thoracolumbar Lumbar splanchnics join superior hypogastric plexus

Superior hypogastric plexus

Is located just below the bifurcation of the aorta

Is a continuation of the intermesenteric/aortic plexus Contains no parasympathetic fibers

Contains visceral afferents ascending from the pelvis

Continues into the pelvis as loose collections of fibers in either side of the anterior sacrum = right and left hypogastric nerves Spreads out in pelvis as inferior hypogastric (pelvic) plexus

Functions

Vasomotor, pilomotor, secretoryto sweat glands to skin of perineum and lower limb Controls ejaculation (in males)

Innervates smooth muscle of blood vessels in pelvis and smooth muscle of some organs Parasympathetic innervation

From pelvic splanchnic nerves

Outflow from 2nd to 4th sacral spinal cord segment Contain visceral afferents

Controls micturition, erection, and defecation

Innervates descending and sigmoid colon via fibers ascending in the sigmoid mesenteryand the parietal peritoneum of the left side of the posterior abdominal wall

Inferior Hypogastric (Pelvic) Plexus

General structure

Formed from fibers of hypogastric nerves, sacral splanchnic nerves, and pelvic splanchnic nerves Lies on posterolateral pelvic wall internal to internal iliac arteries and their branches

Gives off

Middle rectal plexus

Uterovaginal plexus (females)

Vesical plexus

Deferential plexus

Prostatic plexus (males

Cavernous nerves (nervi erigentes) to erectile tissue of penis and clitoris

Except for cavernous nerves, nerves in the various plexuses reach the viscera in companywith relevant branches of the internal iliac artery Branches transmit efferents from both sympathetic and parasympathetic nervous systems and visceral afferents

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Contents

Preand postsynaptic sympathetic fibers from superior hypogastric plexus via hypogastric nerves Postsynaptic sympathetic fibers from sacral splanchnic nerves

Sympathetic ganglia

Visceral afferent fibers that either ascend along the hypogastric nerves or return to the CNS via the pelvic splanchnic nerves, depending on the information carried.

Presynaptic parasympathetic fibers from the pelvic splanchnic nerves

Enteric parasympathetic ganglia where the plexus lies on or near a pelvic viscus Postsynaptic parasympathetic fibers from the enteric ganglia to the viscus innervated

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