Autonomic Innervation of Abdomen and Pelvis in a Nutshell

Autonomic review:

Visceral efferents = motor innervation to smooth muscle, cardiac muscle, exocrine glands.

• Sympathetic
  • Decrease motility
  • Vasoconstriction of blood vessels in abdomen. This is done when blood is called for in extremities (during exercise etc.).
• Parasympathetic
  • Increase motility
  • Stimulus for Urination / Defecation

Both divisions require a 2 cell chain which is interrupted by an autonomic ganglion.

• The first neuron in chain = pre-ganglionic neuron
• The second neuron = post-ganglionic neuron. This neuron will end in whatever the effector is.

Paravertebral ganglion: non visceral sympathetic innervation

• Axon of preganglionic uses ventral root to exit spine, uses the white ramus (is myelinated) to enter paravertebral ganglion where it synapses with post ganglionic neuron.
• The post GN leaves through gray ramus and then leaves both ventral / dorsal rami

Sympathetic innvervation of abdominal viscera:

• The  preGN will leave cord via ventral root, passes through white rami, passes through paraVG, leaves into the preVG where it synapses with the postGN.
• The postGN then goes out to the effector. It will wrap around blood vessels which lead to the organ.

Names of prevertebral ganglia (normally paired):

• Celiac
• Superior mesenteric
• Inferior mesenteric
• Aorticorenal

They are associated with major unpaired vessels of the abdominal aorta, and also the renal vessels which are large.

Note that there are Right and Left ganglion for all of these.  There are R and L greater splanchnic nerves, which will go out to these.

Sympathetic chain:

• T5-T7 contribute to greater splanchinic nerve.
  • The GSN supplies: Organs supplied by the celiac trunk will be supplied by the celiac ganglion.
  • Lower esophagus, stomach, pre-ampulla duodenum, liver, gallbladder
• T10-11 make up the lesser splanchnic nerve
  • Supplies all structures supplied by superior mesenteric artery.
  • Uses superior mesenteric ganglion.
  • Post-ampulla duodenum, small intestine, cecum, ascending colon, part of transverse colon
• L1-2 make up the lumber splanchnic nerve
  • Supplies all structures supplied by inferior mesenteric artery.
  • Uses inferior mesenteric ganglion
  • Rest of transverse colon, sigmoid colon, upper rectum
• T12: make up least splanchnic nerveà goes to aoriticorenal ganglionà supplies the kidneys and the gonads.

Sympathetic

Celiac ganglia and superior mesenteric ganglia will form a unified ring like structure around the aortic branches.  These are hard to differentiate from each other.  They invest the branches of the aorta.

Parasympathetic innervation to the abdomen:

• The vagus distributes its preganglionic fibers into the wall of the organs supplied by both the celiac trunk and the superior mesenteric artery.
• The post ganglionic neuron is short- in the wall of the organ itself.
• The pelvic splanchnic nerve (aka sacral outflow) will supply those structures of the hindgut (i.e. those supplied by the inferior mesenteric).
• Very important: There are no paravertebral ganglion associated with parasympathetic.

Parasympathetic

How does pelvic splanchnic leave?

• The preganglionic fiber does use the ventral root, but it ignores the paravertebral ganglion.
• It enters the ventral ramus and comes off by itself to form the pelvic splanchnic nerve components.
• Thus, a correct statement would be that the pelvic splanchnic nerve (as it goes for other splanchnic nerves) is a collection of branches from appropriately numbered ventral rami.

Sympathetic innervation of the pelvis: Slightly more complicated

• The preganglionic neurons for pelvic viscera arise from the lowest levels possible, L1-L2 (are the last ones possible to use) lateral horns.
• These sympathetic fibers use the lumbar splanchnic nerves.
• These are other nerves that travel on the abdominal aorta (are distinct from those innervating inferior mesenteric organs) they travel along the aorta and are called the aortic plexus.
• When the abdominal aorta bifurcates, they go onto the sacral promontory.
  • Bifurcation occurs at L4-L5
  • Thus these nerves travel onto these vertebrae and onto the sacral promontory to create the superior hypogastric plexus.
• This appears at the level of the sacral promontory and is a continuation of the aortic plexus.
• The aorta bifurcated, allowing the superior hypogastric plexus to continue downward.

Within the superior hypogastric plexus:

• There are small ganglia (are a continuation of prevertebral ganglia, but aren’t the same) associated with the superior hypogastric plexus.
• These preganglionic fibers split into right and left hypogastric nerves and descend into the pelvis until reach branches of the internal iliac where they form a plexus = inferior hypogastric plexus. From here, they follow the blood vessels to the organ.
• Syperior hypogastric plexus is preganglionicà hypogastric nerves are postganglionic.
• Thus, superior hypogastric plexus is the meeting place between fibers of preganglionic neurons and the postganglionic neurons.

Dual pathways:

• The sympathetic chain continues into the pelvis all the way to the coccyx.
• The sympathetic innervation uses the sympathetic chain as well as the plexus system.
• Example: A neuron from L1 would leave and travel to a sacral paravertebral ganglion to synapse.
  • From here, the postganglionoic fiber (now called sacral splanchnic nerve) travels to the inferior hypogastric plexus.
  • The function is the same for both pathways.
  • Thus, if something disrupted the sacral splanchnic nerve, function would still be supported by the lumbar splanchnic pathway.

Sympathetic innervation to the viscera of the pelvis

Parasympathetics:

• Will be using system similar to that used in the hindgut.
• Pelvic splanchnic nerves (just as in abdomen)
  • Will be taken to the wall of the organ in the pelvis.
  • They follow blood vessels to the organs where they find short post vertebral ganglia in the organ wall.
• Note: Parasympathetic fibers also get intertwined in the inferior hypogastric plexus (amidst the sympathetic fibers).
  • This is done for economy of space, thus blood vessels to the pelvic organs will be invested with both sympathetic and parasympathetic fibers.
  • Thus, the inferior hypogastric plexus receives post ganglionic sympatheic fibers from the lumbar splanchnic and hypogastric splanchnic- and receives preganglionic parasympathetic fibers from the pelvic splanchnic nerve.

Parasympathetic autonomic innervation to the viscera of the pelvis

Visceral afferents: sensory fibers

• The visceral afferent nerves are not considered part of the autonomic nervous system, although they do share the pathways.
• As a general rule, parasympathetic pathways carry reflex afferents, whereas sympathetic pathways convey pain afferents.
  • Afferent reflex neurons travel along with the vagus fibers to the brain (the gastric filling reflex), as well as along pelvic splanchnics to the spinal cord (micturation reflex).
  • The bladder distends, there are afferents that sense distension→ when a point is reachedà stimulate efferent to contract bladder.  Afferents are carrying information that will result in a reflex.  Reflexes follow the parasympathetic pathways.
  • Pain and pressure afferents travel along the thoracic splanchnics (sympathetic), lumbar splanchnics (sympathetic), and pelvic splanchnics (parasympathetic- the exception to the rule) to the spinal cord.  Pain afferents follow sympathetic pathways.

• • • This arrangement explains the basis for referred pain.
      1. Nociceptive input from the viscera at any thoracic or lumbar level cannot be distinguished from that of somatic origin.
      2. The pain is referred to the somatic dermatome associated with the particular splanchnic nerve.
    • Knowing approximate visceral innervation patterns is essential to understanding referred pain and making accurate diagnoses.

1. • Because the pain fibers run along the sympathetic pathways, sympathectomy is undertaken to relieve visceral pain such as untreatable carcinoma.

Pathways:

• Receptor in the wall of the viscera is attached to a neuron that has its cell body located in the dorsal root ganglion, and thus information will enter through the dorsal root.
• Here, somatic afferent cell bodies and visceral afferent cell bodies live.
• This pathway taken by the receptor neuron is the same as the sympathetic pathway

Example:

• Gastric ulcer is destroying lining of the stomach and is stimulating pain afferentsà having sensation of pain.
• Painful afferents follow course of sympathetic neurons to that structure.
• The sympathetic path to the stomach is along the greater splanchnic nerve.
• The afferent returning along this path returns through the white ramus and into the ventral ramusà into the dorsal root ganglion (soma located here)à into the ventral root.
• Note: only find visceral afferents in dorsal root ganglions T1-L2.
  • This is the basis for referred pain.
  • There is some cross wiring as return up sensory pathway.  Thus, pain is interpreted along body wall.
  • Note: there is no synapse along the pathway.

Reflexes follow parasympathetic pathway.

• Example: stomach senses distension.
• Vagus nerve is the parasympathetic path to the stomach.
• The afferent has a cell body located in a ganglion associated with the vagus nerve but does not synapse there.  It has no name.