hi let's talk about abdominal autonomics in this video we'll talk about the autonomics
of the abdominal portion of the gastrointestinal system
the gi tube is served by the enteric nervous system or the ens which is sometimes described
as a part of the autonomic nervous system but the enteric nervous system may also function
independently from the autonomic nervous system the enteric nervous system consists of
two plexuses there's the myenteric plexus also known as Auerbach's plexus which is found
between the layers of the muscularis so there are circular and longitudinal layers of the uh of the
muscularis or the smooth muscle of the wall of the gut tube and Auerbach's plexus is found within
these layers the myenteric plexus predominantly acts upon this layer by either contracting or
relaxing the smooth muscle within the other component of the ENS is the submucosal plexus
also known as Meissner's plexus the submucosal plexus is found within the submucosa and
it is going to serve the glands within the submucosa and mucosa as well as the smooth
muscle layer of the mucosa so for instance it will either contract or relax that smooth muscle
or stimulate or inhibit the glands from secreting now the autonomic influence upon the enteric
nervous system is going to come by way of perivascular plexuses these are going to be
fibers that are coming in along the uh the arteries which are going to serve these portions
of the gut tubes and these perivascular plexuses are named for the arteries which convey them
so for instance we're going to see that there is a celiac plexus in association with
the celiac trunk a superior mesenteric plexus in association with the superior mesenteric artery
and so on and so forth along the way these autonomic fibers are also going to be able to
affect the tunica media that would be the smooth muscle layer of the arteries
which are conveying them they will have especially an impact at the level of the arterial where the
the tunica media is at its greatest proportional thickness
and they will also have an effect upon the enteric nervous system as we've discussed
now the sources for these various autonomic plexuses are varied but we need to think of
where the sympathetics are coming from and where the parasympathetics are coming from now i've
i've color-coded sympathetics as orange parasympathetics as blue and anything green is
mixed meaning it's both parasympathetic and sympathetic and let's have a look at some of
these sources now coming down from the thoracic sympathetic chains and ganglia are going to be
three thoracic splanchnic nerves so if you recall thoracic splanchnic nerves are preganglionic
fibers that are entering into the sympathetic trunk but not synapsing in the ganglia
passing through and then synapsing in what we call pre-aortic ganglia so for instance
here we would have the greater thoracic splanchnic nerve
entering in to the abdomen and those fibers are synapsing in the celiac ganglia there are also
going to be fibers from the posterior vagal trunk which are going to move through those ganglia but
they will not synapse because they're going to synapse in the walls of the target organs
and so these presynaptic vagal fibers and then post-synaptic sympathetic fibers are going to form
these plexuses in this case a celiac plexus which is going to be transmitted out along the
celiac trunk and its branches to
regions of the foregut we'll also see that there are other thoracic splanchnic nerves
such as the lesser and infrequently the least that are going to hit target ganglia and be
dispersed among plexuses as well so for instance kind of contiguous sharing fibers between
or from the celiac plexus is also going to be the superior mesenteric plexus there is a ganglion
for the superior mesenteric plexus here that the greater thoracic splanchnic nerve will also
serve and this superior mesenteric plexus will be distributed out along the sma to the mid gut
we'll see that there are also lumbar splanchnic nerves that are distributed from
lumbar ganglia out to various plexuses and generally we see that the first two lumbar
splanchnic nerves are heading out towards what is known as the intermesenteric or the aortic
plexus we call it the inter mesenteric plexus frequently because it's located between the ima
plexus and the superior mesenteric plexus hence inter mesenteric plexus
so these fibers are going to ascend and descend in and amongst these various plexuses
at the base of the aortic plexus we see a concentration
of fibers called the superior hypogastric plexus the superior hypogastric plexus
is contiguous with that aortic plexus it's uh innervated by the third lumbar splanchnic nerve
as we can see here and it's also continuous with what are known as the hypogastric nerves
hypogastric nerves are going to be condensations of fibers between the superior hypogastric plexus
and inferior hypogastric plexuses in the pelvis um before we move on from this slide i'd just like
to point out that one thing not represented here is the anterior vagal trunk the vagal trunks are
coming into the abdomen through the esophageal hiatus and they have a very close relationship
with the esophagus and the anterior vector trunk comes in on the abdominal part
of the esophagus on that anterior portion of it and it's going to send branches directly to
the stomach as well as the liver and gallbladder
now the the pelvis is going to be important here and we'll see this again
when we talk about the autonomics of the reproductive system but let's orient ourselves
to the superior hypogastric plexus so that's here just sort of anterior and possibly a little
inferior from the the bifurcation of the abdominal aorta so we're really talking about that area
there and we have some hypogastric nerve here that's been severed and if allowed to continue
that hypogastric nerve would come down and its fibers would be contiguous with this mixed plexus
called the inferior hypogastric plexus which sits laterally to the abdominal viscera the inferior
hypogastric plexus in addition to having inputs and outputs with that superior hypogastric plexus
will also have inputs from two sources within the pelvis the sympathetic source will be from sacral
splanchnic nerves these sacral splenic nerves are branches of the sacral sympathetic trunk which i'm
outlining there that goes down to the ganglion impar just anterior to the coccyx
and we can see in yellow here are various uh ventral primary rami of sacral nerves so that's
s1 and s2 so that would be the first sacral ganglion and that would be the second sacral
ganglion and coming off of these typically from s1 and s2 we have the sacral splanchnic nerves
which input into the inferior hypogastric plexus we will also have pelvic splanctic nerves
pelvic splanchnic nerves arise from ventral primary rami of s2 3 and 4. these pelvic splanchnic nerves
are conveying preganglionic parasympathetic fibers out to the inferior hypogastric plexus
i keep this straight in my mind by thinking sacral splanchnics are sympathetic
whereas pelvic splanchnics are parasympathetic so pelvic parasympathetic
now these pelvic splanchnic nerves are going to be the source of parasympathetic innervation to
the hindgut and generally what will happen is those fibers from the pelvic splanchnic nerves
will ascend out of the pelvis to the hindgut directly independently of the hypogastric nerve
so some of those parasympathetic fibers may be ascending through the hypogastric
nerves and they may get to the hindgut by way of the inferior mesenteric plexus but
some of them are also moving there independently of this
in terms of autonomic regulation of the gi there are four basic parameters to consider
there's vascular flow motility of the bowel the relaxation of sphincters and glandular secretion
when we think of parasympathetic stimulation parasympathetics are the rest and most importantly
digest um so they're going to lead to vasodilation so an increase of blood flow to the gut tube
the stimulation of bowel motility so it will enhance peristalsis and it will relax sphincters
to allow materials to move through the bowel it will also stimulate glandular secretions
for the process of of digestion in terms of the sympathetic response this is the fight or flight
no time for rest or digest when you are running or fighting for your life these responses are
going to be vasoconstriction that's pretty typical for sympathetics the inhibition of bowel
motility the tightening of bowel sphincters and the inhibition of glandular secretion
any sort of referred pain so referred pain is the result of any distension or traction of
the bowel or ischemia to the bowel it's not true pain it's uncomfortable and it's felt
differently than pain itself it's conducted by means of general visceral afferent fibers
that travel with sympathetic fibers are also general visceral afferent fibers that travel with
parasympathetic fibers but in the abdomen these are for reflexes whereas the ones that travel with
sympathetic fibers are for referred pain referred pain is generally
felt as non-localized non-descript pain pain from the foregut is generally referred to
the epigastric region pain from the mid-gut to the periumbilical region
and pain from the hindgut to the suprapubic aka the hypogastric region
now we've discussed the enteric nervous system
the perivascular plexuses which serve the enteric nervous system parasympathetic and sympathetic
responses as well as the parasympathetic and sympathetic inputs to the various
perivascular plexuses and also we touched on referred pain and where referred pain from the
fore mid and hindgut will be experienced this is your summary slide thank you for your time