hello anatomy friends this is dr alsup and we are going to talk about the intestines both the small
and the large intestine so we'll be comparing the structures of the small and the large intestine
and go through some of the big picture high yield concepts with these areas such as the
basics regarding neurovascular supply so let's start with the small intestine occasionally
referred to as the small bowel if only you were talking about the jejunum and the ilium
from proximal to distal the small intestine is divided into three parts
the most proximal part and shortest is the duodenum and we are going to discuss the duodenum
in a later session where we will discuss with the very closely associated pancreas which would be
located right about here intermediate is going to be the jejunum which is located mostly within
that left upper quadrant but as you can see will certainly extend into some of the other quadrants
and the most distal and the longest portion is going to be the ilium which will transition
into the cecum at the ileocecal junction the small intestine the jejunum and ileum is very dominant
and it's very dominantly present within the abdominal cavity it's very hard to miss
so dominant is often referred to as the mesentery that root of the mesentery
is i'm going to attach to the posterior abdominal wall and it will allow branches
from the retroperitoneal aorta to actually reach the intestine so it'll travel through the
mesentery now if you open the jejunum and ilium to view the mucosa like we've done here you will see
fairly prominent circular folds particularly in the proximal portions of the jejunum you
will not have circular folds associated with the distal most or the terminal portion of the ilium
the circular folds are complete meaning it will form a complete circle within unlike what we see
with the semilunar folds of the large intestine and these folds will serve to increase the surface
area of the small intestine that's particularly important in terms of absorption which is an
important role of the small intestine now when you are viewing externally or macroscopically
it is very difficult to differentiate the jejunum from the ilium in fact you can't really do that so
if we were to ask you to differentiate between the two we would have to ask you something well within
the left upper quadrant making that which one the jejunum or the ilium yes the jejunum is going
to be in the left upper quadrant and if we were talking about the right lower quadrant that would
be the ilium again we are not talking about the duodenum quite yet as we move towards vasculature
which when we're talking duodenum will be unique as the duodenum is derived from both the foregut
and the midgut but the jejunum and the ilium are all mid-guts so it follows the rules
of having branches from the superior mesenteric artery or the sma specifically numerous jejunal
and ileal branches that will supply the small intestine this area will be drained by tributaries
of the superior mesenteric vein or the smv which is part of the hepatic portal system
now moving on to everyone's favorite innervation and the gastrointestinal tract is one of the more
complex areas in terms of innervation in fact it has its own not so little nervous system
referred to as the enteric nervous system or the ens and of course it will also have autonomic
supply as well the enteric nervous system is often described as its own system or part of
the larger autonomic nervous system which i'm a bit more partial to but it's certainly unique
in terms of location the ENS is a complex series of nerve fibers and ganglia embedded
in the walls of the viscera of the gi tract and there are two systems of systems of plexuses
now the Meissner or the submucosal plexus is going to be composed of a ganglionic nerve plexus
within or kind of associated with the submucosa whereas Auerbach's or the myenteric plexus
which you can see right kind of here in this region is located between the longitudinal
and circular smooth muscle layers so these differ mainly in terms of their location
collectively these plexuses are concerned with propulsion of contents through synchronized
contractions as well as mucosal functions so very similar to what we will see in terms of the
sympathetic and parasympathetic functions and will be modulated by the autonomic nervous system
i believe you will have a whole lecture on the ENS so this is just a small preview
the primary autonomic innervation is derived from the superior mesenteric plexus
which is an autonomic plexus that has both sympathetic and parasympathetic fibers the
sympathetic fibers are derived from those greater and lesser splenic nerves that we talked about
way back in the posterior mediastinum session and these will synapse in the celiac or superior
mesenteric ganglia so you can see some of that here and around here so very closely associated to
where those arteries branch off of the abdominal aorta that's where those ganglia will be located
and as sympathetics often do they'll play an important role in terms of vasoconstriction
and will be inhibitory to smooth muscle now when we're talking small intestine for the most
part we are talking mid-gut so we know that the parasympathetic source should be the vagus nerve
and it is so that cranial nerve ten i apologize if you heard my dog barking in the background
now the remember parasympathetics are important in resting and digesting so it will play a role
in initiating contraction of the smooth muscle of the gut tube and will be secretomotor to mucosa
now we reviewed the order of the large intestine in a previous video but to make sure we are
on the same page let's go through it one more time let me clear my throat so we can do that okay that
ilium is going to transition into the cecum at the iliocecal junction the ilium is the most proximal
portion and often one of the largest in caliber it will have the vermiform appendix attached to it
it will continue as the ascending colon which will take an abrupt change in direction
at the right colic or hepatic flexure and it will continue to run transversely as the transverse
colon taking another sharp or abrupt change or turn at the left colic or splenic flexure
this will continue as the descending colon and then there will be a few kind of turns
which will indicate that you are in the sigmoid colon and then the large intestine will begin
to straighten out again and that will be the rectum which will transition into the anal canal
at the pelvic diaphragm now recall that the large intestine is derived from both the mid gut
and the hindgut that point of transition is about two-thirds of the way on the transverse colon so
anything from here on would be mid-gut and from here further would be hind gut
or here distal i guess would be a more appropriate term the intraperitoneal portions of the
colon will have specific mesentery such as the transverse mesocolon as well as the mesoappendix
and the sigmoid colon recall is intraperitoneal so it will have a sigmoid mesocolon
there are four structures that are unique to the large intestine that we want you to
be familiar with the first are going to be the taeniae coli or the tenia coli you'll hear it all
kinds of different ways you'll see it spelled all different kinds of ways this is missing an
a in terms of its uh appropriate spelling um these are going to include three bands of muscles you
can only really see one here they're forming the outer layer of the muscular layers and these are
going to send small deviations uh kind of off to the sides that are going to form circumferential
bands around the large intestine at fairly regular intervals and what that will lead to
is the creation of these very obvious hostra or hostrations or circulations which will appear as
these kind of like puckering out of the large intestine throughout almost its entire length
and these will represent sites where the mucosa and submucosa of the clonic wall is infilted
and this will be viewable as semilunar folds if you are looking internally the semilunar folds
do not form complete rings like we saw with the circular folds of the small intestine and lastly
we are going to have what's referred to as omental or epiploic appendages kind of throughout
the the span but not in every single portion of the large intestine
these are going to be attached to the external surface of many parts and these are fat filled
outpouchings of the peritoneum so those are those epiploic or mental appendages
now in terms of arterial supply it will be really helpful to keep in mind in what part of the large
intestine is derived from the mid-gut those are going to be supplied by branches of the sma and
which parts are derived from the hindgut which will be supplied by branches of the
inferior mesenteric artery so let's go over some of the sma branches or the mid-gut derivatives
first you'll have the iliocolic which should have an o here i apologize for the misspelling
the iliocolic is one of the largest in caliber and while dominant dominantly supply the cecum but it
will send a branch or two to the ilium you can see a little bit of that kind of heading off this way
in this image it will also give off the appendicular artery which will supply
as its name would suggest the appendix the right colic is going to supply
the ascending colon whereas the middle colic is going to supply the transverse colon
in terms of the hindgut derivatives these are going to be supplied by the inferior mesenteric
artery and include the left colic which we can see where do we have that here yeah you
can see it's one of the larger ones in terms of caliber the left colic is going to supply
the descending colon in a little bit of the transverse colon because recall that
distal one third is supplied by the inferior mesenteric you will have a few sigmoid arteries so
you typically have more than one sometimes two to three those are going to supply the sigmoid colon
and lastly what i often look looks to me like a continuation of the inferior mesenteric artery is
going to be the superior rectal artery which will supply the superior portions of the rectum the
vascular supply of the rectum is actually quite complex when you add in the more inferior portions
of the rectum and we will talk about that in a bit more detail in the pelvic anatomy sessions
now almost all of those branches that we just discussed will help to form the marginal artery
which i'm just going to kind of outline the basic placement of that and it will extend obviously
over into the left portion of the body as well the marginal artery is an anastomotic artery that
runs along the inferior or medial borders of the large intestine and it's going to send
short arteries to the actual viscera to supply the fact that anastomoses are pre are prevalent here
is important in cases of occlusion of an artery or a branch there will likely be some form of blood
supply that can get to the area based on the fact that the marginal artery is running throughout the
almost the entire length in terms of venous drainage this will include tributaries of the
superior and the inferior mesenteric veins which are both part of the hepatic portal venous system
last but not least is the innervation of the large intestine which is not tremendously
different from the small intestine as you can see here in terms of the functions
very similar to what we talked about with the small intestine the large intestine
certainly has um the enteric nervous system as described with the small intestine as well
the sympathetics when we're talking overall autonomics will come from a multitude of sources
including the sympathetics from that superior mesenteric nerve plexus recall that nerve plexus
had both parasympathetics and sympathetics but it will have those specific fibers will
play a role in terms of innervating the large intestines sorry this should say large right here
uh so that is going to play a role in terms of the sympathetics but we'll also have importantly the
lumbar splenic nerves which are going to innervate the more distal portions of the large intestine
again understanding which parts are derived from the mid gut
so the proximal most portions to the about two-thirds of the way down on the transverse colon
will have its spare sympathetic supply from the vagus nerve so vagus nerve think mid-gut
as well as foregut and then the hindgut is going to have its parasympathetic supply derived from
the pelvic splanchnic nerves via the inferior hypogastric plexus so remember those pelvic
splanchnic nerves are the sacral portion of the parasympathetics from s2 through s4
so we have finally made our way to the summary slide which i really worked to kind of pull out
those high-yield concepts but please don't ignore the context we provided throughout the slides
or the written descriptions please take your time to review and reach out if you
have any questions thank you for your time and attention and have a great rest of your day