hi let's talk about the rectum in the anal canal
in this video we'll discuss the locations muscular layers and structures of the rectum and anal canal
the rectum is a continuation of the sigmoid colon and thus a part of the large intestine
that rectosigmoid junction is approximately at the vertebral level of S3
when we think of the layers of the rectum they're all the same but they're slightly
phenotypically different than the layers of the remainder the large intestine so for instance
the the outer longitudinal muscle of most of the colon is condensed into ribbons of the tiny e coli
which leads also to a circulated appearance of the colon so we see hostry whereas
in the rectum that outer longitudinal layer of muscle is contiguous all the way around so
there's no teniae coli there are no haustra so it's it's not circulated
whatsoever it's going to have more of a smooth appearance from a serosal view from within the
uh the liver we can see uh invaginations of mucosa that are known as rectal folds
these uh these rectal folds or transverse rectal folds are somewhat constant and reliably located
especially even when the rectum is distended with feces as we look also the the distal portions of
the rectum which we can see here are quite dilated and that region is known as the rectal ampulla
the the rectum itself as a continuation or as a part of the the gut tube um is going to pass
through the muscles of the uh the pelvic floor so it's going through uh levator ani
um in in fact the the closest portion of levator ani is going to wrap around the uh the distal
portion of the rectum um and that uh that portion is the puborectalis muscle
and so the uh the rectum will be oriented sort of inferior anteriorly and then the anal canal
is going to take a sharp bend posteriorly because of that puborectalis sling that wraps about it
and we'll we'll see how how that works the the rectum is supplied by um a couple of sources of
arteries so here we can see the abdominal aorta and there is the inferior mesenteric artery
and one of its major branches is the superior rectal artery which we can see here the superior
rectal artery is going to really supply the the entire length of the rectum with with blood coming
off of the internal iliac artery here we can see an anterior branch called the middle rectal artery
that middle rectal artery is going to supply more of the proximal portions of the rectum
with blood sometimes it has branches towards the the distal rectum and the anal canal but
not as significant a source of blood to the distal rectum in anal canal as is the superior
rectal artery in terms of innervation of the rectum still has the enteric nervous
system which is extrinsically controlled by the autonomic nervous system specifically in this
case the inferior mesenteric plexus which has a middle rectal plectus middle rectal plexus which
supplies the the rectum with with innervation in terms of the the sympathetic uh those those
fibers are coming up down from the the superior hypogastric plexus in the inter mesenteric plexus
as well as the inferior mesenteric plexus here we also have some sympathetics that can arise through
the sacral splanchnic nerves but those those are going to be a minor contribution to the the rectum
in terms of the parasympathetics those are coming predominantly through the pelvic splanchnic nerves
pelvic splanchnics are branches from the ventral primary rami of sacral nerves two three and four
and those branches are going to feed into not just the inferior hypogastric plexus but also we'll
have an independent pathway um out of the uh the pelvis to serve the um the hindgut in in totality
now beyond the rectum is the anal canal and so we have this anal rectal junction
and we can see that they are they are faced differently with the rectum more
inferiorly and sometimes even a little anteriorly but that anal canal has a distinct posterior
orientation to it and that's complements of that that puborectalis muscle forming a sling around
that um the animal canal is uh continuous with with the anus it is divided up into a
a proximal portion which we call the proximal two-thirds of the anal canal
and then there is a distal one-third of the anal canal these uh these two regions
have distinct embryological origins and so their pattern of of blood flow and innervation is going
to be distinctly different and we'll address that on uh on a future slide but suffice it to say when
we look at the mucosa of the proximate anal canal so the approximate two-thirds here we can see that
there are mucosal invaginations here and they form these columns here they're called anal columns
they tend to be more constant in younger individuals and more inconstant in older
individuals and the spaces between these anal columns are known as the anal sinuses
see that there when we go down to where the the columns and the sinuses meet we have something
known as the pectinate or dentate line that pectinate dentate line is going to distinguish the
the proximal anal canal from the distal anal canal and various patterns of neurovascular service
now when we look at the the proximal and distal anal canal we can see that they're
both surrounded by musculature and so here we can see this would be the musculature represented by
the uh the colon and outside of that is the musculature represented by the external anal
sphincter and so we're going to see that there are actually two different or two distinct types
of muscles derived from the colon one part of that is going to be the internal
anal sphincter and the other part will be longitudinal muscle and that longitudinal muscle
is going to separate the internal anal sphincter or the ias from the external anal sphincter
or the eas and let's take a look at that now so we can see in this catavaric view here um
we have the internal anal sphincter this internal anal sphincter is a continuation
of the innermost layer of the muscularis of the colon specifically that circular layer
it's not truly circular like most sphincters are it's more of a spiral orientation which
is reflective of its origin from that layer of the muscularis as it is smooth muscle and as
it is of kind of the hind gut um that internal anal sphincter is going to be
under autonomic control specifically from the inferior hypogastric plexus likewise um there's
longitudinal musculature sometimes this is called the conjoined longitudinal musculature this
is also derived from the longitudinal musculature of the hindgut so it's contiguous with that
longitudinal layer of the rectum so it's smooth muscle and it is also autonomically controlled
by the inferior hypogastric plexus and then finally on the outside here we have the external
anal sphincter or the the eas
and that external anal sphincter let me write it here so we can see it
is skeletal muscle it is innervated by inferior rectal nerves which are
branches of the pudendal nerves so from the the sacral plexus
so let's take a look at some of the divisions we have of the anal canal and
the consequences of having different embryological origins here so
let's just point out here is that pectinate or dentate line and
what is above this line so the the proximal uh anal canal is is going to be distinct from what is
distal to this line or the the distal anal canal so um let me go through um sort of
vessel by vessel and and contrast these so proximally the blood supply to the um the proximal
two-thirds of the anal canal is going to be complements of the superior rectal artery um some
sources will say that there are some contributions from the middle rectal artery but the predominant
blood flow to the proximal anal canal is the superior rectal artery versus the inferior
or distal portion of the anal canal is supplied by the inferior rectal artery which is a branch of
the internal pudendal artery superiorly draining the the mucosa we have tributaries that are going
to coalesce into the superior rectal vein this is a part of the portal system whereas inferiorly
those portions are drained by inferior rectal veins which drain into
the um the internal pudendal vein which is cable so portal above the pectinate line caval below
now there can be dilations of both um the the veins within the superior mucosa and
in the inferior mucosa some terminologies uh we'll refer to these dilations as hemorrhoids
so internal versus external hemorrhoids depending on their locations so for instance an external
hemorrhoid would be a dilation of the mucosa complements of dilations from the caval portions
uh these tend to be um itchy um and even painful that that's because we're experiencing true pain
because this is somatic because the innervation is complements of the inferior rectal nerves which
are somatic in origin because they're branches of the pudendal nerve versus quote unquote
internal hemorrhoids which are dilations of the mucosa from tributaries of the superior rectal
vein now there are also issues in um delimiting hemorrhoids from anorectal varices
anaerectal varices are going to be dilations of the veins in the mucosa that are from
portal hypertension oftentimes as a result of cirrhosis and so sometimes these concepts get
confounded between the two and generally people will very frequently refer to internal hemorrhoids
as inorectal varices and this is a very small
incidence in in the general population and it tends to not be as diagnostic as esophageal
varices in diagnosing portal hypertension but nevertheless this uh this tends to be uh discussed
in terms of lymphatics any lymphatic drainage of the proximal anal canal tends to go to the
internal iliac nodes whereas the drainage of lymphatics from the distal anal canal
tends to go to the superficial inguinal nodes and we'll discuss those superficial inguinal nodes
in the context of the lower limb ultimately that lymph from the superficial inguinal
nodes will go to deep inguinal nodes through common inguinal nodes
um so that that lymph is going to reach the the same pathway as the um as the lymph
from the internal iliac nodes just a a different route and we'll we'll see that as we move forward
so we've discussed the uh the rectum and the anal canal uh the the neurovasculature which serves
different portions of uh of the hindgut here um this is your summary slide thank you for your time