Download Functional Anatomy of the Alimentary Canal

April 22, 2005
Introduction and GI Regulation
Ron Lynch, Ph.D.
626-2472
Functional Anatomy of the Alimentary Canal
Objectives:
1. Identify the location of each organ along the alimentary canal, and in general
terms, the contribution each makes to the gastrointestinal system.
2. Know the general organization and components of the cellular layers which make
up the walls of the alimentary canal.
Overview of the Functional Anatomy of the Alimentary Canal: The gastrointestinal system
operates principally to extract energy and metabolic building blocks from the food presented to
it. This is accomplished through a series of coordinated movements that mechanically fragment
food materials and mix them with copious enzymatic secretions which chemically process the
foodstuffs into constituents small enough for absorption and utilization. In order to understand
the integrative aspects of gastrointestinal function, a review of the anatomical and histological
layout of the alimentary canal and its ancillary organs is required.
I.
Gross Anatomy of the Alimentary Canal:
To move food from the mouth to the
stomach, the chewed food must be mixed
with saliva. Saliva is produced by
Salivary glands :
several glands found within the oral
Sublingual
cavity. The primary glands are bilateral Submaxillary
pairs with the parotids, sublingual and
submaxillary being most important. The
chewed and swallowed food, now referred
to as chyme, moves through the esophagus
to the stomach. A reflex which
Liver
Gallbladde r
coordinates muscle relaxation distal to
the bolus of chyme, and contraction
Duodenum
proximal to the bolus is required to
Ascending
efficiently move the chyme into the
colon
stomach. In addition, a region of smooth
Cecum
muscle at the esophageal/stomach junction
Appendix
regulates flow into or back out
(retrograde) of the stomach. This region,
Ileum
referred to as the lower esophageal
sphincter, is also relaxed during a
coordinated swallowing reflex but quickly closes thereafter.
Pharynx
UES
Esophagus
LES
Stomach
Pancreas
Transverse
colon
Descending
colon
Jejunum
Sigmoid
Rectum
Histologically the most proximal portion of the stomach contains mucus secreting “cardiac
glands”. The segment comprising the next 70-80% of the stomach contains acidic
secreting oxyntic glands, and is referred to as either the body or fundus regions. The
segment of the distal stomach prior to the connection with the small intestine is referred to
April 22, 2005
Introduction and GI Regulation
Ron Lynch, Ph.D.
626-2472
as the pylorus. Functionally, the most proximal 1/3 of the stomach has the ability to
increase its diameter to "accommodate" the incoming bolus of chyme. Tonic
contraction/relaxation of the smooth muscle in this region allows for the initial
accommodation, but also is important for slowly contracting down on the chyme forcing
more of it aborally (away from the oral cavity) to mix with acid and begin digestion. The
distal 2/3 of the stomach carries out intense contractile activity while chyme remains
therein. The most distal region of the stomach, the pylorus, acts as a gate (pyloric
sphincter) for entrance of chyme into the small intestine. In addition, the pyloric glands in
this region secrete the hormone gastrin into the blood in response to stimulatory factors
found within the chyme in the stomach lumen.
Full digestion of nutrients in the chyme occurs within the small intestine. Functionally, the
small bowel is delineated into three regions (proximal to distal); the duodenum, jejunum
and ileum. In the duodenum, acidic chyme leaving the stomach is neutralized by HCO3 and
mixed with digestive enzymes. Both HCO3 and the enzymes are produced in the pancreas.
Bile is produced in hepatocytes, and during periods of fasting (overnight) the bile is stored
in the gall bladder where it is concentrated. The secretions from the pancreas are carried to
the duodenum via an extensive ductal system which combines with the bile ductal system
prior to entrance to the small intestine. A competent common bile duct and sphincter are
required for a normal digestive pattern in the small intestine.
As the chyme moves aborally the epithelial cells efficiently absorb the digestion products
in their simplest monomeric and dimeric forms. Most absorption of water and nutrients
occurs within the jejunum. The ileum is a specialized region containing specialized
transporters; absorption of specific vitamins and reabsorption of bile occur here. The
junction between ileum and the colon (ileocecal sphincter) regulates passage of "nondigested" chyme into the colon, but importantly retards retrograde flow of bacteria from the
colon to the ileum.
The large intestine is an important storage organ. Bacteria housed within it further digest
the chyme contents to extract all possible nutrients. The colon also reabsorbs about 5-10%
of the total water, and it is at this level that water resorption is regulated.
II. Blood Flow to the Alimentary Organs: The Splanchnic Circulation.
Splanchnic circulation refers to the vasculature which brings blood to and from the major
abdominal organs including the liver, spleen, stomach, pancreas, large and small intestine. This
vascular system is the major blood reservoir containing between 20-40% of total blood volume.
More than 60% of the total splanchnic blood volume can reside in venules making the
splanchnic venous circulation the primary source for mobilizing blood during crisis (eg.,
hemorrhage). Fluid exchange in the splanchnic system also is very high. Secretions by the
digestive tract amount to 7-8 liters/day with 1-2 liters/day of H2O ingested. This volume is
April 22, 2005
Introduction and GI Regulation
Ron Lynch, Ph.D.
626-2472
approximately 2 to 3 times the body's plasma volume. Thus, changes in fluid exchange between
the GI tract and the systemic circulation can dramatically alter blood plasma volume.
II. General Anatomical
Characteristics
Splanchnic Circulation – refers to
all organs fed by celiac, superior
mesenteric and inferior mesenteric
arteries.
Hepatic Circulation – refers to liver
blood flow.
Mesenteric circulation –- refers to
intestinal blood flow.
Organization of blood flow to and from splanchnic
organs. For most splanchnic organs blood flows to each
through a branch of one of the three major arteries from
the aorta and then collects into the portal vein to drain to
the liver. The liver also receives blood from the hepatic
arterial branch of the celiac artery. Hence, blood passes
both in parallel circuits and in series circuits through the
splanchnic vessels. Numbers in parentheses indicate the
blood flow in milliliters per minute through the named
vessels that might be expected in an average adult male.
Blood
flow
(value)
in
ml/min
Lymphatic Flow from the Alimentary Canal and GI
organs can be up to 25 ml/min
Blood Flow in the Portal Vein. Blood collected from all of the alimentary organs flows into the
portal vein. The flow of blood in the portal vein to the liver is important in metabolic clearance
of ingested substances from the blood such as nutrients (glucose), drugs and toxins. Thus, the
positioning of the liver at the entrance for blood into the systemic circulation is important for
screening substances absorbed from the alimentary canal.
III. Structural Characteristics of the Intestinal Wall
Lymph node
Villus
Epithelium and Mucosa
Muscularis mucosa
Submucosa
(contains most nerves
and bloood vessels)
Circular muscle
Longitudinal muscle
Serosa (connective tissue)
Myenteric plexus
Submucosal plexus
Gland in
submucosa
April 22, 2005
Introduction and GI Regulation
Ron Lynch, Ph.D.
626-2472
The wall of the alimentary canal exhibits a consistent arrangement along its length. The outer
surface is covered with a layer of connective tissue called the serosa. Immediately under the
serosa is a layer of smooth muscle which runs longitudinally along the length of the canal
(longitudinal muscle) used for propulsive movements. The next obvious layer of cells is another
muscle layer which encircles the canal (circular muscle), and is important for changing lumen
diameter including the sphincters. Between the muscle layers is a network of nerves that exhibit
a large amount of cell to cell synapses. This layer of nerves regulates activity in both muscle
layers and is referred to as the myenteric plexus. Under the circular muscle lies a highly
variable layer of tissue with respect to depth and content called the submucosa (i.e., under the
mucosa or lumen lining). The striking feature of the submucosa is the large amount of glandular
tissue in specific areas like the stomach. Also, on the serosal side of the submucosa next to the
circular muscle layer is a second layer of nerve cells (the submucosal plexus). Activity in these
nerves regulates circular muscle contractility, as well as, secretory activity in the glands and
absorptive activity of the epithelial lining. The epithelial cells provide several functional
characteristics to the alimentary tract, but most important these cells are responsible for all
absorption, fluid secretion and acting as a protective barrier to infiltation against bacteria and
toxins.
GASTROINTESTINAL NERVOUS SYSTEM
1. Understand the general organization and function of the enteric nervous system.
2. Determine the differential regulation of GI functions by sympathetic and
parasympathetic innervation.
3. Understand the concept of Summation of Signals and how this relates to the
integration of information determining a response.
I.
Enteric (Intrinsic) Nervous System: Within the wall of the alimentary canal are two
dense layers of neurons which are critically important for coordinating the various
functions required for digestion and absorption of nutrients. The layer of nerves found
beneath the mucosa is referred to as the submucosal plexus. Nerves within this plexus
send out processes which synapse on other nerves within the plexus (inter-neurons), as
well as, effector cells (secretory, endocrine) located within the mucosa and submucosa, and
smooth muscle cells in the circular layer.
April 22, 2005
Introduction and GI Regulation
Myenteric
ganglion
Interganglionic
fiber tract
Ron Lynch, Ph.D.
626-2472
Circular
muscle
Submucosal
ganglion
Longitudinal
200 um
Mucosa
Enteric neurons of the submucosal and myenteric plexuses in the wall of the GI tract. The plexuses consist of
ganglia interconnected by fiber tracts. (Redrawn from Wood JD: In Johnson RL, editor: Physiology of the
gastrointestinal tract, ed 2, New York, 1987, Raven Press).
The myenteric plexus of nerves reside between the circular and longitudenal smooth muscle.
These nerves primarily regulate and coordinate contractility of the muscle layers. Visceral
smooth muscle contractility is generally under inhibitory control. Since the secretory and
contractile functions of the alimentary canal are coordinated within the plexus network of the
gut, the intestinal sensing and regulation of motor activity has been referred to as a "visceral
brain." This network also allows for the elaboration of local "reflex arcs" throughout the gut.
II. Extrinsic (Autonomic) Regulation of Alimentary Functions.
A. Sympathetic Innervation: Pre-vertebral ganglia provide innervation via adrenergic
(norepinephrine) fibers to the enteric plexi, and directly to vascular smooth muscle.
The primary consequence of sympathetic stimulation is a general inhibition of motor
and secretory function which includes enhanced contraction of the alimentary
sphincters via activation through the submucosal plexus. Possibly the most important
response to sympathetic output is vasoconstriction (blood vessels) through direct
innervation of blood vessels.
B. Parasympathetic Innervation. All parasympathetic fibers synapse on neurons within the
enteric plexi of the alimentary canal. The primary response to parasympathetic
stimulation is a general increase in motor and secretory function within the alimentary
canal, and ancillary organs.
Integration of Signals. Signals are integrated primarily at the level of the enteric nervous
system, but also through long reflexes centered in the prevertebral ganglia and spinal cord.
ENDOCRINE CONTROL OF GI FUNCTION:
1.
Identify the four primary GI hormones, their primary physiological actions and
roles in regulating GI function.
2.
Understand the categories of endocrine, paracrine and neurotransmitter, and by
April 22, 2005
Introduction and GI Regulation
Ron Lynch, Ph.D.
626-2472
which of these mechanisms the GI regulatory peptides act.
The GI tract is the largest endocrine organ in the body. It produces a wide variety of
hormones and biologically active peptides. General features of gut hormones include
their peptide nature (generally 50 or so amino acid residues), synthesis in a precursor
form (generally via an N-terminal "signal sequence"), and intracellular or intragranular
processing prior to secretion.
April 22, 2005
Introduction and GI Regulation
Ron Lynch, Ph.D.
626-2472
Characteristics of the Major Gastrointestinal Hormones
I.
A.
B.
Hormones of the GI Tract
Gastrin
1.
Released by G-cells in the antral portion of stomach in response to
neural stimulation, peptides in the stomach, and stretch of the stomach
2.
Actions: Stimulates HCl secretion and growth of stomach and pancreas.
Secretin
1.
secreted by duodenal cells in response to elevated acidity and increased fat
2.
Actions: Stimulates secretion of bicarbonate by pancreas and biliary tract
C.
Cholecytokinin (CCK)
1. secreted by duodenal I-cells; stimulated by certain essential amino acids and
long-chain fatty acids in duodenum, some CCK cells in jejunum and colon.
2. Actions: stimulates pancreatic secretion of enzymes and gall bladder
contraction and promotes growth of pancreatic cells.
D. Glucose-dependent insulinotropic peptide - GIP.
1. Released from duodenal K-cells in response to luminal glucose & fat.
2. Primary action is to promote insulin release.
GI hormones can be delineated into two families. CCK and the gastrins comprise one family. A
5 amino acid stretch at the C-terminus is identical, and all gastrin-like activity is expressed by
this fragment. Gastrin and CCK elevate Ca2+ within target tissues. The second family is related
to secretin. Included in this family are VIP (Vasoactive Intestinal Peptide), GIP and glucagon;
these peptides elevate cAMP in target tissues.
III.
Distribution of GI active peptides.
The primary GI hormones are produced in and secreted by specialized cells scattered
throughout the small intestine (or distal stomach – gastrin) and specifically located within the
crypts of Lieberkuhn. These mucosal endocrine cells structurally resemble mucosal cells
involved in digestion and absorption in having brush borders, but have secretory granules
that are released to the blood (basolateral secretion) upon stimulation.
Many peptides which modulate GI function are located within enteric neurons including CCK,
substance P, neurotensin and VIP and therefore are considered neurotransmitters.
Several other peptides are active modulators of GI function, and act in a paracrine fashion (i.e.
they are secreted by specific cells and act only at sites near their site of release). These include
Somatostatin which decreases gastrin secretion and Histamine which stimulates acid secretion