Description
• Describe the anatomical and functional characteristics of each of the
four layers of the digestive tract wall: mucosa, submucosa, muscu-
laris externa, and serosa
• Distinguish between the two types of gastrointestinal motility: seg-
mentation and peristalsis
• Explain how each of the three types of sensory receptors within the
digestive tract is stimulated: chemoreceptors, osmoreceptors, and
mechanoreceptors
• Explain how the following mechanisms regulate activity of the di-
gestive system: intrinsic nerve plexuses, extrinsic autonomic nerves,
and gastrointestinal hormones
• List the components of saliva and their functions
• Describe how salivary secretion is regulated
• Explain how swallowing takes place
• For the esophagus, stomach, small intestine, and large intestine, de-
scribe:
• Specialized anatomical modifications
• Type of motility and how it is regulated
• Types of secretions and how they are regulated
• Digestive processes that take place
• Absorptive processes that take place
digested and absorbed by the human gastrointestinal tract. For example,
cellulose, the fibrous form of plant carbohydrates, is indigestible by
humans. Normally, about 95% of ingested food materials are made avail-
able for use by the body. Interestingly, as long as food remains within the
digestive tract, it is technically outside the body. Not until the materials
have crossed the epithelium that lines the tract are they considered to have
“entered” the body.
The digestive system consists of:
• Gastrointestinal tract
• Accessory digestive organs
The gastrointestinal tract is essentially a tube that runs through the center of
the body from the mouth to the anus. This tube consists of the following
organs:
• Mouth
• Pharynx
• Esophagus
• Stomach
• Small intestine
• Large intestine
Although these organs are continuous with one another, each has important
anatomical modifications that allow it to carry out its specific functions.
The accessory digestive organs exist outside the gastrointestinal tract; how-
ever, each of these organs empties secretions into the tract that contribute to
the process of digestion. These accessory digestive organs include:
• Salivary glands
• Liver
• Gallbladder
• Pancreas
Digestive tract wall
The digestive tract wall has the same basic structure from the esophagus
through and including the colon. The four major layers within the wall are:
• Mucosa
• Submucosa
• Muscularis externa
• Serosa
Mucosa. The innermost layer of the wall is the mucosa, which consists
of a mucous membrane, the lamina propria, and the muscularis mucosa. The
mucous membrane provides important protective and absorptive functions for
the digestive tract. The nature of the epithelial cells lining the tract varies
from one region to the next. Rapidly dividing stem cells continually produce
new cells to replace worn out epithelial cells. The average life span of these
epithelial cells is only a few days. The lamina propria is a thin middle layer
of connective tissue. This region contains the capillaries and small lymphatic
vessels that take up the digested nutrient molecules. The muscularis mucosa
is a thin layer of smooth muscle. Contraction of this muscle may alter the
effective surface area for absorption in the lumen.
Submucosa. The submucosa is a thick middle layer of connective tissue.
This tissue provides the digestive tract wall with its distensibility and elas-
ticity as nutrient materials move through the system.
Muscularis externa. The outer layer of the wall is the muscularis externa.
In most regions of the tract, it consists of two layers of muscle: an inner
circular layer and an outer longitudinal layer. Contraction of the circular
layer narrows the lumen of the tube. Contraction of the longitudinal layer
causes the tube to shorten.
The muscle of the digestive tract consists of single-unit smooth muscle.
Within each layer, the muscle cells are connected by gap junctions forming
a syncytium. Action potentials generated at a given site travel along the
muscle layer. Furthermore, this muscle is self-excitable; it undergoes slow
but continuous electrical activity producing rhythmic contractions of the
digestive tract wall. The cycles of depolarization and repolarization in the
smooth muscle are referred to as slow-wave potentials. These potentials do
not reach threshold during each cycle, so contraction does not necessarily
occur with each depolarization. Smooth muscle contraction will take place
only when the slow wave actually depolarizes all the way to threshold. At
this point, voltage-gated Ca++ channels open, Ca++ ions enter the cell and one
or more action potentials are generated. These action potentials result in
phasic contractions. The force and duration of muscle contraction is deter-
mined by the number of action potentials generated. Typically, phasic con-
tractions last only a few seconds.
Muscular activity, or gastrointestinal motility, is enhanced by stretching
the muscle, as occurs with the presence of food materials and distension of
Digestive system
Anatomy of GI Tract with special reference to anatomy and functions of
stomach, ( Acid production in the stomach, regulation of acid production
through parasympathetic nervous system, pepsin role in protein
digestion) small intestine and large intestine, anatomy and functions of
salivary glands, pancreas and liver, movements of GIT, digestion and
absorption of nutrients and disorders of GIT
Energetics:
Formation and role of ATP, Creatinine Phosphate and BMR
