Gastrointestinal
Tract
Organs of the digestive system are divided into two main
groups: organs within the alimentary canal and accessory digestive organs.
The alimentary canal, which is also called the
gastrointestinal (GI) tract or gut, is the entire length of tube that winds
through the body from the mouth to the anus. It digests, breaks down and
absorbs food through its lining into the blood.
The muscular
alimentary canal
Mouth
Pharynx
Esophagus
Stomach
Small intestine
Large intestine
Anus
The accessory
digestive organs
Supply secretions contributing to the breakdown of food
Teeth & tongue
Salivary glands
Gallbladder
Liver
Pancreas
The
Digestive Process
Ingestion
–Taking in food through the mouth
Propulsion
(movement of food)
–Swallowing
–Peristalsis – propulsion by alternate contraction
&relaxation
Mechanical
digestion
–Chewing
–Churning in stomach
–Mixing by segmentation
Chemical
digestion
–Complex food molecules (carbohydrates, proteins and lipids)
broken down into chemical building blocks (simple sugars, amino acids, and
fatty acids and glycerol)
–Carried out by enzymes secreted by digestive glands into
lumen of the alimentary canal
Absorption
– Transport of digested end products into blood and lymph in
wall of canal
Defecation
– Elimination of indigestible substances from body as feces.
Layers of GI Wall
Four main layers. From inside (the lumen) to outside the
are:
Mucosa,Sub mucosa, Muscularis
(external), Serosa ( visceral peritoneum).
Mucosa is the innermost,
moist, epithelial membrane that lines the entire digestive tract.
(1) It secretes mucus, digestive enzymes, and hormones;
(2) absorbs digestive
end products into the blood and
(3) protects against
infectious disease.
•Consists of a lining epithelium, a lamina propria, and a
Muscularis mucosa.
•Epithelium - simple columnar epithelium and goblet cells
•Lamina propria - areola C.T. with capillaries and lymphoid
follicles
•Muscularis mucosa - thin layer, produces local movements of
the mucosa.
Sub mucosa is a moderately dense connective tissue layer
containing blood and lymphatic vessels, lymphoid follicles, and nerve fibers.
Muscularis (external) typically consists of smooth
muscle and is responsible for peristalsis and segmentation. Contains the my
enteric plexus of Auerbach, the other major intrinsic nerve plexus. Located
between the two layers of smooth muscle, controls motility of the G.I. tract.
Serosa ( visceral peritoneum) the
protective outer layer of the intraperitoneal organs, is the visceral
peritoneum.
Electrical
Activity Of GIT Smooth Muscle
It is consist of two
types:
Slow waves or basic electrical rhythm (BER) and Spike
Potentials.
Slow waves or basic electrical rhythm
(BER)
These are
slow, undulating changes in resting membrane potential of GIT smooth muscle.
Caused By:
Slow,
undulation of the pumping activity of Na-K Pump.
Functions:
The basic
electrical rhythm (BER) is a slow spontaneous depolarized wave in the GI.
BER of
smooth muscle cells form the basis (action potential) of stomach contractions.
The ability of BER to induce smooth muscle contraction is
altered by the activity of extrinsic
nerves and hormones, brought into play by stomach and intestine receptors.
Slow waves are not true action potential, but show
undulating changes in the resting membrane potential.
Spike
Potentials
These are true Action Potentials that occur when Resting Membrane
Potential of GIT smooth muscle rises above-40mV.
CAUSED BY:
Opening of slow Ca-Na
channels.
FUNCTIONS:Ca++
ions that enter GIT smooth muscle fiber during spike potential, cause GIT
smooth muscle to contract (i.e. Peristalsis).
Potential
Changes Of GIT Smooth Muscle Membrane
Under normal conditions, the resting membrane potential
averages about -56 mill volts, but multiple factors can change this level. When
the potential becomes less negative, which is called depolarization of the
membrane, the muscle fibers become more excitable. When the potential becomes more negative, which is called hyper polarization, the
fibers become less excitable.
Factors that
depolarize the membrane —that's make it
more excitable—are :
(1) stretching of the
muscle,
(2) stimulation by acetylcholine,
(3) stimulation by parasympathetic nerves that secrete
acetylcholine at their endings, and
(4) stimulation by several specific gastrointestinal
hormones.
Important factors
that make the membrane potential more negative—that is, hyperpolarize the
membrane and make the muscle fibers less excitable—are:
(1) the effect of nor epinephrine or epinephrine on the
fiber membrane and
(2) stimulation of the sympathetic nerves that secrete
mainly nor epinephrine at their endings.
Calcium Ions
and Muscle Contraction.
Smooth muscle contraction occurs in response to entry of
calcium ions into the muscle fiber. Calcium ions, acting through a calmodulin
control mechanism, activate the myosin filaments in the fiber, causing
attractive forces to develop between the myosin filaments and the actin
filaments, thereby causing the muscle to contract.
The slow waves do not cause calcium ions to enter the smooth
muscle fiber (only sodium ions). Therefore, the slow waves by themselves
usually cause no muscle contraction. Instead, it is during the spike
potentials, generated at the peaks of the slow waves, that significant
quantities of calcium ions do enter the fibers and cause most of the
contraction.
Tonic Contraction of Some
Gastrointestinal Smooth Muscle
Some smooth muscle of the gastrointestinal tract
exhibits tonic contraction as well as
or instead of rhythmical contractions.
Tonic contraction is continuous, not associated with the
basic electrical rhythm of the slow waves but often lasting several minutes or
even hours. The tonic contraction often increases or decreases in intensity.
Tonic contractions
(continued)- Caused by:
Continuous
repetitive spike potential
Hormonal
effects
Continuous
entry of Ca.
Neural Control of Gastrointestinal
1. Enteric Nervous System
2. Autonomic Nervous System
3. Sensory Nervous System
Enteric Nervous System It is
Intrinsic system of GIT extending from
esophagus up to anus. And it is
subdivided into two.
Myenteric or Aurbach’s Plexus. Lies between longitudinal and
circular muscles layer. And its
function is to control GIT movements.
submucosal
plexus or Meissner’s plexus: Lies in submucosa. And its function is
to control GIT secretions and blood flow. Sub serves sensory functions by receiving signals from
GIT Epithelium and from stretch receptors of GIT wall.
Autonomic Nervous System
Activity is control by sympathetic and parasympathetic nervous system.
Parasympathetic
Nervous System: Vagus Nerve innervates GIT
from esophagus up to proximal 2/3
of transverse colon.
Pelvic
Parasympathetic Nerve:(S2-S4)Innervates GIT from distal 1/3 of transverse
colon.
Functions:
Increase peristalsis
and tone.
Relaxes Sphincters.
Increase digestive
secretions.
Sympathetic Nervous System:
Preganglionic sympathetic originate in T5 TO L2 segment of
spinal cord, Pass thru sympathetic chain, synapse with
Post ganglionic neuron in celiac and mesenteric and hypo
gastric ganglia. Postganglionic fibers innervates in GIT.
Functions
Decrease peristalsis and tone
Contract sphincters
Decrease digestive secretion
Sensory Nervous System
Sensory nerve endings originate in the gastrointestinal
epithelium or gut wall and send afferent fibers to both plexuses of the enteric
system, as well as
(1) To the
prevertebral ganglia of the sympathetic nervous system,
(2) To the
spinal cord, and
(3) In the
Vagus nerves all the way to the brain stem.
Functions
These sensory nerves can elicit local reflexes within the
gut wall itself and still other reflexes that are relayed to the gut from
either the prevertebral ganglia or the basal regions of the brain.
Movements in the
Gastrointestinal Tract
Two types of movements occur in the gastrointestinal
tract:
1.Mixing
movements: ( Segmentation Contraction)
2 Propulsive
movements: (Peristalsis)
Mixing
Movements
First the intestinal wall is in relaxing state when it is
distended with chyme
(food which is mixed with secretion)
The distension of intestinal wall causes contraction in
intestine and these contraction divides the intestine into segments
Each segment is 1-2cm long As one set of segmentation
contraction relaxes, a new set often begins but the contraction this time
appears at new sites between the previous contraction.
These contraction help in chopping of chyme and promotes
mixing of food particles with the secretion of small intestine.
Segmentation contraction occurs 12time/min in duodenum and 8time/min in the
ileum.
These contraction last for 5-6sec, they occur through out
the digestive period.
These segmentation contraction are controlled by the
Myenteric plexuses of Autonomic Nervous
System.
Propulsive
peristaltic movements:
Chyme is propelled or pushed through small intestine by
peristaltic waves.
These waves are produced and response to stretch .This is
called Myenteric Reflex.
They occur in any part of small intestine and move at a velocity of 0.2-2cm/sec. They
usually dies off after travel this distance.
From here it starts a new peristalsis state and move a chyme
in forward direction, thus several peristaltic waves occur one after the other
and push the small intestine contents at the distal end of the small intestine.
Therefore due to the slow movement of peristalsis waves
usually 3-5hours are required for passage of chyme from pyrolysis to the
ileocecal valve.
These peristaltic wave don't occur alone but are
superimposed upon segmental movement of small intestine. Thus both segmental and peristalsis movements
occurs simultaneously.
