BHS 116.2 – Physiology
Notetaker: Vivien Yip
Date: 1/16/2013, 1st hour
Page1
Final is 60-65% new material (post Test 2 material – G.I. physiology and pathology)
Lecture 13 Intro to G.I. Tract, Stomach, Functions and G.I. Motility
The Alimentary Tract
- Food is not considered “internal” to the body until nutrients are absorbed into the blood
o Bacterial in GI tract, food is still external
- Provides body with water, electrolytes and nutrients
- Food is moved through the alimentary tract
o Motility
- Food is digested in alimentary tract through secretion of digestive juices
o Digestion
o CHO, fats, amino acids
- Digested food products are absorbed and carried away by the circulation
o Absorption
- All of these processes require specific secretions in different regions of GI tract that help with the
different processes
- Nervous and hormonal systems control these functions
o Stimulating/inhibiting
Describe the layers of a typical GI tract cross section
Typical cross section of the gut
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Mucosa
o Inner to outer layers:
o Mucous membrane (epithelium),
direct contact with food products
o Lamina propria
o Muscularis mucosa, smooth
muscle
Submucosa
o Connective tissue
o Larger vessels
Muscularis externa
o Circular muscle (inner)
o Longitudinal muscle (outer), runs
length of the tube
o Allow for different types of
movements
Serosa
o Serous covering that lines the GI tract
o In contact with abdominal cavity
Same regions along entire length of GI tract, different thicknesses of different layers
BHS 116.2 – Physiology
Notetaker: Vivien Yip
Date: 1/16/2013, 1st hour
Page2
Describe GI motility- depolarization, hyperpolarization, BER, spike potential, hormonal control of
gastric motility, peristalsis
GI Motility
- GI smooth muscle
o Functions as a syncytium, muscle cells contract together in unison
o Normal resting potential is -56mV
o Excited by continual intrinsic electrical activity
 Always has some type of resting tone to the muscle
o Main function: Mixes and moves contents of digestive tract
Slow Waves
- Set by basic eletrical rhythm (BER) of GI tract
- Slow undulating changes in the RMP (resting membrane potential)
- Intensity varies from 5-15mV
- Frequency varies from 3-12 waves per minute depending upon location in the GI tract
- Not action potentials
o All subthreshold potentials (below -40mV)
o Never trigger the threshold potential
- Baseline fluctuation in the membrane potential
o Do not usually cause muscle contraction, only gives basal tone
- Initiated and maintained by non-contractile cells called Interstitial cells of Cajal
o Pacemakers
o Cyclic variations in Ca++
- Held together via gap junctions and allow movement of ions from cell to cell,
- Can get small changes in membrane potential as calcium comes in, get upward depolarization, as
calcium leaves, potential goes down
- Calcium is what contributes to the tone of smooth muscle
o Control the appearance of spike potentials
- Depending on where we are in the slow wave, it will determine whether an external stimulus will
trigger an action potential or spike
potential (at the peak or at the valley of
the slow wave)
Spikes
- Action potentials
- Occur automatically when RMP
(resting membrane potential) of GI
smooth muscle becomes greater than 40mV
o When slow waves reach
threshold
o Occur in presence of an
external stimulus (neuronal,
hormonal, or stretch)
o If trigger occurs at the peak of
slow wave potential, will get an action potential
- At the valley of a slow wave potential, the stimulus may not be strong enough to elicit threshold
to be reached and thus an action potential
- Slow waves do control the appearance of an action potential (AP)
- Actional potentials are due to calcium-sodium channels
o APs resemble those of cardiac pacemaker cells
o Slow to open and close resulting in long action potentials (10-20ms)
BHS 116.2 – Physiology
Notetaker: Vivien Yip
Date: 1/16/2013, 1st hour
Page3
Tonic Contraction
- Seen in some smooth muscle sphincters of the GI tract
- Continuous contraction NOT associated with slow waves
- Constant influx of calcium to allow for constant closure of sphincters
- Caused by
o Continuous repetitive spike potentials (action potentials)
o Hormones which bring about depolarization without action potentials
o Continuous entry of calcium into the cell
Depolarizing & Hyperpolarizing factors on GI smooth muscle
Depolarizing
Stretching of the smooth muscle (food entering
region of GI tract will stretch the muscle)
Acetylcholine (from intrinsic nerves)
Parasympathetic nerves secretion of acetylcholine
(from extrinsic nerves)
GI hormones
Cause contraction of GI smooth muscle
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Hyperpolarizing
Norepinephrine (intrinsic nerves)
Sympathetic nerve secretion of norepinephrine
(extrinsic nerves)
Adrenal secretion of epinephrine
Inhibitory to contraction, puts membrane potential
further away from threshold, will not be able to
trigger AP in a hyperpolarized state
Intrinsic nerves are within and enclosed from the GI tract
Extrinsic nerves are outside the GI and can stimulate it
Will not get digestion during a fight/flight reaction in sympathetic state
Describe the components and function of the enteric nervous system
Enteric Nervous System (Intrinsic Nervous System)
- Neurons are entirely in the wall of the gut from the esophagus to anus (intrinsic)
- Gets sensory input from sensory nerves
with endings in the GI epithelium
- Can function independently of CNS
(extrinsic nervous system)
o Depend only on the intrinsic
system
- Have afferent and efferent neurons of
walls of GI tract
- Even though independent of CNS, they
are still innervated by extrinsic
sympathetic and parasympathetic
systems
o Intrinsic can function on its own but still has input from sympathetic and parasympathetic
neurons
- Inhibitory and excitatory
o Depending on which neurons are stimulated
- Function in motility and secretion of GI tract
BHS 116.2 – Physiology
Notetaker: Vivien Yip
Date: 1/16/2013, 1st hour
Page4
Enteric Nervous System has 2 plexii
1. Myenteric plexus (Auerbach’s)
- Located between longitudinal and circular muscle layers
- Controls GI contractions and movements
- Major role in moving contents along length of GI tract
- Myenteric = movement
- Stimulates and inhibits tonic contraction or tone on the gut wall
- Modifies intensity and rate of the rhythmical contractions (depending on other influences:
extrinsic nervous system or hormones in the GI tract)
- Results in faster/slower movement of the peristaltic waves
- Excitatory and inhibitory contractions
2. Submucosal plexus (Meissner’s)
- Major role: Controls GI secretion
- Minor role: absorption and blood flow
- Submucosal = secretion
GI Reflexes
- Triggered by chemoreceptors, mechanoreceptors and osmoreceptors all along the GI tract
- Reflexes that are integrated entirely within the enteric nervous system (short reflexes)
o Control secretion, peristalsis (movement of food) and mixing contractions
o From one adjacent portion of GI tract to the other
- Reflexes from the gut/GI tract to the sympathetic ganglia and then back to the GI tract (long
reflexes)
o Involve the extrinsic nervous system
o Control long distance signals
o Ex. Signals from the colon to inhibit stomach motility
 Going from one region to a distal region of the GI tract
o Ex. Colon to spinal cord up to brain, back down spinal cord to the stomach
- Reflexes from gut to spinal cord or brain and back to the gut (long reflexes)
o Control gastric motor and secretory activity
Hormones also control GI motility:
CCK, Choleocystokinin
- Secreted by cells in the mucosa of the duodenum and jejunum (first two parts of small intestine)
- Secreted in response to breakdown products of FATs
- Increases contractility of the gall bladder resulting in bile secretion into small intestines
o Bile helps with the absorptive process of fats and breakdown of fats
- Inhibits stomach (gastric) motility (primarily emptying)
o Prevent further food to be moved from stomach down to GI tract
Secretin
- Secreted by the cells in the mucosa of the duodenum
- Secreted in response to acidic gastric juice emptying into the duodenum
- Inhibits gastric motility (primarily emptying)
o Prevent movement of food further down the small intestines
BHS 116.2 – Physiology
Notetaker: Vivien Yip
Date: 1/16/2013, 1st hour
Page5
GIP, Gastric Inhibitory Peptide
- Secreted by cells in the mucosa of the upper small intestine
- Secreted in response to (major) fatty acids, (minor response) to amino acids and carbohydrates
- It can inhibit gastric motility
o But, Main function is to stimulate insulin release
- One of the incretins discussed previously in stimulating insulin release, primes the blood for
glucose and amino acids
GI Tract Movement
1. Propulsive movement (peristalsis)
- Move food length wise down GI tract
- Controlled by longitudinal muscle layer
- Causes food to move forward to an appropriate rate to accommodate digestion and absorption
- Usual stimulus is distension of the gut
- Requires an active myenteric plexus
2. Mixing movement
o Keeps intestinal content mixed
o Controlled by circular muscle layer
o Primary role is to make sure everything is mixed up in small intestines (where most
digestion takes place here)
o Have to mix enzymes around to get optimal contact between molecules of protein
carbohydrates and fat
o Ensure proper digestion
Describe the splanchnic circulation and countercurrent blood flow
One of the main blood source to GI:
The Splanchnic Circulation
- Blood supply to stomach and esophagus
- Consists of the blood vessels that supply the (lower) GI tract along with the spleen, pancreas, and
liver
- Blood courses through the gut, spleen, or pancreas then goes to the portal vein of the liver
- When food is absorbed into bloodstream, goes through liver to be processed before it enters
general circulation
- Liver will have a chance to detoxify any bacteria before releasing into the blood
- Increased GI activity causes increased blood flow to the GI tract
- Increased metabolic rate of the GI tissue (due to peristalsis, absorption, mixing movement)
decreases oxygen concentration resulting in vasodilation of the local blood vessels
o Vasodilation to bring more oxygen rich blood to metabolic organ
- GI mucosa and glands secrete vasodilators during the digestive process along the GI epithelium
o Include vasoactive intestinal peptide (VIP) and bradykinin
BHS 116.2 – Physiology
Notetaker: Vivien Yip
Date: 1/16/2013, 1st hour
Page6
Describe the process of food ingestion from mouth to stomach – voluntary, pharyngeal & esophageal
steps
Ingestion of Food
1. Mastication (chewing)
- Chewing reflex
o Food in the mouth initiates a relaxation of the jaw muscles resulting in lower jaw
dropping
o Jaw drop initiates a stretch reflex in the jaw muscles (receptors here that trigger
contraction) resulting in a rebound contraction
o This contraction compresses the food against the wall of the mouth which inhibits the jaw
muscle again
 Rebound chewing reflex
 This is repeated again and again
o Teeth are designed for cutting (incisors) and grinding (molars) to break down food into
more easily digestible fragments so stomach and itnestines don’t have to work so hard
Why is chewing important?
- When eating fruits and vegetables it breaks down undigestable cellulose to release nutrient
portions
o Can extract some nutrients from cellulose
o We cannot break down the fiber portion of cellulose
- It increases the surface area of the food making it easier for digestive enzymes to work
o Important for enzymes to access and digest them properly
- It also makes it easier for food to pass down the GI tract
Ingestion of Food
2. Swallowing (deglutition)
- 3 stages
o Voluntary stage (oropharyngeal)
 Initiates the swallowing process
 Food is squeezed into the pharynx (back of the mouth region) by the tongue
 Opening of our throat
 From this point on swallowing becomes entirely (involuntary) automatic and
cannot be stopped
o Pharyngeal stage
 Involuntary
 Food passes through the pharynx into the esophagus
 Occurs in less than 2 seconds & interrupts respiration
- What prevents food from going down to trachea or through nose?
- Impulses pass to the brain stem initiating a series of automatic pharyngeal muscle contractions
o Autonomic reflex
o Involuntary movement:
 Soft palate pulls upward preventing reflux of food into the nasal cavity
 Movement of the vocal cords and epiglottis prevent food from entering the
trachea
- The upper esophageal sphincter (pharyngoesophageal) relaxes allowing food to move easier into
the esophagus
- Once food moves through the sphincter, there is an automatic reaction that occurs:
o Relaxation of pharynx allows bolus of food to start moving to esophagus
o Contraction of pharynx contract and squeeze food down into esophagus
- The entire wall of the pharynx contracts propelling food into the esophagus
BHS 116.2 – Physiology
Notetaker: Vivien Yip
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Date: 1/16/2013, 1st hour
Page7
Esophageal stage
 Involuntary
 Occurs in less than 8-10 seconds
 Promotes passage of food from the pharynx to the stomach
Primary peristalsis
 Caused by stretching of esophagus by food
 Continuation of the peristaltic wave that began in the pharynx
 Longitudinal muscle layer
 Enough to get food down to the stomach, if any food left that doesn’t make it all
the way down, can get a secondary wave of peristalsis to help move anything left
over
Secondary peristalsis
 Results from distention of the esophagus by retained food
 If there is anything left over from the primary peristalsis movement
Gastroesophageal sphincter
 Normally remains tonically constricted
 Not a “true” sphincter
 A valve like closure of the lower esophagus prevents reflux of stomach acid
 A receptive relaxation of the sphincter precedes the peristaltic wave allowing
food into the stomach
Clicker Q
The myenteric plexus controls GI movement/motility
- True