Post on 03-Jun-2018
8/12/2019 GIT Physiology CHAPTER NO 62 Guyton by Dr. Roomi
1/41
Physiology of GITChapter No=62
By
Dr. Mudassar Ali Roomi (MBBS, M.Phil.)
Assistant Professor Physiology
8/12/2019 GIT Physiology CHAPTER NO 62 Guyton by Dr. Roomi
2/41
GIT
Function:The GI tract provides a continuoussupply of water, nutrients and electrolytes to thebody.
To achieve this target we need:1. Movement (Motility)
2. Secretion of glands
3. Absorption of nutrients
4. Circulation of blood (splanchnic circulation)5. Control of all these functions (nervous and
hormonal)
8/12/2019 GIT Physiology CHAPTER NO 62 Guyton by Dr. Roomi
3/41
Two Types of Digetsive Systems
Incomplete digestivesystem
One-way, saclikedigestive cavity
Complete digestive
system A tube with an
opening at each end
8/12/2019 GIT Physiology CHAPTER NO 62 Guyton by Dr. Roomi
4/41
Major Components of GIT
Mouth (oral cavity)
Pharynx (throat)
Esophagus (food pipe)
Gut
Stomach Small intestine
Duodenum jejunum ileum
Large intestine Cecum appendix colon Rectum Anus
8/12/2019 GIT Physiology CHAPTER NO 62 Guyton by Dr. Roomi
5/41
8/12/2019 GIT Physiology CHAPTER NO 62 Guyton by Dr. Roomi
6/41
Accessory Organs
Salivary glands (sublingual, submandibular and parotidglands)
Secrete saliva.
Pancreas: Secretes digestive enzymes.
Liver:
Secretes bile. Gallbladder:
Stores and concentrates the bile.
8/12/2019 GIT Physiology CHAPTER NO 62 Guyton by Dr. Roomi
7/41
Typical cross section of intestine
A typical cross section
shows:
1. Mucosa.
2. Submucosa.
3. Muscularis externa:
Circular muscle layer.
Longitudinal musclelayer.
4. Serosa.
8/12/2019 GIT Physiology CHAPTER NO 62 Guyton by Dr. Roomi
8/41
Mucosa: First Layer (innermost layer)
Mucosa= epithelium andlamina propria
Small amount of smoothmuscle (muscularis
mucosae) is also present
Lumen: passageway.
Epithelium may havemucus secreting glands
(also secrete digestiveenzymes)
Secretes and absorbs.
8/12/2019 GIT Physiology CHAPTER NO 62 Guyton by Dr. Roomi
9/41
Second Layer: Submucosa
Contains loose
Connective Tissue.
Contains Glands, blood
vessels, lymphatics andnerves.
Carries away absorbed
materials.
8/12/2019 GIT Physiology CHAPTER NO 62 Guyton by Dr. Roomi
10/41
Third Layer: Muscularis externa
Provides the tube
movements (peristalsis)
Has two coats of
smooth muscle tissue: Inner Circular fibers
(increase and decrease
tube diameter)
Outer Longitudinal fibers(lengthen and shorten
the tube)
8/12/2019 GIT Physiology CHAPTER NO 62 Guyton by Dr. Roomi
11/41
Fourth Layer: Serosa (serous layer)
Outer covering of the
tube.
Visceral peritoneum.
Secretes serous fluid tokeep the outside of the
organs lubricated and
moist, thus performs a
protective function.
8/12/2019 GIT Physiology CHAPTER NO 62 Guyton by Dr. Roomi
12/41
Neural Control of the GI Function
Neural Controls: Extrinsic (3)
Parasympathetic. Sympathetic. Somatic.
Intrinsic (2) Myenteric plexus. Submucosal plexus.
SANS and PANSmodulate the enteric
nervous system asopposed to directlycontrolling the smoothmuscle of the bowel.
8/12/2019 GIT Physiology CHAPTER NO 62 Guyton by Dr. Roomi
13/41
Extrinsic control
Autonomic Neural Pathways:Parasympathetics
Upper/Cranial via Vagus nerve:
Innervates the upper segments of GI tract to Splenic flexure
along with the pancreas.
Lower/Sacral via Pelvic Splanchnic nerves (nervi erigentes)
S2-S4to the descending colon, rectum and anus.
Post Ganglionic Neurons:
Located mainly in the Myenteric and Submucosal plexuses.
Neurotransmitter:
Acetylcholine. Function:
Stimulates GI secretion, motor activity.
Relaxes sphincters
8/12/2019 GIT Physiology CHAPTER NO 62 Guyton by Dr. Roomi
14/41
Extrinsic controlAutonomic Neural Pathways:
Sympathetics Fibers originate in the spinal cord between segments T-5 and L-2.
Preganglionic fibers enter the Sympathetic chains, then pass on toCoeliac and Mesenteric ganglia (PREVERTEBRAL ganglia)
Post Ganglionic Fibers:
They originate in these ganglia and pass on to all parts of the GITthrough sympathetic nerves.
Neurotransmitter:
Norepinephrine.
Some amounts of Epinephrine.
Function: Inhibition of GI secretion, motor activity.
Contraction of GI sphincters and blood vessels.
8/12/2019 GIT Physiology CHAPTER NO 62 Guyton by Dr. Roomi
15/41
8/12/2019 GIT Physiology CHAPTER NO 62 Guyton by Dr. Roomi
16/41
Intrinsic/ Enteric Nervous System:
Submucosal (Meissners)plexus.
Myenteric (Auerbachs)
plexus.
Regulate segment-to-segment movement of
the gastrointestinal
tract.
May be considered a 3rd
part of the ANS.
8/12/2019 GIT Physiology CHAPTER NO 62 Guyton by Dr. Roomi
17/41
Enteric Nervous SystemMyenteric plexus (Auerbachsplexus):
Locatedbetween thelongitudinal and circularlayers of muscle in the wallof the GIT.
Controls tonic and
rhythmic contractions. Exerts control primarily
over digestive tractmotility.
Principal effects: Increased tone.
Increased intensity ofcontractions.
Increased rate ofcontractions.
Increased velocity ofconduction.
8/12/2019 GIT Physiology CHAPTER NO 62 Guyton by Dr. Roomi
18/41
Myenteric plexus (Auerbachsplexus):
It is not totally excitatory in nature.
Some of the neurons of the myenteric nervous
system secrete VIP. This is an inhibitory peptide.
These inhibitory signals are useful for controlling
the intestinal sphincters e.g. pyloric sphinter
Partially controlled by autonomic nervous
system.
8/12/2019 GIT Physiology CHAPTER NO 62 Guyton by Dr. Roomi
19/41
Enteric Nervous System
Submucosal plexus (Meissnersplexus)
Buried in the Submucosa.
Senses the environmentwithin the lumen.
Regulates GI blood flow.
Controls epithelial cellfunction (local intestinalsecretion and absorption).
May be sparse or missingin some parts of GI tract.
Partially controlled byautonomic nervoussystem.
8/12/2019 GIT Physiology CHAPTER NO 62 Guyton by Dr. Roomi
20/41
Enteric Nervous System
Types of Neurons in the Enteric System:
1. Sensory neurons: Chemoreceptors sensitive to acid, glucose and amino acids have
been demonstrated which, in essence, allow "tasting" of luminalcontents. Sensory receptors in muscle respond to stretch andtension.
2. Motor neurons:
Control GI motility and secretion, and possiblyabsorption.
3. Interneurons: Largely responsible for integrating information from
sensory neurons and providing it to motor neurons.
8/12/2019 GIT Physiology CHAPTER NO 62 Guyton by Dr. Roomi
21/41
Types of Enteric Neurotransmitters in enteric
neurons
Acetylcholine: Excitatory
Stimulates smooth muscle contraction.
Increases intestinal secretions.
Releases enteric hormones.
Dilates the blood vessels.
Norepinephrine: Derived from extrinsic sympathetic neurons.
Inhibits the GI activity.
Causes vasoconstriction
Others (some are excitatory some are inhibitory) Adenosine tri phosphate (ATP), Serotonin, Dopamine,
Cholecystokinin, VIP, Somatostatin, Substance P,
Leu-enkephalin, Met-encephalin and Bombesin.
8/12/2019 GIT Physiology CHAPTER NO 62 Guyton by Dr. Roomi
22/41
Electrical activity in the Smooth Muscle of GIT
Smooth muscle in the GIT is excitable.
Slow, intrinsic electrical activity takes place
along the membranes of this smooth muscle.
The GI smooth muscle acts as a functional syncytium.
Due to presence of Gap junctions.
Therefore, whenever an action potential is generated
within the muscle mass, it travels in all directions.
8/12/2019 GIT Physiology CHAPTER NO 62 Guyton by Dr. Roomi
23/41
Electrical activity in GIT
The RMP is -50 to -55
mV. This can be
changed to different
levels to control themotor activity of the
gut.
There are two types of
electrical waves: Slow waves-
Spike potentials.
8/12/2019 GIT Physiology CHAPTER NO 62 Guyton by Dr. Roomi
24/41
Slow Waves/ Basic Electrical Rhythm of the gut.
These are spontaneous,rhythmic fluctuations inthe RMP between -55 to -40 mV.
These are not true actionpotentials, but are
localized/graded potential They do not cause muscle
contraction in the GIT. Frequency of slow waves
in different parts of gut: 3/ min in the body of the
stomach. 12/ min in the duodenum. 8-9/ min in the terminal
ileum.
8/12/2019 GIT Physiology CHAPTER NO 62 Guyton by Dr. Roomi
25/41
Cause of Slow Waves
Pacemakercells/Interstitial cells ofCajal:
These pacemaker cells
undergo cyclic changesin the membranepotential due to uniqueNa ion channels which
periodically open andproduce inward,pacemaker currantswhich in turn generateslow waves.
8/12/2019 GIT Physiology CHAPTER NO 62 Guyton by Dr. Roomi
26/41
Slow Waves
Function: They lead the RMP to
threshold value so that thespike potential can take place.
Stimulated by(which causeshypopolarization):
Stretch. Acetylcholine. Parasympathetic nervous
system (vagus) Parasympathomimetic drugs
e.g. Pilocarpine Slow wave activity is greatly
reduced by Vagotomy,Norepinephrine andsympathetics (which causeshyperpolarization)
8/12/2019 GIT Physiology CHAPTER NO 62 Guyton by Dr. Roomi
27/41
Spike Potentials
These are true action potentials.
They occur whenever the RMP of the smooth musclebecomes more positive than -40 mV (threshold)
Higher the slow wave potential, greater is the frequency ofthe spike potentials
Each spike lasts for up to 10- 20 m sec which is 10- 40 timeslonger than the action potential in the nerves fibers.
Ionic basis of action potentials/spike potential: In GIT, the ion channels involved are slow Calcium- Sodium
channels. The slowness accounts for the long duration of the action
potential in the GI smooth muscle.
8/12/2019 GIT Physiology CHAPTER NO 62 Guyton by Dr. Roomi
28/41
Three types of Gastrointestinal Reflexes
1. Reflexes that are integrated entirely within the enteric nervoussystem. These are the reflexes which control: Secretion.
Peristalsis.
Contractions etc.
2. Reflexes from the gut to the prevertebral sympathetic ganglia andthen back to the GI tract. Gastrocolic reflex.
Enterogastric reflex.
Colonoileal reflex.
3. Reflexes from the gut to the spinal cord or brain stem and thenback to the GI tract. Reflexes from stomach and duodenum to the brain stem and back via
the vagus nerve.
Pain reflexes.
Defecation reflexes.
8/12/2019 GIT Physiology CHAPTER NO 62 Guyton by Dr. Roomi
29/41
8/12/2019 GIT Physiology CHAPTER NO 62 Guyton by Dr. Roomi
30/41
1. GASTRIN:
Secretion: By G cells in gastric pits of the mucosa.
Stimulus: Stomach distention , the products ofproteins and Gastrin releasing peptide, which isreleased by the nerves of the gastric mucosa as aresult of Vagal stimulation.
Actions:
1. Increases HCl production in stomach.
2. Increases gastric motility. 3. Stimulates growth of gastric mucosa.
4. Contracts lower esophageal sphincter.
5. Relaxes pyloric sphincter.
8/12/2019 GIT Physiology CHAPTER NO 62 Guyton by Dr. Roomi
31/41
2. CHOLECYSTOKININ (CCK):
Secretion: By I cells in the mucosa of the duodenumand jejunum.
Stimulus: Chyme rich in digestive products oftriglycerides and fatty acids and monoglycerides and
amino acids.Actions: Contracts the gallbladder.
Opens the Sphincter of Oddi.
Increases secretion of pancreatic juice rich in digestiveenzymes.
Inhibits gastric secretion and motility.
May reduce hunger.
8/12/2019 GIT Physiology CHAPTER NO 62 Guyton by Dr. Roomi
32/41
3. SECRETIN
Secretion: By S cells in the mucosa of theduodenum.
Stimulus: Acid chyme (H+), fatty acids in the smallintestine causes secretion of Secretin.
Actions: Stimulates the watery secretion of pancreatic juice and
bile that is rich in bicarbonate ions. MCQ
Inhibit production of HCl in stomach.
Promotes growth and maintenance of the pancreas. Enhances the effects of Cholecystokinin (CCK)
8/12/2019 GIT Physiology CHAPTER NO 62 Guyton by Dr. Roomi
33/41
8/12/2019 GIT Physiology CHAPTER NO 62 Guyton by Dr. Roomi
34/41
5. Motilin
Secretion: By the cells of the upper
duodenum.
Stimulus: It is released during fasting.
Actions:
Increases the gastrointestinal motility.
Causes formation of Interdigestive myoelectric
complexes/Migratory motor complexes (MMC) in afasting person.
House keeping function
8/12/2019 GIT Physiology CHAPTER NO 62 Guyton by Dr. Roomi
35/41
8/12/2019 GIT Physiology CHAPTER NO 62 Guyton by Dr. Roomi
36/41
Functional Types of Movements in the GIT
1. Propulsive movements/peristalsis
2. Mixing movements.
Both are brought about by the Enteric nervous system, but
are influenced by the Extrinsic nervous system, especially theParasympathetic nervous system.
8/12/2019 GIT Physiology CHAPTER NO 62 Guyton by Dr. Roomi
37/41
Propulsive movements/peristalsis
It is an inherent property of many syncitial smoothmuscle tubes.
Occurs in the gut, bile ducts, glandular ducts andureters.
Stimulus: Distension of the gut, physical or chemical irritation of the
surface epithelium and stimulation by the Parasympatheticnervous system.
Controlled by: Actual peristalsis is brought about by the Myenteric
nervous system.
But it is influenced by the Parasympathetic nervoussystem.
8/12/2019 GIT Physiology CHAPTER NO 62 Guyton by Dr. Roomi
38/41
Propulsive movements/peristalsis
Mechanism of myenteric
reflex or the peristalticreflex: Distension of the gut-
Stimulation of the entericnervous system.
The gut wall contracts 2-3 cmbehind the point of stimulus-formation of a contractilering.
Initiation of a wave ofcontraction- travels in the
anal direction. Relaxation of the gut wall
distal to the point ofstimulation- receptiverelaxation.
8/12/2019 GIT Physiology CHAPTER NO 62 Guyton by Dr. Roomi
39/41
Function of the Myenteric plexus inperistalsis:
Weak or no peristalsis in areas of the gut where
the Myenteric plexus is absent ( e.g. congenitalmegacolon/Hirschprungsdisease)
If the Cholinergic nerve endings of the Myentericplexus are blocked by Atropine- No peristalsis.
Therefore, effectual peristalsis requires an intactfunctional Myenteric plexus.
8/12/2019 GIT Physiology CHAPTER NO 62 Guyton by Dr. Roomi
40/41
Properties of peristalsis:
1. Directional movement: Theoretically, it can occur in both the orad and anal directions.
But the wave of peristalsis dies out if it travels in the orad direction.
Exact reason is not known.
Probably because of the fact that the Myenteric plexus is itself polarized.
2. Receptive Relaxation: Part of the gut immediately after the peristaltic ring relaxes to allow the
easy movement of food.
Is a function of the Myenteric plexus.
This relaxation takes place only the anal direction, ahead of the contractilering.
Therefore, peristalsis moves towards the anus.
3. The anal ward moving complex in association with the Myentericplexus is called the Myenteric/ peristaltic reflex. The peristaltic reflex Plus the anal direction of the movement is called the
Law of the Gut.
8/12/2019 GIT Physiology CHAPTER NO 62 Guyton by Dr. Roomi
41/41
Mixing Movements
Differ in most parts of the gut. Segmentations- in smallintestine
Haustrations- in large intestine In many cases, most of the
mixing is provided byperistalsis itself, especially
when it pushes the foodagainst a closed sphincter. Local , intermittent,
constrictive contractionsoccur at some places in thegut. They are present afterevery few cms.
They last for 5 to 10 secs.
Then new contractionsappear at new places.
This pattern is calledchopping and shearing offood.