Mark Louie D. LopezCollege of SciencePolytechnic University of the Philippines

GASTROINTESTINAL

PHYSIOLOGY

OUTLINE

• Control of the GIT

• The Upper GIT – motility and secretions

– Mouth and Phayrnx

– Stomach

• Small Intestine Motility

• Migrating Myoelectric Complex (MMC)

• Intestinal Reflexes

• Large Intestine Motility

• Defecation/ Retrosphincteric Reflex

ANIMAL PHYSIOLOGY

ANIMAL PHYSIOLOGY

ANIMAL PHYSIOLOGY

CONTROL OF GASTROINTESTINAL TRACT

• Neurocrine

– Autonomic Nervous System

– Enteric Nervous System

– Somatic Nervous System

• Endocrine – Hormones

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CONTROL OF GASTROINTESTINAL TRACT

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BASIC ELECTRICAL RHYTHM OF GIT

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BASIC ELECTRICAL RHYTHM OF GIT

• Slow waves do not always reach the threshold

– When they do, they produce action potential (spikes)

• Stretching of the stomach will lead to increased

stimulation of the colon to discharge its contents.

• Acetylcholine & Parasympathetics = Increase

spiking

• Norepinephrine & Sympathetics = decreased the

activity of GIT

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BASIC ELECTRICAL RHYTHM OF GIT

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BASIC ELECTRICAL RHYTHM OF GIT

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STOMACH

• More layers compared to the rest of the GIT aside

from the longitudinal and circular layers, it has an

additional oblique layer

• Wider, thicker and has a good capability of further

crushing the food.

• Cardiac, Fundus, Pylorus

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STOMACH

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GASTRIC MOTILITY : STORAGE FUNCTION

• As food enters the stomach, the fundus and body

of the stomach enlarges – receptive relaxation

• Gastric emptying time depends on the chemical

makeup of the food

• Vagovagal reflex – muscle tone in the stomach is

reduced in order to accommodate more food

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GASTRIC MOTILITY : MIXING FUNCTION

• Food in the body and antrum increases contraction

through peristalsis and retropulsion

• Peristalsis – wavelike contractions where contents

are moved forward

• Retropulsion – upstream squeezing action

• When it comes close to pylorus, the pylorus closes;

so even though antral contractions propel food to

pylorus, it goes back to proximal area, hence there

is mixing of food

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GASTRIC MOTILITY: MIXING FUNCTION

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GASTRIC MOTILITY: EMPTYING FUNCTION

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GASTRIC MOTILITY: EMPTYING FUNCTION

• Chyme (food + gastric secretion) is propelled

through the pyloric sphincter into the intestine

• Pyloric sphincter allows only the smallest particles

to pass through, until such time that nothing is left

in the stomach

ANIMAL PHYSIOLOGY

GASTRIC MOTILITY: EMPTYING FUNCTION

• Local Reflexes

– Excitatory – stretching of antrum

– inc volume of liquid

– Inhibitory – enterogastric reflexes

• Hormones

– Excitatory – Gastrin

– Inhibitory – Cholecystokinin, Secretin:

• hypotonic/hypertonic, high caloric content (esp. fat), acidity, consistency

• Interdigestive Motor Cycle or Migrating Myoelectric

Complex

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GASTRIC JUICES COMPOSITION

• Mucus and Bicarbonate (HCO3-)

• Enzymes: Pepsins

• Hydrochloric acid (HCl)

• Intrinsic Factor (IF)

– binds Vitamin B12 and aids its

– absorption in the ileum

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PHASES OF GASTRIC SECRETION

GASTROINTESTINAL TRACT

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SMALL INTESTINE

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• The small intestine is thick-walled while large

intestine is thin-walled. Longitudinal layer of large

intestine is just a thin strip.

• The small intestine has three parts: duodenum,

jejunum and ileum.

• Small intestine is attached to a mesentery.

SMALL INTESTINE

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LARGE INTESTINE

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• The large intestine is puffy. It has a long circular

layer and a short longitudinal layer. Outpouchings

of the large intestine are known as haustra.

• The large intestine has a blind pouch, the cecum.

LARGE INTESTINE

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INTESTINES

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SMALL INTESTINE MOTILITY

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• Mixes the chyme with the digestive juices (from the

pancreas, liver, and small intestine) and bile to

facilitate digestion and absorption

• Key to proper digestion is proper mixing of chyme

with the digestive juices

• Propels the chyme from the duodenum to the colon

(aboral direction -Direction opposite the mouth)

PERISTALSIS

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SEGMENTATION

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ILIOCECAL JUNCTION

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• Close link between terminal ileum and cecum by

ileocecal ligaments – functional valve

• Prevents backflow of cecal contents

• Keeps small intestinal bacterial concentrations at

usual low levels

– Colon is the common site of intestinal bacteria; Small intestine is relatively cleaner

ILIOCECAL JUNCTION

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ILIOCECAL JUNCTION

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MIGRATING MYOELECTRIC COMPLEX (MMC)

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• Peristaltic motor patterns that occur during fasting

• Motility in interdigestive period

• Begins at the distal part of the stomach

Phases:

1. Quiescent phase

2. Increasing action potential frequency

3. Peak electrical and mechanical activity

- associated with increased GI secretions

MIGRATING MYOELECTRIC COMPLEX (MMC)

ANIMAL PHYSIOLOGY

• Phase 1: Quiescent phase

– There is no significant electrical and mechanical activity

– Small intestine is still

• Phase 2: Increasing action potential frequency

– Periods of small disorganized contractions

MIGRATING MYOELECTRIC COMPLEX (MMC)

ANIMAL PHYSIOLOGY

• Phase 3: Peak electrical and mechanical

activity

– Period in which large or intense contractions propagate along the length of the intestine

– Recall that this is the interdigestive period, there is nothing in the small intestine. What is acted upon is the residue → cleansing when there is no food being processed

– Body is capable of cleaning the small intestine

– stimulated by motilin

– hormone responsible for the Phase 3 of MMC

– secreted by upper duodenum during fasting

MIGRATING MYOELECTRIC COMPLEX (MMC)

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MIGRATING MYOELECTRIC COMPLEX (MMC)

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LARGE INTESTINE MOTILITY

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• Enhances the efficiency of water and electrolyte

absorption

• Promotes excretion of fecal material

• No distinction between fed and fasting state

• Display nocturnal suppression and can be

stimulated by a meal (especially oily food)

LARGE INTESTINE MOTILITY

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• Non-propulsive segmentation contractions (2-4

cycles/minute) result to haustrations

– Segmentation contractions chop and mix the ingested food, presenting it to the mucosa where absorption occurs, making the ingested food drier and drier every cycle. These contractions are quite prominent and form sacculations in the colon known as haustra.

LARGE INTESTINE MOTILITY

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• Mass Peristalsis (High Amplitude Propagated

Contractions / HAPCs)

– Goes up to 20-50 cm, 1 to 3 times a day

– Includes weak retrograde patterns allowing additional opportunity for absorption of water and electrolytes

– Known also as giant migrating contractions, this pattern of

motility is like a very intense and prolonged peristaltic contraction which strips an area of large intestine clear of contents

LARGE INTESTINE MOTILITY

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• Rectal motor complex

– Contractions between sigmoid colon and rectum during sleep

LARGE INTESTINE MOTILITY

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DEFECATION STIMULATION

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DEFECATION STIMULATION

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• When there is fecal material in the rectum,

stretching sends a signal to the spinal cord center

• Stretching brings about a stimulus to the rectum (even sometimes to the sigmoid) that will increase

contraction that will open the internal anal sphincter(smooth muscle)

• However, there is a higher center stimulus that can

go to the external anal sphincter (somatic,

voluntary muscle) so one can delay release of stool

by contracting or closing it.

DEFECATION STIMULATION

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DEFECATION STIMULATION

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