GASTROINTESTINAL PHYSIOLOGY

ALIMENTARY TRACT

• Provides body with a continual supply of water, electrolytes, vitamins and nutrients

ALIMENTARY TRACT requires

1. Movement of food through the alimentary tract

2. Secretion of digestive juices and digestion of the food

3. Absorption of water, electrolytes, vitamins and digestive products

4. Circulation of blood through GI organs to carry away absorbed substance

5. Control of all these functions by local, nervous and hormone

• GI smooth muscle functions as syncytium

• Longitudinal and circular muscle• Muscle fibers are electrically connected

through gap junctions allowing low resistance movement of ions from one cell to another

• Muscle bundle fuse with one another• AP elicited anywhere within the muscle

mass, it travels in all directions of the muscle

ELECTRICAL ACTIVITY OF GI SMOOTH MUSCLE

• Excited by continual slow, intrinsic electrical activity along its membrane

• 2 basic types of electrical wave• SLOW WAVES

- most of GI contractions occurring rhythmically

- not AP, slow changes in the resting membrane potential

- 3 min body of stomach; 12min duodenum; 9min terminal ileum

- Caused by complex interaction of smooth muscle and specialized cells

ELECTRICAL ACTIVITY OF GI SMOOTH MUSCLE

• SLOW WAVES - unknown cause

- Caused by complex interaction of smooth muscle and specialized cells (interstitial cells of Cajal - electrical pacemaker of smooth muscle cells)

- Do not alone cause muscle contraction- Excite intermittent spike potentials

ELECTRICAL ACTIVITY OF GI SMOOTH MUSCLE

• SPIKE POTENTIALS- True action potentials- Occurs automatically when resting

membrane potential becomes more positive (more than -40mV)

- RMP : -50mV and -60mV- 10 to 20 milliseconds- Calcium-sodium channels- Much slower to open and close- TONIC CONTRACTION > continuous

repetitive spike potential caused by hormones, or a continuous entry of calcium

ENTERIC NERVOUS SYSTEM – neural control of the GI function

• Lies entirely in the wall of the gut from esophagus all the way to anus

• 2 plexus

– MYENTERIC PLEXUS (Auerbach’s plexus)

– SUBMUCOSAL PLEXUS (Meissner’s plexus)

ENTERIC NERVOUS SYSTEM – neural control of the GI function

• MYENTERIC PLEXUS (Auerbach’s plexus)• Outer plexus• Lies between longitudinal and circular muscle• Linear chains of many interconnecting neurons• Controls mainly the GI movements

• Increase tonic contractions• Increase intensity of rhythmical contractions• Increase rate of rhythm of contractions• Increase conduction velocity of excitatory waves

ENTERIC NERVOUS SYSTEM – neural control of the GI function

• SUBMUCOSAL PLEXUS (Meissner’s plexus)• Inner plexus that lies in the submucosa• Controls mainly GI secretions and local blood

flow• Controls function ( secretion, absorption, local

contraction)

NEUROTRANSMITTERS SECRETED B Y ENTERIC NERVOUS SYSTEM

• Acetylcholine (excitatory)• Norepinephrine (inhibitory)• Adenosine triphosphate• Serotonin• Dopamine• Cholecystokinin• Substance P• somatostatin

HORMONAL CONTROL OF GI MOTILITY

• Released into the portal circulation• Exert physiological actions on target

cells with specific receptor

HORMONESHORMONE STIMULI FOR

SECRETIONSITE OF

SECRETIONACTIONS

Gastrin ProteinDistentionNerve (GRP)

G cells of antrum, duodenum, jejunum

Stimulates gastric acid secretion and mucosal growth (gastric mucosa)

Cholecystokinin

ProteinFatAcid

I cells of duodenum, jejunum, ileum

Stimulates pancreatic enzyme secretionPanncreatic bicarbonate secretion, gallbladder contraction,Inhibits gastric emptying

Secretin AcidFat

S cells of duodenum,jejunum,ileum

Stimulates pepsin secretion, biliary bicarbonate secretion, growth of exocrine pancreasInhibits gastric emptying

Gastric Inhibitory peptide (GIP)

ProteinFatCarbohydrate

K cells of duodenum and jejunum

Stimulates insulin releaseinhibits gastric acid secretion

Motilin FatAcidnerve

M cells of duodenum and jejunum

Stimulates gastric motility and intestinal motility

FUNCTIONAL TYPES OF MOVEMENT IN THE GIT

• PROPULSIVE MOVEMENT• Cause food to move forward along the tract

at an appropriate rate to accommodate digestion and absorption

• Peristalsis – basic propulsive movement of the GIT> inherent property of many syncytial smooth muscle> usual stimulus of intestinal peristalsis is distention the gut> occurs weakly or not at all with an absence of myenteric plexus

FUNCTIONAL TYPES OF MOVEMENT IN THE GIT

• MIXING MOVEMENT• Keeps the intestinal contents thoroughly

mixed at all times• Peristaltic contractions causes most of the

mixing

PROPULSION and MIXING OF FOOD IN THE ALIMENTARY

TRACT

INGESTION OF FOOD

• Hunger – determines intrinsic amount of food

• Appetite – determines the type of food preferentially being seek

Mechanism important in maintaining an adequate nutritional supply

MASTICATIONo To lubricate food• To chop feed into smaller pieces• Teeth – chewing • Anterior teeth (incisors) strong cutting

action• Posterior teeth (molars) grinding action

• Muscle mostly innervated by CN V motor branch

• Chewing process controlled mainly from nuclei in the brainstem

MASTICATION• Important in digestion• Indigestible cellulose membranes

around nutrient that must be broken before digestion

• Digestive enzymes act on the surfaces of food particles

• Chewing reflex – presence of bolus of food in the mouth initiates reflex inhibition of muscle of mastication allowing lower jaw to drop ---- initiates stretch reflex of jaw muscles leading to rebound contraction

DEGLUTITION ( swallowing)• Complicated mechanism• Pharynx subserves respiration and

swallowing• Converted for only a few seconds at a

time for propulsion of food• STAGES OF SWALLOWING

1. Voluntary stage – initiates swallowing process2. Pharyngeal stage – involuntary and

constitutes the passage of food from pharynx into esophagus

3. Esophageal stage – involuntary phase transporting food from pharynx to the stomach

DEGLUTITION ( swallowing)1. VOLUNTARY STAGE

Food ready fro swallowing, rolled into the pharynx posteriorly by the pressure of tongue upward and backward against the palate

2. PHARYNGEAL STAGE Trachea is closed Esophagus is opened Fast peristaltic wave initiated by the nervous

system of the pharynx forces bolus of food into the upper esophagus

Principally a reflex act Occurs less than 6 seconds temporarily

interrupting respiration

DEGLUTITION ( swallowing)3. ESOPHAGEAL STAGE

Functions to conduct food rapidly from esophagus to stomach

2 types of peristaltic movement PRIMARY PERISTALSIS

Continuation of the peristaltic wave beginning in the pharynx spreading into the esophagus during pharyngeal stage

SECONDARY PERISTALSIS Results from distention of the esophagus itself

by the retained food continues until all foood emptied into the stomach

STOMACH• Motor functions of the stomach

1. Storage of large quantities of food until it can be processed in the duodenum and lower intestinal tract

2. Mixing of food with gastric secretions until it forms a semi fluid mixture called CHYME

3. Slow emptying of chyme from stomach into the small intestine for proper digestion and absorption

STOMACH• Digestive juices secreted by gastric glands• Comes in contact with the stored food against

the mucosal surface• Weak peristaltic constrictor waves (mixing

waves) begins in the upper stomach• CHYME – resulting mixture that passes down

the gut after food thoroughly mixed with gastric secretions

• HUNGER CONTRACTIONS – occurs when stomach has been emptied for several hours

STOMACH• STOMACH EMPTYING• Promoted by intense peristaltic

contractions in the stomach antrum• Rate of emptying is regulated by

signals from both the stomach and duodenum

• GASTRIC FACTORS promoting emptying

• Increase food volume• Effect of gastrin (causes secretion of

highly acidic gastric juice)

SMALL INTESTINE• Movements• MIXING CONTRACTIONS (Segmentation

contractions)• Portion of small intestines becomes distended with

chyme• Elicits stretching of intrestinal wall• Contraction causes segmentation of the small intestine

• PROPULSIVE MOVEMENT• Chyme propelled through small intestine by peristaltic

movement

• ILEOCECAL VALVE • Prevent the backflow of fecal contents from

colon to small intestines

COLON

• Functions• Absorption of water and electrolytes from

chyme to form solid feces• Storage of fecal material until it can be

expelled

• Proximal half of colon – absorption• Distal half of colon – storage• Movements usually sluggish

COLON

• MIXING MOVEMENTS (haustrations)• Combined contractions of circular and

longitudinal muscle causes the unstimulated portion of thelaarge intestine to bulge outwards into baglikes.

• PROPULSIVE MOVEMENTS (Mass movements)• Propulsion in the cecum and ascending colon

resulting from slow but persistent haustral contractions

• Usually persists for 10 to 30mins and cease when they have forced a mass of feces into the rectum

DEFECATION

• Occurs when a mass of feces are moves in the rectum

• Reflex contraction of the rectum and relaxation of anal sphincter

• Defecation reflex• An intrinsic reflex mediated by local enteric

nervous system in the rectal wall

DEFECATION

• Voluntary activity1. Voluntary relaxation of external sphincter2. Elevation of intra abdominal pressure by

deep breathing moving diaphragm downwards

3. Elevation of intra thoracic pressure by closure of glottis, contraction of respiratory muscles on full lungs

4. Reflex relaxation of puborectalis muscle which aligns the rectum and anal canal