GASTROINTESTINAL PHYSIOLOGYChapter-III

(Gastrointestinal Motility)Ass. Prof. Dr. Emre Hamurtekin

EMU Faculty of Pharmacy

PERISTALTISPeristaltis is a

reflex response initiated when the gut wall is stretched.

It occurs in all parts of the GI tract.

SEGMENTATIONA segment of bowel contracts at both ends and a second

contraction occurs in the center of the segment. It is designed to retard the movement of the intestinal

contents. It provides mixing of the intestinal contents with the

digestive juice.

BASIC ELECTRICAL ACTIVITYThe spontaneous rythmic fluctuations of the

membrane potential of the smooth muscle of GI tract between about -65 and -45 mV is called ‘basic electrical rhythm, BER’.

The BER itself rarely causes muscle contraction.Spike potentials over the depolarizing portion of the

BER waves increase muscle tension.BER coordinates peristaltic activity and setting the

rhythm of segmentation.Ach increases the number of spikes and the tension

of the smooth muscle, but epinephrine decreases.

MIGRATING MOTOR COMPLEXA modification in the electrical and motor activity

in smooth muscle of GI tract between periods of digestion.

The cycles (MMC) of motor activity migrate from the stomach to the distal ileum.

Phases of MMC:Phase I: No spike potentials, no contractionsPhase II: Irregular spike potentials & contractionsPhase III: Regular spike potentials & contractions

The MMCs are initiated by motilin.

MOUTH & ESOPHAGUS

MASTICATION & SWALLOWINGMastication (chewing):

Breaks up large food particlesMixes the food with the salivary secretions

Swallowing is a reflex response triggered by afferent impulses in,

trigeminal,glossopharyngeal,vagus nerve

efferent impulses in trigeminal, facial,hypoglossal nerve

LOWER ESOPHAGEAL SPHINCTERLES is tonically active

but relaxes on swallowing.

The tone of the LES is under neural control.

Ach ...intrinsic sphincter contraction

NO & VIP…....intrinsic

sphincter relaxation

STOMACH

GASTRIC MOTILITY and EMPTYINGReceptive relaxation:

Relaxation of the fundus and the upper part of the body of the stomach to accommodate the food, when food enters the stomach.

After receptive relaxation, peristaltis begins and mixes the food and permits the semiliquid portions to pass through the pylorus.

CCK and secretin stimulates the pyloric sphincter and prevents regurgitation.

Gastric emptying rate depends on;Type of food ingested

(carbohydrate rich > protein rich > fat rich)Osmotic pressure of the material entering the duodenum

Hyperosmolality decreases the gastric emptying

Fats, carbohydrates and acid in the duodenum inhibit gastric acid / pepsin secretion and gastric motility.

CCK also can inhibit gastric emptying.

VOMITINGSome main reasons that trigger vomiting:

Irritation of the upper GI tract mucosaMotion sicknessEmotionally charged stimuliChemical agents in the circulation stimulating CTZ

Chemoreceptor Trigger Zone (CTZ):Located in the Area Postrema in the medullaNot protected by blood-brain barrierIncludes chemoreceptor cells that can be stimulated

by certain chemical agents.Vomiting is an example of central regulation of GI

motility.

SMALL INTESTINE

INTESTINAL MOTILITYThere are 3 types of smooth muscle contractions:

PeristaltisSegmentation contractionsTonic contractions

Tonic contractions are relatively prolonged contractions that in effect isolate one segment of the intestine from another.

Segmentation contractions retard

Tonic contractions transit time

COLON

COLON MOTILITYMotility in this segment is slow. Ileocecal valve is shut by the increases in colonic pressure

and opened increases in ileal pressure.Gastroileal reflex:

Relaxation of cecum and increase in the passage of small

intestinal content when food leaves the stomach.3 types of colonic smooth muscle contractions:

Peristaltic waves Segmentation contractions Mass action contraction*** (occurs only in the colon)

Mass action contraction:

Simultaneous contractions over large confluent areas occurs

about 10 times / day that move the material from one portion of

the colon to another.

ILEOCECAL VALVE

DEFECATION

DEFECATIONDistension of rectum with feces initiates reflex

contractions and desire to defecate.Internal anal sphincter relaxes when the rectum is

distended.Internal anal sphincter:

Sympathetic: excitatoryParasympathetic: inhibitory

External anal sphincter:Skeletal musclePudendal nerve

Gastrocolic reflex:Contractions of the rectum and desire to defecate initiatedby distention of stomach by food

THE END