GASTROINTESTINAL PHYSIOLOGY Chapter-III (Gastrointestinal Motility) Ass. Prof. Dr. Emre Hamurtekin EMU Faculty of Pharmacy

PERISTALTIS Peristaltis is a reflex response initiated when the gut wall is stretched. It occurs in all parts of the GI tract.

SEGMENTATION A 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 ACTIVITY The 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 COMPLEX A 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 contractions Phase II: Irregular spike potentials & contractions Phase III: Regular spike potentials & contractions The MMCs are initiated by motilin.

MOUTH & ESOPHAGUS

MASTICATION & SWALLOWING Mastication (chewing):  Breaks up large food particles  Mixes 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 SPHINCTER LES 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 EMPTYING Receptive 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.

VOMITING Some main reasons that trigger vomiting:  Irritation of the upper GI tract mucosa  Motion sickness  Emotionally charged stimuli  Chemical agents in the circulation stimulating CTZ Chemoreceptor Trigger Zone (CTZ): Located in the Area Postrema in the medulla Not protected by blood-brain barrier Includes chemoreceptor cells that can be stimulated by certain chemical agents. Vomiting is an example of central regulation of GI motility.

SMALL INTESTINE

INTESTINAL MOTILITY There are 3 types of smooth muscle contractions:  Peristaltis  Segmentation contractions  Tonic 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 MOTILITY Motility 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

Distension of rectum with feces initiates reflex contractions and desire to defecate. Internal anal sphincter relaxes when the rectum is distended. Internal anal sphincter: Sympathetic: excitatory Parasympathetic: inhibitory External anal sphincter: Skeletal muscle Pudendal nerve Gastrocolic reflex: Contractions of the rectum and desire to defecate initiated by distention of stomach by food

THE END