Anatomically, the stomach is usually divided into two major parts: (1) the body (2) the antrum. Physiologically, it is more appropriately divided into the “orad” portion, comprising about the first two thirds of the body the “caudad” portion, comprising the remainder of the body plus the antrum

Basic electrical rhythm They are the slow,undulating changes in the RMP of smooth muscle of GIT. Intensity-5 to 15 mv Frequency-3 to 12/min 3/min-In body of stomach 12/min-In duodenum 8 to 9/min-In terminal ileum

Causes of slow waves- Complex interactions among smooth muscle cells and interstitial cells of Cajal. Interstitial cells undergo cyclic changes in membrane potential due to unique ion channels that periodically open and produce inward current which generate slow wave activity.

Peristaltic wave It is the basic propulsive movement of GIT and is an inherent property of syncytial smooth muscles. Stimulation at any point causes a contractile ring to appear which moves forward and food in front of ring also moves in forward direction Stimulus is distention of gut-stretching stimulates enteric nervous system to cause contraction 2 to 3 cm behind this point

Receptive relaxation of stomach When peristalsis appears, the gut relaxes several centimeters in analward direction which facilitates the propulsion of food in forward direction. Mechanism:-stretching – Vagovagal reflex – impulses go to brain stem and back to stomach - inhibitory neurons of the myenteric plexus mediate this receptive relaxation

Motor Functions of the Stomach Storage of large quantities of food Secretions of gastric glands Mixing of this food with gastric secretions until it form Chyme Digestion Absorption Slow emptying of the chyme from the stomach into the small intestine

Storage Formation of concentric circles Stretch Vagovagal reflex Decreased tone in the muscular wall of stomach-Bulging of wall-Accomodates 0.8 to 1.5 litre of food in relaxed stomach

Secretion Lined by simple columnar epithelium Mucus glands Gastric (oxyntic) glands Pyloric glands

Four different types of cells make up the gastric glands: Mucous cells Parietal cells Chief cells Endocrine cells

Mixing and Propulsion of Food Presence of food in stomach causes weak peristaltic constrictor wave called slow wave to begin Begin in mid to upper portions of stomach wall and moves towards antrum Appears once every 15 to 20 seconds

Initiated by gut basic electrical rhythm When constrictor wave progresses from body towards antrum, becomes more intense, some are so intense that they provide peristaltic action potential It pushes the antral contents under high pressure towards pylorus

It digs deeply into the food contents in the antrum Opening of pylorus is small and allows only few ml or less of antral contents to empty into duodenum Also when peristaltic wave approaches pylorus, pyloric muscle contracts which narrows the pyloric opening

Retropulsion results in mixing of food Resulting in antral contents to squeeze upstream towards body of stomach Retropulsion results in mixing of food

Peristaltic Wave

Chyme It is the murky semifluid or paste that results from thorough mixing of food with gastric secretions Fluidity depends on:- Relative amount of food Water Stomach secretion Degree of digestion

Hunger Contractions Rhythmical peristaltic contractions in the body of the stomach Strong tetanic- lasts for 2 to 3 minutes Intensity is increased young healthy individuals Low BSR Hunger pangs

Digestion Digestion of carbohydrates- starts in mouth by enzyme ptyalin – only 5% of starch will be hydrolyzed in mouth Digestion continues in fundus and body of stomach for almost 1 hour Then activity of salivary amylase is blocked by acid 30 to 40% of starch is hydrolyzed to form maltose

Digestion of proteins- Pepsin is most active at pH 2 to 3 and inactive at pH 5.HCl is secreted at pH of 0.8 It digests protein collagen- the major constituent of intercellular connective tissue of meats

Absorption Stomach is the poor absorptive area of GIT b/c it lacks typical villus type of absorptive membrane and presence of tight junctions b/w epithelial cells Highly lipid soluble substances like alcohol and aspirin are absorbed in small quantity

Intrinsic factor Secreted by the parietal cells Necessary for absorption of vitamin B12 in ileum

Stomach Emptying Promoted by intense peristaltic contractions in the stomach antrum. Emptying is opposed by varying degrees of resistance to passage of chyme at the pylorus.

Pyloric Pump Peristaltic waves, in addition to causing mixing in the stomach, also provide a pumping action called the “pyloric pump” and it forces several milliliters of chyme into the duodenum

Mostly weak contractions- cause mixing. (20% of the time the food stays in the stomach) strong intense contractions very tight ring like constrictions which squeeze the contents of stomach- cause stomach emptying

Role of the Pylorus in Controlling Stomach Emptying Distal opening- pylorus Increased thickness of circular muscles Tonically contracted Pyloric sphincter The degree of constriction  nervous-humoral reflex signals from stomach & duodenum Allow passage of water and other fluids