Anatomically, the stomach is usually divided into two major parts: (1) the body (2) the antrum.Physiologically, it is more appropriately divided intothe 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 rhythmThey are the slow,undulating changes in the RMP of smooth muscle of GIT.Intensity-5 to 15 mvFrequency-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 waveIt 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 directionStimulus is distention of gut-stretching stimulates enteric nervous system to cause contraction 2 to 3 cm behind this point

Receptive relaxation of stomachWhen 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 theStomachStorage of large quantities of food Secretions of gastric glandsMixing of this food with gastric secretions until it form ChymeDigestionAbsorptionSlow emptying of the chyme from the stomach into the small intestine

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

SecretionLined by simple columnar epitheliumMucus glands Gastric (oxyntic) glandsPyloric glands

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

Mixing and Propulsion of Food

PeristalticWave

ChymeIt is the murky semifluid or paste that results from thorough mixing of food with gastric secretionsFluidity depends on:-Relative amount of foodWaterStomach secretionDegree of digestion

Hunger ContractionsRhythmical peristaltic contractions in the body of the stomachStrong tetanic- lasts for 2 to 3 minutesIntensity is increased young healthy individuals Low BSRHunger pangs

DigestionDigestion of carbohydrates- starts in mouth by enzyme ptyalin only 5% of starch will be hydrolyzed in mouthDigestion continues in fundus and body of stomach for almost 1 hourThen activity of salivary amylase is blocked by acid30 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 0.8It digests protein collagen- the major constituent of intercellular connective tissue of meats

AbsorptionStomach 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 cellsHighly lipid soluble substances like alcohol and aspirin are absorbed in small quantity

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

Stomach EmptyingPromoted 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 EmptyingDistal opening- pylorusIncreased thickness of circular musclesTonically contractedPyloric sphincterThe degree of constriction nervous-humoral reflex signals from stomach & duodenumAllow passage of water and other fluids

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