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 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 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 theStomach

• 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 reflexDecreased 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

PeristalticPeristaltic

WaveWave

Chyme

• It is the murky semifluid or paste that results from thorough mixing of food with gastric secretions

Fluidity depends on:-1.Relative amount of food2.Water3.Stomach secretion4.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