1. Saliva can be emitted through two reflexes known as simple salivary reflex and conditioned reflex. Simple salivary reflex happens when chemoreceptors are stimulated by the presence of food and send signal to the medulla oblongata which thusly signal the arrival of more saliva. The conditioned salivary reflex happens when there is no oral incitement. Seeing hearing or smelling nourishment can trigger this reflex.
The salivary organs create saliva and secrete it. The salivary glands are affected by the autonomic sensory system. The ANS consist of the sympathetic nervous system and the parasympathetic system. The sympathetic happens when there is no adrenalin which enacts the alpha and beta adrenergic receptors. This results in decrease of saliva secretion. The parasympathetic happens when there is presence of food. Acetylcholine is discharged and the acinar cells secrete saliva. The more parasympathetic incitement more saliva secreted.
2. Cephalic stage
The cephalic phase is controlled by the brain, phase starts before food is ingested generally when the food is still in the mouth. This is initiated by sight, smell of food and the taste of food that stimulate appetite and the more appetite an individual has, the greater the stimulation. The cerebral cortex is stimulated and transmit signal to the hypothalamus, the medulla and the parasympathetic sensory system through the vagus nerve and to the stomach through the gastric organs in the stomach. This stage represents 20% of gastric secretion related with eating food. This part of the cephalic reflex is inhibited when an individual has no appetite. This cause Enterochromaffin-like cells to secret histamine and increase hydrochloric acid in the stomach. There will likewise be an impact on G cells to expand gastrin circulation.
When food enters the stomach the gastric glands secrete gastric juice. The stomach extends starting stretch receptors that sends signal to the medulla and back to the stomach through the vagus nerve. This reason the gastric organs to secrete more gastric juice. In part processed protein activates the G cells that are found in the pyloric area of the stomach to secret gastrin. The stimulation of G cells fortifies the gastric organs to secrete gastric juice. Little peptide and amino acids from the breakdown of proteins buffer the stomach acid to keep the pH from dropping to much As digestion continues, the peptides empty the stomach, causing the pH to drop low. When the pH reaches below 2, G cells and pariental cells are inhibited and the gastric phase is slowed down and the hydrochloric acids and pepsin decreases. Close to two third of gastric secretion occurs during this phase. Food that has been ingested stimulate gastric secretion in two ways: By stretching the stomach which activates two reflexes: a short reflex and a long reflex. And the second gastric activity is by raising the pH.
3. The landing of chyme in the duodenum start the intestinal phase and it controls the gastric movement through hormones and nervous reflexes. The duodenum help aid secretion of gastric juice and afterward inhibit it once more. The duodenum stretches vagal reflexes that invigorate the stomach peptide and amino acids in the chyme that stimulate G cells to secrete gastrin. However, the acid and semi digested fats in the duodenum invigorate the enterogastricc reflex. The stomach gets inhibitory signal from the duodenum through the enteric sensory system and furthermore medulla gets inhibitory signs from the duodenum to restrain vagal stimulation. A decrease in the vagal stimulation stimulate sympathetic neurons that sends inhibitory signal to the stomach. Two hormones are engaged with the duodenum in particular the cholecystokinin and secretin. Cholecystokinin is found in the duodenum where it is incorporated and secreted by the enteric endocrine cells. Partially digested proteins and fats in the small intestine stimulate its production. At the point when chyme enters the small intestine in huge amounts the cholecystokinin is released into the blood and it bind to the receptors on pancreatic acinar cells initiating a secretion of large amount of digestive enzymes. The second hormone secretin is found in the epithelium of the small intestine produced by the endocrinocytes. Presence of acid in the duodenum cause secretion of secretin because of the stream of acid loaded chyme from the stomach through the pylorus. The primary function of secretin is to stimulate duct cells to secret water and bicarbonate. The enzymes secreted by the acinar cells are flown out of the pancreas, through the pancreatic duct into the duodenum.
4. Chemical Digestion in the Small Intestine
Proteins are large molecules that are needed by the body but cannot be absorbed directly. The protein should be broken down into amino acids then those amino acids recombined to form specific proteins for example enzymes, antibodies and hormone. The breakdown of protein happens from the stomach and proceed to the small intestine. Pepsin start the breakdown of protein by breaking peptide bonds holding protein together. Proteins are then broken down into polypeptide in the stomach which at that point move to the small intestine. Enzymes such as trypsin, chymotrypsin and carboxypeptidase are secreted from the pancreas and enters the duodenum with the assistance of brush border enzymes. The peptide bonds holding the polypeptide are broken down into little peptides (two amino acids). The enzymes continue to breaking down molecules into amino acids which are small and are then absorbed through the small intestine lining and into the circulatory system.
5. Ruminants are given this name because they ruminate (chew the cud). Ruminants cannot digest their own food (grass) therefore, depend on microbes to digest complex polysaccharides for them. They have evolved their stomach to be able to house these microbes. Microbes are important because they produce enzymes such as cellulase and other enzymes necessary to break down plants material. The first microbe is the Fibrobacter succinogenes which break down glucose into acetate and succinate as by product. Ruminoccus flavifaciens which is involved in the digestion of plant cell wall because it contain high concentration of cellulase and hemicellulase activities. The product produced are hydrogen, acetate which is used as an oxidasable substrate and the succinate as a growth substrate. Megasphaera elsdenii is found in young ruminant and ferments glucose used for gluconeogenesis. Most of these microbes produces enzymes that break down glucose in the host’s stomach for energy. Cellulose broken down to fatty acids can be absorbed by the rumen wall. Most bacteria use cellulase enzyme complexes that bind to bacteria surface to digest polysaccharide. Starch and cellulose are broken down into glucose while hemicellulose and pectin are fermented. Fungi release more soluble cellulase than bacteria and are therefore more successful in fermenting plant particles. Both the fungi and archaea are lost from the ruminoreticulum at a slower rate than bacteria because they attach to food bolus. This is a better way of ensuring their survival because they reproduce at a slower rate than bacteria. Microbes and ruminants have a symbiotic relationship. The microbes in the gut benefit by receiving nutrient from the ruminant and the ruminant benefit by digestion of its food and the relationship is mutualistic.