saliva can be secreted through two reflexes known as simple salivary reflex and conditioned reflex. Simple salivary reflex occurs when chemoreceptors are stimulated by the presence of food and send signal to the medulla oblongata which in turn signal the release of more saliva. The conditioned salivary reflex occurs when there is no oral stimulation. Seeing, hearing or smelling food can trigger this reflex.
The salivary glands produce saliva and secrete it. The salivary glands are under the influence of the autonomic nervous system. The ANS consist of the sympathetic nervous system and the parasympathetic system. The sympathetic happens when there is no adrenalin which activates the alpha and beta adrenergic receptors. This cause a decrease in production of saliva. The parasympathetic occurs when there is presence of food. Acetylcholine is released and the acinar cells secrete saliva. The more parasympathetic stimulation, more saliva secreted.
2. Cephalic Phase
The cephalic phase is controlled by the brain, the phase begins before food is ingested usually 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 nervous system through the vagus nerve and to the stomach through the gastric glands in the stomach. This phase accounts for 20% of gastric secretion associated with eating food. This enhanced secretion is known as the conditioned reflex because the secretory activity is initiated by sight or smell of food and occurs when we want 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 also be an influence on G cells to increase gastrin circulation.
When food enters the stomach, the gastric glands secret gastric juice. The stomach stretches initiating stretch receptors that sends signal to the medulla and back to the stomach through the vague nerve. This cause the gastric glands to secrete more gastric juice. Partially digested protein activate the G cells that are found in the pyloric region of the stomach to secret gastrin. The activation of G cells stimulates the gastric glands to secrete gastric juice. Small peptide and amino acids from the breakdown of proteins buffer the stomach acid to prevent the pH from dropping o 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 arrival of chyme in the duodenum initiate the intestinal phase and it controls the gastric activity through hormones and nervous reflexes. The duodenum firstly assist in secretion of gastric juice and then inhibit it again. The duodenum stretches masks vagal reflexes that stimulate the stomach, peptide and amino acids in the chyme that stimulate G cells to secret more gastrin. However, the acid and semi digested fats in the duodenum stimulate the enterogastricc reflex. The stomach receives inhibitory signals from the duodenum via the enteric nervous system and also medulla receives inhibitory signals from the duodenum to inhibit vagal stimulation. A decrease in the vagal stimulation stimulates the sympathetic neurons that sends inhibitory signal to the stomach. Two hormones are involved in the duodenum namely the cholecystokinin and secretin. Cholecystokinin is found in the duodenum where it is synthesised and secreted by the enteric endocrine cells. Partially digested proteins and fats in the small intestines stimulate its production. When chyme enters the small intestine in large quantities, the cholecystokinin is released into the blood and it binds 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 due to the flow of acid laden chyme from the stomach through the pylorus. The main 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 needs to be broken down into amino acids then those amino acids recombined to form specialized proteins such as enzymes, antibodies and hormone. The breakdown of protein occurs from the stomach and continue to the small intestine. Pepsin initiate the breakdown of protein by breaking peptide bonds holding protein together. It is an active protein digesting enzyme of the stomach. Proteins are then broken down into polypeptide in the stomach which then move to the small intestine. Enzymes such as the trypsin, chymotrypsin and carboxypeptidase are secreted from the pancreas and enters the duodenum with the help of brush border enzymes. The peptide bonds holding the polypeptide are broken down into small peptides (two or more amino acids). The enzymes continue to break down molecules into amino acid which are very small and are then absorbed through the small intestine lining and into the blood stream.
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.