 This video will cover the following objective from digestive physiology. List the factors that regulate the activity of the digestive system, autonomous smooth muscle, mechanical and chemical receptors, hormones, intrinsic nerves, and extrinsic nerves. This slide shows us an illustration of the structure of the alimentary canal which is organized into four layers. The deepest layer, known as the mucosa, is lining the lumen where the food is traveling through. The mucosa is a mucous membrane and this is where nutrient absorption occurs. The nutrients move across the epithelium of the mucosa into the small capillaries as well as lymphatic blood vessels called lacteals which carry nutrients throughout the body. The next layer superficial to the mucosa is the submucosal layer. The submucosal layer consists of mostly connective tissue that provides some structural support for the alimentary canal and also provides a passageway for larger blood vessels and nerves to travel through. Superficial to the submucosa is the muscularis which consists of primarily single unit smooth muscle tissue. Then superficial to the muscularis is the serosa which functions to anchor the alimentary canal to surrounding tissues to help stabilize the position of the digestive organs and also provides a route for the travel of large blood vessels and nerves. There is nervous tissue within the alimentary canal known as the enteric nervous system. The enteric nervous system consists of the my enteric plexus a network of nervous tissue found in the muscularis and the submucosal plexus a network of nervous tissue found in the submucosal layer. The enteric nervous system includes sensory receptors such as chemoreceptors that can detect the presence of nutrients and mechanoreceptors that can detect stretching of the alimentary canal. This sensory information can be processed by integration centers that are local within the alimentary canal to enable local reflexes that function as intrinsic control mechanisms. But this sensory information can also be relayed through afferent fibers into the central nervous system enabling extrinsic control mechanisms where the autonomic nervous system regulates the functions of the alimentary canal. The smooth muscle in the muscularis layer of the wall of the alimentary canal is single-unit smooth muscle tissue that is regulated by the autonomic nervous system but contains an intrinsic pacemaker cell known as the interstitial cell of cahal that functions to stimulate the action potential in an autonomous contraction mechanism so that the action potential is generated spontaneously within the wall of the alimentary canal independent from the activity of the nervous system. This illustration shows us the interstitial cells of cahal that are located in the pacemaker region just adjacent to the smooth muscle tissue in the muscularis. The interstitial cells of cahal have a spontaneous activation mechanism that produces an action potential which spreads as a slow wave of depolarization into the smooth muscle fibers stimulating opening of L-type calcium channels calcium rushes into the smooth muscle fibers and stimulates contraction. The slow wave is the rhythmic spontaneous variation in the membrane potential of smooth muscle fibers in the muscularis of the alimentary canal. The slow wave spreads from interstitial cells of cahal through the smooth muscle tissue via gap junctions between the interstitial cells and smooth muscle fibers and gap junctions between adjacent smooth muscle fibers and the single unit smooth muscle tissue. When the slow wave stimulates depolarization to threshold for opening of L-type calcium channels calcium rushes into the cell stimulating contraction. When acetylcholine is released into the muscularis of the alimentary canal it binds to muscarinic acetylcholine receptors on the interstitial cells of cahal stimulating an increased rate of spontaneous depolarization and this leads to an increased frequency of action potentials. Acetylcholine also binds to muscarinic acetylcholine receptors on smooth muscle fibers leading to increased calcium influx with each action potential. Together these mechanisms produce an increased force of contraction increasing the muscle tone in response to acetylcholine. Calcium binds to the calcium sensor protein Calmodulin and Calmodulin then activates myosin light chain kinase. Then phosphorylates myosin light chain stimulating the power stroke cycle of the sliding filament theory to produce contraction. Acetylcholine is the primary excitatory neurotransmitter that stimulates both contraction as well as secretion from the glands in the alimentary canal. Acetylcholine is released by the excitatory neurons of the enteric nervous system to stimulate both contraction and secretion. Acetylcholine is also released by the postganglionic parasympathetic fibers in an extrinsic control mechanism. Acetylcholine is released by both extrinsic and intrinsic control mechanisms leading to increased contraction of the smooth muscle in the muscularis and also leading to increased secretion of glands in the submucosa. These glands produce mucus that helps to lubricate the alimentary canal but also carry enzymes into the alimentary canal that are important for chemical digestion. Intestinal hormones also regulate the functions of the alimentary canal. The enteroendocrine cells are cells found in the mucosal layer in the alimentary canal in response to chemical stimuli such as nutrients detected by the hemoreceptors of the enteric nervous system or stretching of the alimentary canal detected by the mechanoreceptors of the enteric nervous system. The enteroendocrine cells will be stimulated to release hormones. There are three major hormones released by the enteroendocrine cells in the intestine. There are enteroendocrine eye cells that release the hormone cholecystokinin which is abbreviated CCK. There are enteroendocrine S cells that release the hormone secretin and enteroendocrine G cells that release the hormone intestinal gastrin. These hormones can either stimulate or inhibit the contraction of the muscularis and they can either stimulate or inhibit secretions from glands in the alimentary canal. We will see an example of a control mechanism regulating secretions from gastric pits in the stomach where the intestinal gastrin hormone will function to stimulate secretion whereas the hormones CCK and secretin will function to inhibit secretion. There are also enteroendocrine cells in the mucosa of the stomach that produce gastric hormones. Gastrin is the primary gastric hormone that will stimulate the secretion of gastric juice and also stimulate the contractions of the smooth muscle to produce churning. Gastrin stimulates the secretion of gastric juice that is rich in the digestive enzyme pepsin that will initiate the chemical digestion of protein and gastrin also stimulates churning and this way gastrin stimulates both chemical digestion and mechanical digestion in the stomach. Gastrin is similar to the intestinal gastrin but this gastrin that's a gastric hormone is produced by enteroendocrine cells within the stomach as an intrinsic control mechanism whereas the intestinal gastrin produced by the enteroendocrine G cells of the small intestine function as a extrinsic control mechanism to stimulate the activity of the stomach. The other major gastric hormone produced by enteroendocrine cells in the mucosa of the stomach is ghrelin. Ghrelin has the function of stimulating hunger and in response to stretching of the stomach and nutrients detected within the stomach the enteric nervous system will inhibit the release of ghrelin. So in response to food entering the stomach stretching the stomach activating the mechanoreceptors of the enteric nervous system and nutrients from that food stimulating the chemoreceptors of the enteric nervous system there will be a decrease in the amount of ghrelin being released by the enteroendocrine G cells of the stomach and ghrelin has the function of binding to receptors in the hypothalamus and stimulating hunger so as there's decreasing levels of ghrelin after eating a meal you'll feel less hungry and satiety is the opposite of hunger the feeling of fullness after eating is known as satiety and so decreasing levels of ghrelin is one thing that will promote satiety but there's another hormone, an intestinal hormone that's important for promoting satiety in response to stretching of the small intestine and in response to nutrients detected by chemoreceptors in the small intestine the enteroendocrine I cells within the small intestine release the intestinal hormone cholecystokinin abbreviated CCK then CCK binds to receptors in the hypothalamus promoting satiety and decreasing hunger