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Gastric activity involved in digestion is divided into three phases of digestion known as the cephalic phase, the gastric phase, and the intestinal phase. These phases overlap and all three can occur simultaneously. [1] A fourth phase of acid secretion is known as the basal state which occurs in the times between meals (interdigestive phase).
The major processes that occur in the GI tract are: motility, secretion, regulation, digestion and circulation. The proper function and coordination of these processes are vital for maintaining good health by providing for the effective digestion and uptake of nutrients.
Layers of the Alimentary Canal.The wall of the alimentary canal has four basic tissue layers: the mucosa, submucosa, muscularis, and serosa. The enteric nervous system in humans consists of some 500 million neurons [11] (including the various types of Dogiel cells), [1] [12] 0.5% of the number of neurons in the brain, five times as many as the one hundred million neurons in the human spinal ...
The vagovagal reflex is active during the receptive relaxation of the stomach in response to swallowing of food (prior to it reaching the stomach). When food enters the stomach a "vagovagal" reflex goes from the stomach to the brain, and then back again to the stomach causing active relaxation of the smooth muscle in the stomach wall.
The first stage, the cephalic phase of digestion, begins with secretions from gastric glands in response to the sight and smell of food. This stage includes the mechanical breakdown of food by chewing , and the chemical breakdown by digestive enzymes , that takes place in the mouth .
In ruminants, because dietary carbohydrates tend to be metabolized by rumen organisms, gluconeogenesis occurs regardless of fasting, low-carbohydrate diets, exercise, etc. [8] In many other animals, the process occurs during periods of fasting, starvation, low-carbohydrate diets, or intense exercise.
The sympathetic nervous system is responsible for setting off the fight-or response. [3] The parasympathetic nervous system is responsible for the body's rest and digestion response. [3] In many cases, both of these systems have "opposite" actions where one system activates a physiological response and the other inhibits it.
[1] [2] Absence of enteropeptidase results in intestinal digestion impairment. [ 3 ] Enteropeptidase is a serine protease ( EC 3.4.21.9 ) consisting of a disulfide-linked heavy-chain of 82-140 kDa that anchors enterokinase in the intestinal brush border membrane and a light-chain of 35–62 kDa that contains the catalytic subunit. [ 4 ]