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Homework answers / question archive / The activity of the digestive system is remarkably complex, and largely beyond our conscious control or awareness
The activity of the digestive system is remarkably complex, and largely beyond our conscious control or awareness. On a day-to-day basis, with the exception of ingestion, defecation, and emesis (vomiting!), we are mostly unaware of all that is taking place in our digestive organs. The chemical breakdown of food, secretion of acid, enzymes, bile, regulatory hormones, mucus and bicarbonate, absorption of digested nutrients; it happens without our conscious control or recognition. Sometimes, however, even without illness, we do become aware of the motility of our digestive tract, when our stomachs "growl" or we are able to feel the movements of peristalsis. As with most other digestive processes, peristalsis and the rate of transit through the gut tube are tightly regulated, via both neural and endocrine mechanisms. Chemical regulation of gut motility involves both paracrine (chemical message is released and acts locally) and endocrine (chemical message is released to the blood stream) effects. For example, peptides released from the gut tube itself regulate contraction of the gallbladder, constriction of sphincters, affect intestinal motility, and inhibit gastric emptying. Four of the six main digestive hormones play a role in motility regulation; cholecystokinin (CCK), secretin, motilin, and glucagon-like peptide 1 (GLP), (gastrin at high concentrations also can affect stomach motility). They are released from cells in the small intestine in response to the presence of food, or because of input from the autonomic nervous system (ANS). They act on smooth muscle in the gallbladder, pancreas, stomach, small intestine, as well as associated ducts and sphincters, to affect movement both of food and other digestive secretory products (such as bile from the gall bladder). Hormones from other sources such as the brain, as well as signals from the nervous system are also important in regulating motility. Nervous regulation of digestion is achieved both through the autonomic nervous system (ANS) and the enteric nervous system (ENS). The ANS is the visceral motor division of the peripheral nervous system, and as such, responds to information integrated by the central nervous system. The ENS is essentially the "brain" of the digestive system - it is a local collection of sensory, integrating, and motor nervous fibers that communicate with the ANS, that senses, integrates, and directs changes in motility (and secretion). There are two divisions of the ANS that dually innervate digestive structures; the Sympathetic Nervous System (SNS) and the Parasympathetic Nervous System (PSNS). Increased activity in the SNS is generally inhibitory to digestive function, while increased
PSNS activity supports digestive function (increases motility and secretion). Through the connection between the ANS and the ENS, external stimuli, such as the smell of bread or emotional distress can affect gut activity. In today's exercise, we will be detecting waves of electrical activity associated with the rhythmic contraction of the smooth muscle of the stomach. Unlike the rest of the alimentary canal, which typically has only two layers of smooth muscle, the wall of the stomach contains 3 layers. The innermost layer runs oblique to the long axis of the stomach, the middle layer runs around the short axis of the stomach and is called the circular layer, and the outermost layer runs parallel to the long axis of the stomach and is called the longitudinal layer. These 3 layers of muscle contract at regular intervals - about 3 contractions per minute, whether the stomach is full or empty, although with a full stomach, stimuli such as stretch and increased PSNS activity will intensify the contractions. These waves of contractions are called peristallic waves. Pacemaker cells in the longitudinal layer spontaneously depolarize to set the rhythm, and the depolarization spreads through adjacent cells via gap junctions (they are electrically coupled) (called single unit smooth muscle). These pacemaker cells generate subthreshhold depolarizations, which are enhanced by neural and hormonal factors to strengthen them to threshold, at which point they generate action potentials that propagate through the electrically coupled cells. Factors such as distension of the stomach, which stimulates stretch receptors, as well as endocrine regulation, determine the strength of the pacemaker depolarization. Thus, the more food present in the stomach, the more intense gastric contraction will be. However, as food moves from the stomach to the small intestines, increased distension of the duodenum and the presence of fat, amino acids, and low ph stimulate feedback mechanisms that inhibit stomach motility and slow stomach emptying. During digestion, each peristaltic wave usually "squirts" only about 3 mls of chyme (the creamy paste formed by the churning of ingested food with gastric juice) into the duodenum. The wave of contraction actually shuts the pyloric valve, so that most of the material propelled toward the pyloric sphincter splashes back, contributing to the mixing and mechanical digestion of stomach contents. The material that is propelled into the duodenum passes through the pyloric sphincter before the peristaltic wave actually gets to the sphincter, closing it. Within about 4 hours of a meal, peristalsis succeeds in completely emptying the stomach. Factors such as meal size and water content affect the rate of emptying (larger meals and higher liquid content speed emptying). Fluids pass quickly into the small intestine, while solid material is prevented from passing through the small opening of the pyloric sphincter, ensuring it stays in the stomach until it is further mechanically and chemically digested. The contents of the duodenum also greatly affect the speed of gastric emptying - receptors in the wall of duodenum sense the volume and nature of the entering chyme (acid, carbohydrates and fats slow digestion) and release endocrine signals that inhibit gastric emptying.
Il. Data and Calculations (1 pt) A. Frequency of peak EGG activity in the unfed state: _48.8_mHz B. Frequency of peak activity in the fed state: 44.4mHz C. Average magnitude of EGG waves before eating: 0.412 mV D. Average magnitude of EGG waves after eating: 0.875 mV E. Difference in magnitude, pre-to-post consumption: 0.463 mV III. Questions (2 pts) 1. (1 pt) Was there a difference in frequency of contraction before and after eating? Whether or not you observed a difference in your subject, what mechanisms might cause a frequency change, given what factors determine initiation of gastric contraction?
