Introduction to Gastrointestinal Physiology What is the GI system? Long tube open at both ends (lips and anus), the gut Associated glands How is it organized? Some of what we ingest gets into our bloodstream Some comes out the other end GI system converts what we ingest into usable forms. It’s a batch processor. Batch processors hold contents in a chamber, do something to it, move it to another chamber, do something else to it, etc.

What does the GI system do? Breaks ingested chunks into small pieces. Converts large molecules to small molecules (digestion, by hydrolysis) that can be absorbed. Secretes fluids into lumen of gut. Absorbs nutrients from lumen of gut into circulation. Mixes and propels contents.

What will we deal with in the GI section? How it propels and mixes contents (motility) What gets secreted into the lumen What gets digested (hydrolyzed) in the lumen What gets absorbed from the lumen Maybe most important, how are those things coordinated in time?

Gut is layered through it’s entire length Skeletal muscle to about one-third of the way down the length of the esophagus, again at the external anal sphincter. Everything else is smooth muscle

Splanchnic Circulation All venous blood from stomach, intestines, and pancreas go to liver before entering the general circulation. Therefore, liver gets exposed to higher levels of anything in venous effluent from those organs than anyplace else does. Guess what? Liver regulates plasma concentrations of stuff absorbed from stomach and intestines, largely under the influence of hormones secreted from the pancreas Guess what else? Liver is an important site of detoxification of ingested compounds that are absorbed in the small intestine

Basic GI Functions Motility (mixing and propulsion of gut contents) Secretion (of various fluids into gut lumen) Digestion (hydrolysis of macromolecules into absorbable units) Absorption (of fluids and of nutrients)

Motility The important functions of muscle activity in the gut (“motility”) are mixing secretions with contents and exposing all contents to mucosa for absorption, and propulsion of contents. When longitudinal muscle contracts, gut segment shortens. When circular muscle contracts, gut diameter decreases. There’s one muscle layer that is only in the small intestine, the muscularis mucosae. We’ll deal with it when we get to that. Most propulsion of gut content is done by circular muscle.

Peristalsis Peristalsis is a ring of contraction of circular muscle that travels (not extends) analward in the gut. Although not shown in this cartoon, some gut contents are propelled in each direction. Note that gut diameter ahead of peristaltic ring is larger than it is behind. Therefore, flow resistance is lower higher in analward direction (resistance varies as fourth power of diameter!). Therefore, there is propulsion as well as mixing by peristalsis.

Secretion Sources: Glands in wall of gut that secrete into gut lumen via ducts Secretory cells in gut mucosa External glands that secrete into the gut lumen via ducts Two major kinds of secretions are serous (watery) and mucous (viscous, containing mucus) fluids. Mucous secretions are lubricants. One source is goblet cells in the mucosa of every part of the GI tract. Serous fluids serve a number of functions: Some contain digestive enzymes Some contain strong acid Some are alkaline, neutralizing the acid Some are essentially salt and water, contributing to the fluidity

Digestion Most of what we eat is in the form of macromolecules (proteins, polysaccharides, nucleic acids, fat) that are too big to absorb. They are hydrolyzed to amino acids, monosaccharides (simple sugars), nucleotides, fatty acids and glycerol, respectively, mostly through hydrolysis by digestive enzymes. Absorption Nutrient, electrolyte and water reabsorption involve a number of specialized transport systems in the mucosa.

Control of GI Functions Efficiency requires activities be coordinated. Why expend energy by using muscles in or secreting fluids into an empty gut segment? How are activities controlled? 1.Local stimulation by gut contents. Local mechanical stimulation by gut contents is how goblet cells are stimulated to secrete mucous, for example. 2.“Short reflex” neural mechanisms involving receptors on mucosal surface that synapse with one or both plexuses, which transmit information along length of the gut and elicit responses. Plexuses in gut are sometimes called the “little brain” because they are so independent of the CNS.

3.“Long reflex” mechanisms involving the autonomic nervous system. Sympathetic stimuli (splanchnic nerve) are usually inhibitory, parasympathetic stimuli (vagus nerve) usually excitatory. 4.Humoral control via polypeptide gut hormones. Neural mechanisms act nearly instantaneously and are local. Humoral mechanism have gradual onset and end, act globally.

The Oral Cavity - Motility Chewing – Why bother? 1.The typical bite of food is too big to swallow safely because esophagus is behind trachea; tracheal cartilage is in front and sides only. So, reducing particle size is essential 2.The nutritious part of some vegetable material is enclosed in a cellulose hull and can’t be digested unless that hull is fractured. 3.Chewing brings food into contact with entire surface of the oral cavity, which initiates a number of GI reflexes as well as stimulating salivary secretion.

The Chewing Reflex Chewing is more formally called mastication. Feel free to titter. “Chewing reflex” is really two consecutive reflexes: 1.Pressure of bolus of food on palate stimulates pressure receptors on palate. Pressure receptor stimulation reflexively relaxes jaw muscles; lower jaw drops. That’s the first reflex arc. 2.Drop of lower jaw stretches jaw muscles, stimulating stretch receptors. Stretch receptors reflexively cause jaw muscles to contract. This is the second reflex arc. Contraction of jaw muscles raises jaw. If there is still food in the mouth, it re-establishes pressure on the palate, starting the sequence again.

Salivary Secretion Three pairs of salivary glands: parotid, submandibular, sublingual Resting rate of secretion of around 0.5 ml/min keeps mouth lining moist. Important for speech, and opposes sensation of thirst. Saliva is alkaline, important to neutralize acid produced by bacteria around base of teeth, which can erode tooth enamel. Stimuli to salivary secretion: Local mechanical pressure (why do you suppose tobacco chewers generate so much saliva?) Low pH in oral cavity (acidity in mouth promotes secretion of alkaline solution into it)

Stimuli to salivary secretion (cont’d) Neural stimuli – cholinergic and catecholaminergic transmitters both stimulate. Cholinergic is more important because parasympathetic innervation of salivary glands is much more extensive than sympathetic. Parasympathetic stimulation results from any sensation associated with eating (Pavlov’s discovery of conditioned reflexes is a good example)

Saliva Resting level of secretion = 0.5 ml/min; up to 5 ml/min. Average = 1 to 2 liters/day, nearly all of which enters gut. Kinds of saliva 1. Serous (watery) fluid, maintains normal oral pH and contributes to fluidity of ingested food. 2. Mucous fluid, lubricates. 3. Digestive, one enzyme that breaks down starch to smaller pieces, including simple sugars. Doesn’t do much oral cavity – not enough time. Sublingual and submandibular glands both produce serous and mucous fluids. Parotid gland produces salivary amylase (also called ptyalin).

Swallowing Formally = deglutition. Not nearly as titter-worthy as mastication. Includes two major phases, voluntary and involuntary (Sherwood uses oropharyngeal and esophageal, respectively) Voluntary phase consists of front of tongue pressing hard palate, which rolls the bolus of food toward the pharynx. Everything beyond this is involuntary. That’s why you can give a dog a pill by throwing it into the back of his mouth

Involuntary phase of swallowing: Muscles in back of throat force food into pharynx. Larynx elevates and entrance is blocked by epiglottis. Vocal cords narrow to a slit, limiting the size of the opening to anything that got past the epiglottis. Respiration is inhibited. Pharyngeal muscles contract, forcing food toward esophagus. Note that respiration is inhibited when entrance to trachea is blocked. Overriding this inhibition can allow food into trachea.

Esophagus Sphincters at pharyngeal and gastric ends. Upper, normally closed, prevents swallowing air during breathing. Kids learn how to force air past it, and can burp at will. Lower is normally constricted and prevents reflux of gastric contents. When pharyngeal muscles force food toward esophagus, upper sphincter relaxes and a peristaltic wave begins, traveling all the way to the stomach. If there’s still food in the esophagus, local distension initiates secondary wave. There can be tertiary and quaternary waves until esophagus is empty. It normally covers the length of the esophagus in about 10 seconds Since skeletal muscle can’t transmit cell-to-cell action potentials, it’s obvious that the myenteric plexus must control rate and direction of peristaltic wave in upper third of esophagus. In fact, it controls the rate and direction in the lower two-thirds as well, and the wave travels much faster than cell-to-cell excitation in smooth muscle The only secretion into the esophagus is mucous, from goblet cells

Esophagus (cont’d) Skeletal muscle can’t transmit action potentials, so myenteric plexus must control rate and direction of peristalsis from pharynx through upper third of esophagus. In fact, it controls rate and direction of peristalsis over the entire length of esophagus. Rate of conduction of action potentials in smooth muscle is much too slow to sccount for rate of peristaltic travel in esophagus.

Heartburn Heartburn is caused by gastric reflux. Gastroesophageal sphincter is normally tightly closed. That’s why you can dive head first without gastric contents getting into your mouth. Increasing abdominal pressure by contracting abdominal muscles doesn’t force gastric content into esophagus. Why? Because gastroesophageal sphincter is below diaphragm, in abdominal cavity, so increased pressure tends to force it closed. Hiatal hernia is a condition in which opening in diaphragm is large enough to let upper end of stomach (and gastroesophageal sphincter) poke up into thorax. When abdominal pressure rises, it can force gastric content into esophagus. That’s heartburn.

Esophageal Secretion The only secretion in the esophagus is mucous, secreted by goblet cells. Esophageal Digestion and Absorption There is no digestion except a trivial amount of starch hydrolysis catalyzed by salivary amylase. There is no absorption in the esophagus.