Gastrointestinal Physiology

• The lecture is an introduction to the gastrointestinal system, also known as the GI tract.

Physiology 210 Information

• The lecturer is Dr. Elaine M. Leslie; she can be contacted via email at [email protected] or by phone at 780-492-9250.
• Lecture notes are available on eClass.
• An approved letter of accommodation is needed to enable the recording of lectures.
• "Vander's Human Physiology: The Mechanisms of Body Function, 16th edition" is recommended as a reading resource, specifically Chapter 15.
• Exam questions are based on lecture notes.
• Lecture material and slides are for personal use only and should not be shared, distributed, or posted for monetary benefit, as this constitutes a copyright infringement.

Lecture Topics

• General introduction to the GI system
• Mouth and esophagus
• Stomach
• Pancreas
• Liver and biliary system
• Small intestine
• Large intestine

General Introduction to the Gastrointestinal System

• The lecture addresses major functions of the GI system and their structural relationships.
• Blood supply and unusual aspects of the blood circulatory pathway are explained.
• The roles of the autonomic and enteric nervous systems in regulating GI activity are explored.
• The difference between intrinsic and extrinsic innervation is explained.
• The role of hormones in regulation of GI activity, and the meanings of neurocrine, endocrine, paracrine, and autocrine regulation are covered.
• Types of muscle activity in the GI tract, properties of smooth muscle, and the "basic electrical rhythm" will all be discussed.

Functions of the Gastrointestinal Tract (GIT)

• The GIT transfers digested organic nutrients, minerals, and water from the external to the internal environment.
• Digestion involves the chemical alteration of food into absorbable molecules, aided by GI motility, pH changes, biological detergents, and enzymes.
• Absorption refers to the movement of digested food from the intestine into the blood or lymphatic system.
• Excretion eliminates non-absorbable components of food, bacteria, intestinal cells, and hydrophobic molecules.
• GIT serves as a host defense, being continuous with the exterior of the body.
• A highly developed immune system is present.

Components of the GIT

• Mouth, pharynx, esophagus, stomach, small intestine (duodenum, jejunum, ileum), and large intestine.
• Accessory organs include the pancreas, liver, gall bladder, and salivary glands.

Structure of the GIT

• A long muscular tube stretching from the mouth to the anus.
• Its composition is similar from mid-esophagus to anus.
• The layers of the GIT include the mucosa, submucosa, muscularis externa, and serosa.

Layers of the Mucosa

• The epithelial layer, the smallest layer, lines all body cavities and surfaces, and being "polarized" means that it possess different characteristics between its basolateral and apical arrangement.
• Tight junctions confine transport proteins to specific membrane regions.
• Epithelial layer facilitates selective uptake of nutrients, electrolytes, and water, and prevents passage of harmful substances.
• The surface area is amplified by villi and crypts (+ microvilli).
• Stem cells in crypts divide and daughter cells migrate upwards towards the villous (17 billion daily).
• Paracellular pathway is limited by tight junction seal.
• Transcellular pathway, a two step process, requires a transport protein on the apical and basolateral surface of the cell.
• Lamina propria is connective tissue containing small blood vessels, nerve fibers, and lymphatic vessels responsible for fat absorption
• Muscularis mucosa is a thin layer of smooth muscle not involved in GIT contraction but important in villi movement

Submucosa

• This layer contains a plexus of nerve cell bodies relays information to and from the mucosa.
• It is composed of connective tissue, blood, and lymphatic vessels.

Muscularis Externa

• It has a thick inner layer of circular muscle oriented to cause narrowing of the lumen.
• The outer longitudinal layer's fibers are oriented to shorten the tube.
• The myenteric nerve plexus regulates muscle function.

Serosa

• It is connective tissue that encases the intestine and connects it to the abdominal wall.

Blood Supply to the GIT

• It is critical for carrying away water soluble absorbed nutrients to sites in elsewhere in the body.
• Blood perfuses intestine and flows to the liver via the portal vein.
• The portal vein drains blood from the digestive tract into the liver, providing nutrient-rich blood to the liver, where harmful substances can be removed and nutrients can be processed.
• The liver receives blood from both venous and arterial circulation.
• The liver receives less oxygenated blood (~25%), but blood is more nutrient-rich than other organs (75% is derived from the portal vein, which is oxygen poor).

Regulation of GI Processes

• GI processes are governed by the volume and composition of lumen contents; by the volume of luminal contents, osmolarity and pH of contents, and concentrations of specific digestion contents.
• Reflexes are also initiated by monosaccharides, fatty acids, peptides, and amino acids.
• Mechanoreceptors are activated by mechanical stimuli like pressure and stretch.
• Osmoreceptors are activated by changes in osmolarity.
• Chemoreceptors are activated by the binding of specific chemicals.
• Intrinsic Neural Regulation features the "brain of the gut", a dense and complex network of neurons contained completely within the walls of the GIT, equivalent to the spinal cord.
• It controls the activity of secretomotor neurons for motility and secretory function, and functions independently of the CNS as it is critical for involuntary functions.
• The enteric nervous system, a two nerve netowr comprised of the myenteric plexus and submucosal plexus influences smooth muscle and secretion, respectively.

Extrinsic Neural Regulation of GI Processes

• Regulation is through the autonomic nervous system (parasympathetic and sympathetic).
• Regulation influences the motility and secretion of the GIT
• Hunger, sight/smell of food, and emotional state all affect regualtion. Long and short neural reflex pathways can be either intrinsic or extrinsic.

Cateogries of Chemical Messengers

• Endocrine messenger - Hormones travel through the blood to target cells.
• Paracrine messenger - Substances released by a cell affect nearby target cells.
• Autocrine messenger - Substances affect the same cell that secreted it.
• Neurocrine messenger - Neurotransmitters are released by nerve cells to affect other neurons or effector cells.
• Endocrine cells are scattered throughout the epithelium of the stomach and small intestine with one surface of each endocrine cell exposed to the GI lumen.
• Chemical substances in lumen stimulate cell to release hormones across opposite surface of cell into blood vessels in lamina propria.
• The four best understood GI hormones —secretin, cholecystokinin (CCK), gastrin, and glucose dependent insulinotropic peptide (GIP)— all peptides participate in a feedback system that regulates some aspect of the GI lumen.

CCK (Cholecystokinin)

• Fatty acids and amino acids in the small intestine triggers CCK secretion from cells in small intestine into blood.
• Pancreas increases digestive enzyme secretion, along with gall bladder contraction and release of bile acids for fat breakdown.
• Fats and amino acids are absorbed and CCK production halts.

Major GI Hormones

• Gastrin is released from the stomach antrum G cells in response to peptides/amino acids and parasymp nerves.
• Secretin is released from small intestine S cells in response to acid and causes decreased HCl release from the stomach.
• CCK is released from small intestine I cells in response to digested fat/protein, and causes decreased stomach motility.
• GIP is released from the small intestine K cells in response to glucose or fat and causes an increase in insulin.

Intestinal Motility

• Contraction and relaxation of two outer smooth muscle layers of the GIT allows movement of contents.
• Peristalsis (propulsion) are circular muscle contractions to longitudinal relaxation to advance contents.
• Segmentation (mixing) contraction and relaxation of intestinal segments allows mixing of chyme and a slowed transit of food resulting in more absorption.

Basic Electrical Rhythm

• The GIT features pacemaker cells throughout smooth muscle cells.
• These are constantly undergoing spontaneous depolarization-repolarization cycles and slow waves.
• Slow waves propagated through the circular and longitudinal muscle layer through gap junctions.
• Spontaneous slow waves do not result in significant contractions.
• Excitatory hormones/neurotransmitters further depolarize the membrane to threshold facilitating contraction.
• The number of action potentials fired is proportional to the force of contraction.
• Frequency of contraction is maintained by the basic electrical rhythm, whereas the force of contraction is mediated by neuronal and hormonal input.

This quiz covers the role of hormones, receptors, and the enteric nervous system in controlling gastrointestinal processes. It explores mechanisms of hormone release, initiation of reflexes, and the impact of malabsorption on hormone secretion. It also assesses understanding of specific hormones like cholecystokinin (CCK).