GIT Physiology PDF Lecture 1

Gastrointestinal tract (GIT) Physiology Dr.Ahmed talib Lect. 1 — The gastrointestinal tract is a continuous tube that stretches from the mouth — to the anus. Its primary function is to serve as a portal whereby nutrients an...

Gastrointestinal tract (GIT) Physiology Dr.Ahmed talib Lect. 1 — The gastrointestinal tract is a continuous tube that stretches from the mouth — to the anus. Its primary function is to serve as a portal whereby nutrients and water can be absorbed into the body. The alimentary tract provides the body with a continual supply of water, electrolytes, and nutrients, To achieve this requires : — (1) Movement of food through the alimentary tract. — — (2) Secretion of digestive juices and digestion of the food. — — (3) Absorption of water, various electrolytes, and digestive products. — (4) Circulation of blood through the gastrointestinal organs to carry away the — — absorbed substances. — (5) Control of all these functions by local, nervous, and hormonal systems. — Figure below shows the entire alimentary tract. Each part is adapted to its specific — functions: some to simple passage of food, such as the esophagus; others to temporary storage of food, such as the stomach; and others to digestion and absorption, such as — the small intestine. — 1  General Principles of Gastrointestinal Motility — -Physiologic Anatomy of the Gastrointestinal Wall — Figure below shows a typical cross section of the intestinal wall, including the following layers from outer surface inward: — (1) The serosa. — (2) A longitudinal muscle layer. — (3) A circular muscle layer. — (4) The submucosa. — (5) The mucosa. — In addition, sparse bundles of smooth muscle fibers, the mucosal muscle, lie in the deeper layers of the mucosa. The motor functions of the gut are performed by the different layers of smooth muscle. — — — —Electrical Activity of Gastrointestinal Smooth Muscle The smooth muscle of the gastrointestinal tract is excited by almost continual slow, intrinsic electrical activity along the membranes of the muscle fibers. This activity has two basic types of electrical waves: (1) slow waves and (2)spikes  Slow Waves. Most gastrointestinal contractions occur rhythmically, and this rhythm is determined mainly by the frequency of so- called “slow waves” of smooth muscle membrane potential. The precise cause of the slow waves is not completely understood, although they appear to be 2 caused by complex interactions among the smooth muscle cells and specialized cells, called the interstitial cells of Cajal, that are believed to act as electrical pacemakers for smooth muscle cells. These interstitial cells form a network with each other and are interposed between the smooth muscle layers, with synaptic-like contacts to smooth muscle cells. The interstitial cells of Cajal undergo cyclic changes in membrane potential due to unique ion channels that periodically open and produce inward (pacemaker) currents that may generate slow wave activity. The slow waves usually do not by themselves cause muscle contraction in most parts of the GIT, except perhaps in the stomach. Instead, they mainly excite the appearance of intermittent spike potentials, and the spike potentials in turn actually excite the muscle contraction.  Spike Potentials :-The spike potentials are true action potentials. They occur automatically when the resting membrane potential of the GITsmooth muscle becomes more positive. The most features of the action potentials of the GIT is The spike potentials last 10 to 40 times as long in GIT muscle as the action potentials in large nerve fibers. Another important difference between the action potentials of the GIT smooth muscle and nerve fibers. In nerve fibers, the action potentials are caused almost entirely by rapid entry of sodium ions through sodium channels to the interior of the fibers. In GIT smooth muscle fibers, the channels responsible for the action potentials they allow especially large numbers of calcium ions to enter along with smaller numbers of sodium ions and therefore are called calcium-sodium channels. These channels are much slower to open and close than are the rapid sodium channels of large nerve fibers. The slowness of opening and closing of the calcium-sodium channels accounts for the long duration of the action potentials. Factors that depolarize the membrane of GIT that is, make it more excitable are (1) stretching of the muscle (2) stimulation by acetylcholine released from the endings of parasympathetic nerves. (3) stimulation by several specific gastrointestinal hormones. Important factors that make the membrane potential more negative (Hyperpolarization) More negative hyperpolarization make the muscle fibers less excitable are (1) The effect of norepinephrine or epinephrine on the fiber membrane (2) Stimulation of the sympathetic nerves that secrete mainly norepinephrine at their endings. 3  Tonic contraction Some smooth muscle of the GIT exhibits tonic contraction, as well as, or instead of, rhythmical contractions. Tonic contraction is continuous, not associated with the basic electrical rhythm of the slow waves but often lasting several minutes or even hours. The tonic contraction often increases or decreases in intensity but continues. —Tonic contraction is sometimes caused by —1- Continuous repetitive spike potentials the greater the frequency, the greater the degree of contraction. — 2- Tonic contraction is caused by hormones or other factors that bring about continuous partial depolarization of the smooth muscle membrane without causing action potentials. — 3- Continuous entry of calcium ions into the interior of the cell brought about in ways not associated with changes in membrane potential. (The details of these mechanisms are still unclear). Nerve Supply to GIT GIT has two types of nerve supply: 1- Intrinsic nerve supply 2- Extrinsic nerve supply. Intrnsic Nerve Supplay Enteric Nervous System Intrinsic nerves to GIT form the enteric nervous system that controls all the secretions and movements of GI tract. Enteric nervous system is present within the wall of GI tract from esophagus to anus. enteric nervous system. The system contains about 100 million sensory neurons, interneurons, and motor neurons in humans as many as are found in the whole spinal cord and the system is probably best viewed as a displaced part of the central nervous system (CNS) that is concerned with the regulation of gastrointestinal function. It is sometimes referred to as the "little brain" for this reason. It is connected to the CNS by parasympathetic and sympathetic fibers but can function autonomously without these connections Nerve fibers of this system are interconnected and form two major networks called (1) An outer plexus lying between the longitudinal and circular muscle layers, called the myenteric plexus or Auerbach’s plexus. (2) An inner plexus, called the submucosal plexus or Meissner’s plexus, that lies in the submucosa. These nerve plexus contain nerve cell bodies, processes of nerve cells and the receptors. The receptors in the GI tract are stretch receptors and chemoreceptors. Enteric nervous system is controlled by extrinsic nerves. 4 A- Auerbach Plexus OR Myenteric nerve plexus Auerbach plexus is also known as myenteric nerve plexus. It is present in between the inner circular muscle layer and the outer longitudinal muscle layer. Functions of Auerbach plexus is Major function of this plexus is to regulate the movements of GIT Some nerve fibers of this 1- plexus accelerate the movements by secreting the excitatory neurotransmitter substances like acetylcholine, serotonin and substance P. 2- Other fibers of this plexus inhibit the GIT motility by secreting the inhibitory neurotransmitters such as vasoactive intestinal polypeptide (VIP), neurotensin and encephalin. The resulting inhibitory signals are especially useful for inhibiting some of the intestinal sphincter muscles that impede movement of food along successive segments of the gastrointestinal tract, such as the pyloric sphincter, which controls emptying of the stomach into the duodenum, and the sphincter of the ileocecal valve, which controls emptying from the small intestine into the cecum. B- Meissner Nerve Plexus OR submucus nerve plexus Meissner plexus is otherwise called submucus nerve plexus. It is situated in between the muscular layer and submucosal layer of GIT. Function of meissner plexus is the regulation of secretory functions of GI tract. These nerve fibers cause constriction of blood vessels of GI tract. Note :- - Acetylcholine most often excites gastrointestinal activity. - Norepinephrine almost always inhibits gastrointestinal activity. — Extrinsic Nerve Supply Extrinsic nerves that control the enteric nervous system are from autonomic nervous system. Both sympathetic and parasympathetic divisions 5  of autonomic nervous system innervate the GIT. The intestine receives a dual extrinsic innervation from the autonomic nervous system, with parasympathetic cholinergic activity generally increasing the activity of intestinal smooth muscle and sympathetic noradrenergic activity generally decreasing it while causing sphincters to contract. 1- Sympathetic Nerve Fibers Preganglionic sympathetic nerve fibers to GIT arise from lateral horns of spinal cord between fifth thoracic and second lumbar segments (T5 to L2). From here, the fibers leave the spinal cord, pass through the ganglia of sympathetic chain without having any synapse and then terminate in the celiac and mesenteric ganglia. The postganglionic fibers from these ganglia are distributed throughout the GI tract. Functions of sympathetic nerve fibers are inhibit the movements and decrease the secretions of GIT by secreting the neurotransmitter noradrenaline. It also causes constriction of sphincters. 2- Parasympathetic Nerve Fibers Parasympathetic nerve fibers to GI tract pass through some of the cranial nerves and sacral nerves. The preganglionic and postganglionic parasympathetic nerve fibers to mouth and salivary glands pass through facial and glossopharyngeal nerves. Preganglionic parasympathetic nerve fibers to esophagus, stomach, small intestine and upper part of large intestine pass through vagus nerve. Preganglionic nerve fibers to lower part of large intestine arise from second, third and fourth sacral segments (S2, S3 and S4) of spinal cord and pass through pelvic nerve. All these preganglionic parasympathetic nerve fibers synapse with the postganglionic nerve cells in the myenteric and submucus plexus. Functions of parasympathetic nerve fibers is accelerate the movements and increase the secretions of GI tract. The neurotransmitter secreted by the parasympathetic nerve fibers is acetylcholine (Ach). Afferent sensory nerve fibers from the gut. — Many afferent sensory nerve fibers innervate the gut. Some of them have their cell bodies in the enteric nervous system itself and some in the dorsal root ganglia of the spinal cord. These sensory nerves can be stimulated by: (1) irritation of the gut mucosa. (2) excessive distention of the gut. (3) presence of specific chemical substances in the gut. — Signals transmitted through the fibers can then cause excitation or, under other conditions, inhibition of intestinal movements or intestinal secretion. In addition, other sensory signals from the gut go all the way to multiple areas of the spinal cord and even the brain stem. For example, 80 6 percent of the nerve fibers in the vagus nerves are afferent rather than efferent. These afferent fibers transmit sensory signals from the gastrointestinal tract into the brain medulla, which in turn initiates vagal reflex signals that return to the gastrointestinal tract to control many of its functions. Gastrointestinal reflexes. — The anatomical arrangement of the enteric nervous system and its connections with the sympathetic and parasympathetic systems support three types of gastrointestinal reflexes that are essential to gastrointestinal control. They are the following: 1. Reflexes that are integrated entirely within the gut wall enteric nervous system. These include reflexes that control much gastrointestinal secretion, peristalsis, mixing contractions, local inhibitory effects, and so forth. 2. Reflexes from the gut to the prevertebral sympathetic ganglia and then back to the gastrointestinal tract. These reflexes transmit signals long distances to other areas of the gastrointestinal tract, such as signals from the stomach to cause evacuation of the colon (the gastrocolic reflex), signals from the colon and small intestine to inhibit stomach motility and stomach secretion (the enterogastric reflexes), and reflexes from the colon to inhibit emptying of ileal contents into the colon (the colonoileal reflex). 7 3. Reflexes from the gut to the spinal cord or brain stem and then back to the gastrointestinal tract. These include especially (1) Reflexes from the stomach and duodenum to the brain stem and back to the stomach by way of the vagus nerves to control gastric motor and secretory activity. (2) Pain reflexes that cause general inhibition of the entire gastrointestinal tract. (3) Defecation reflexes that travel from the colon and rectum to the spinal cord and back again to produce the powerful colonic, rectal, and abdominal contractions required for defecation (the defecation reflexes). Hormonal control of gastrointestinal motility The gastrointestinal hormones are released into the portal circulation and exert physiological actions on target cells with specific receptors for the hormone. The effects of the hormones persist even after all nervous connections between the site of release and the site of action have been severed. 1. Gastrin — Is secreted by the “G” cells of the antrum of the stomach in response to stimuli associated with ingestion of a meal, such as distention of the stomach, the products of proteins, and gastrin releasing peptide, which is released by the nerves of the gastric mucosa during vagal stimulation. The primary actions of gastrin are: (1) Stimulation of gastric acid secretion. (2) Stimulation of growth of the gastric mucosa. 2.Cholecystokinin (CCK) — Is secreted by “I” cells in the mucosa of the duodenum and jejunum mainly in response to digestive products of fat, fatty acids, and monoglycerides in the intestinal contents. This hormone strongly contracts the gallbladder, expelling bile into the small intestine where the bile in turn plays important roles in emulsifying fatty substances, allowing them to be digested and absorbed. Cholecystokinin also inhibits stomach contraction moderately. Therefore, at the same time that this hormone causes emptying of the 8  gallbladder, it also slows the emptying of food from the stomach to give adequate time for digestion of the fats in the upper intestinal tract. — 3.Secretin — Was the first gastrointestinal hormone discovered and is secreted by the “S” cells in the mucosa of the duodenum in response to acidic gastric juice emptying into the duodenum from the pylorus of the stomach. Secretin has a mild effect on motility of the gastrointestinal tract and acts to promote pancreatic secretion of bicarbonate which in turn helps to neutralize the acid in the small intestine. 4.Gastric inhibitory peptide — Is secreted by the mucosa of the upper small intestine, mainly in response to fatty acids and amino acids but to a lesser extent in response to carbohydrate. It has a mild effect in decreasing motor activity of the stomach and therefore slows emptying of gastric contents into the duodenum when the upper small intestine is already overloaded with food products. 5.Motilin — Is secreted by the upper duodenum during fasting, and the only known function of this hormone is to increase gastrointestinal motility. Motilin is released cyclically and stimulates waves of gastrointestinal motility called interdigestive myoelectric complexes that move through the stomach and small intestine every 90 minutes in a fasted person. Motilin secretion is inhibited after ingestion by mechanisms that are not fully understood. Functional types of movements in the gastrointestinal tract Two types of movements occur in the gastrointestinal tract: (1) Propulsive movements, which cause food to move forward along the tract at an appropriate rate to accommodate digestion and absorption, (2) Mixing movements, which keep the intestinal contents thoroughly mixed at all times. 1. Propulsive movements Peristalsis — The basic propulsive movement of the gastrointestinal tract is peristalsis, A contractile ring appears around the gut and then moves forward; this is analogous to putting one’s fingers around a thin distended tube, then constricting the fingers and sliding them forward along the tube. Any material in front of the contractile ring is moved forward. Peristalsis is an inherent property of many syncytial smooth muscle tubes; stimulation at any point in 9 the gut can cause a contractile ring to appear in the circular muscle, and this ring then spreads along the gut tube. (Peristalsis also occurs in the bile ducts, glandular ducts, ureters, and many other smooth muscle tubes of the body). — The usual stimulus for intestinal peristalsis is distention of the gut. That is, if a large amount of food collects at any point in the gut, the stretching of the gut wall stimulates the enteric nervous system to contract the gut wall 2 to 3 centimeters behind this point, and a contractile ring appears that initiates a peristaltic movement. Other stimuli that can initiate peristalsis include chemical or physical irritation of the epithelial lining in the gut. Also, strong parasympathetic nervous signals to the gut will elicit strong peristalsis. Function of the myenteric plexus in peristalsis. — Peristalsis occurs only weakly or not at all in any portion of the gastrointestinal tract that has congenital absence of the myenteric plexus. Also, it is greatly depressed or completely blocked in the entire gut when a person is treated with atropine to paralyze the cholinergic nerve endings of the myenteric plexus. Therefore, effectual peristalsis requires an active myenteric plexus. 2. Mixing movements — Mixing movements differ in different parts of the alimentary tract. In some areas, the peristaltic contractions themselves cause most of the mixing. This is especially true when forward progression of the intestinal contents is blocked by a sphincter, so that a peristaltic wave can then only churn the intestinal contents, rather than propelling them forward. At other times, local intermittent constrictive contractions occur every few centimeters in the gut wall. These constrictions usually last only 5 to 30 seconds; then new constrictions occur at other points in the gut, thus “chopping” and “shearing” the contents first here and then there. These peristaltic and constrictive movements are modified in different parts of the gastrointestinal tract for proper propulsion and mixing. 11