General Principles of Gastrointestinal Function PDF

General Principles of Gastrointestinal Function Dr. ArzuTemizyürek OUTLINE 1. General Principles of Gastrointestinal Function— Motility, Nervous Control, and Blood Circulation 2. Propulsion and Mixing of Food in the Alimentary Tract 3. Secretory Functions of the Alimentary Tract 4. Digestion and...

General Principles of Gastrointestinal Function Dr. ArzuTemizyürek OUTLINE 1. General Principles of Gastrointestinal Function— Motility, Nervous Control, and Blood Circulation 2. Propulsion and Mixing of Food in the Alimentary Tract 3. Secretory Functions of the Alimentary Tract 4. Digestion and Absorption in the Gastrointestinal Tract 5. Physiology of Gastrointestinal Disorders Gastrointestinal (GI) tract [Alimentary canal] a continuous muscular digestive tube  Digests:  breaks food into smaller fragments  Absorbs:  digested material is moved through mucosa into the blood  Eliminates:  unabsorbed & secreted wastes ORGAN SYSTEMS  Includes:  Mouth, pharynx & esophagus  Stomach  Small intestine  Large intestine  Accessory digestive organs: teeth, tongue, gall bladder, salivary glands, liver & pancreas Figure 23.1 Gastrointestinal (GI) tract [Alimentary canal] (1) Movement of food through the alimentary tract (2) Secretion of digestive juices and digestion of the food (3) Absorption of water, various electrolytes, vitamins, 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 John E. Hall; Guyton & Hall Physiologic Anatomy of the GI Wall- Layers from the outer to inner 1.Serosa 2.Longitudinal smooth muscle layer 3. Circular smooth muscle layer 4. Submucosa 5. Mucosa CANAL Figure 23.6 General Principles of GI Motility GI Smooth Muscle Functions As a Syncytium ▪ Individual smooth muscle fibers in the GI tract are arranged in bundles and they are arranged in bundles of as many as 1000 parallel fibers ▪ In the longitudinal muscle layer, the bundles extend longitudinally down the intestinal tract ▪ In the circular muscle layer, they extend around the gut ORGANIZED INTO TWO LAYERS (LONGITUDINAL AND CIRCULAR) OF CLOSELY APPOSED FIBERS  When the longitudinal layer contracts the organ dilates and shortens  When the circular layer contracts the organ is elongated.  Peristalsis – alternating contractions and relaxations of smooth muscles that mix and squeeze substances through the lumen of organs General Principles of GI Motility GI Smooth Muscle Functions As a Syncytium ▪ Within each bundle, the muscle fibers are electrically connected with one another through large numbers of gap junctions allowing rapid movement of electrical signals for contraction. NERVE SUPPLY  Smooth muscles are innervated by the autonomic nerves, both sympathetic as well as parasympathetic.  The two have opposite effects.  In some organs sympathetic stimulation causes contraction and parasympathetic stimulation causes relaxation of smooth muscles. While in some other organs a reverse action is seen. Electrical Activity of GI Smooth Muscle Slow Waves . Most gastrointestinal contractions occur rhythmically, and this rhythm is determined mainly by the frequency of “slow waves” of smooth muscle membrane potential. • Usually do not by themselves cause muscle contraction in most parts of the gastrointestinal tract, except in the stomach. • Instead, they mainly excite the spike potentials, and the spike potentials in turn actually excite the muscle contraction. Electrical Activity of GI Smooth Muscle Spike Potentials • The spike potentials are true action potentials. • They occur automatically when the resting membrane potential of the gastrointestinal smooth muscle becomes more positive. • The higher the slow wave potential rises, the greater the frequency of the spike potentials, usually ranging between 1 and 10 spikes per second. Spike potentials appear on these peaks DIFFERENCE OF ACTION POTENTIAL BETWEEN GI SMOOTH MUCLE AND NERVE FIBERS 1. The spike potentials last 10 to 40 times as long in gastrointestinal muscle as the action potentials in large nerve fibers, with each gastrointestinal spike lasting as long as 10 to 20 milliseconds. II. 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 gastrointestinal smooth muscle fibers, they allow especially large numbers of calcium ions to enter along with smaller numbers of sodium ions and therefore are called calcium-sodium channels. III. 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. Also contracts the muscle fibers in the intestine. Electrical Activity of GI Smooth Muscle Changes in Voltage of the Resting Membrane Potential. CHANGES IN VOLTAGE OF THE RESTING MEMBRANE POTENTIAL. Factors that depolarize the GI membrane (1)Stretching of the muscle (2)Stimulation by acetylcholine (released from the endings of parasympathetic nerves), (3) Stimulation by several specific gastrointestinal hormones. Smooth muscle contraction occurs in response to entry of calcium ions into the muscle fiber Electrical Activity of GI Smooth Muscle Tonic Contraction of Some Gastrointestinal Smooth Muscle. Some smooth muscle of the gastrointestinal tract exhibits tonic contraction as well as, or instead of, rhythmical contractions. Tonic contraction is continuous Causes of tonic contraction I. Continuous repetitive spike potentials the greater the frequency, the greater the degree of contraction. III. Continuous entry of II. Hormones or other factors that bring . calcium ions into the about continuous partial depolarization of the interior of the cell brought smooth muscle membrane without causing about in ways not associated action potentials. with changes in membrane potential COPYRIGHT 2009, JOHN WILEY & SONS, INC. PREVERTEBRAL GANGLIA PARAVERTEBRAL GANGLIA (TRUNCUS SYMPATHICUS) NEURAL CONTROL OF GASTROINTESTINAL FUNCTION— ENTERIC NERVOUS SYSTEM The gastrointestinal tract has a nervous system all its own called the enteric nervous system. Beginning in the esophagus and extending all the way to the anus. This highly developed enteric nervous system is especially important in controlling gastrointestinal movements and secretion. The enteric nervous system is composed of two plexuses (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, which lies in the submucosa. NEURAL CONTROL OF GASTROINTESTINAL FUNCTION— ENTERIC NERVOUS SYSTEM The myenteric plexus controls mainly the gastrointestinal movements The submucosal plexus controls mainly gastrointestinal secretion and local blood flow. NEURAL CONTROL OF GASTROINTESTINAL FUNCTION— ENTERIC NERVOUS SYSTEM The extrinsic sympathetic and parasympathetic fibers connect to both the myenteric and submucosal plexuses. The enteric nervous system can also function independently NEURAL CONTROL OF GASTROINTESTINAL FUNCTION— ENTERIC NERVOUS SYSTEM There are also sensory nerve endings that originate in the gastrointestinal epithelium or gut wall and send afferent fibers to both plexuses of the enteric system, as well as (1) To the prevertebral ganglia of the sympathetic nervous system, (2) To the spinal cord (3) To the brain stem (within the vagus nerves) Local reflexes within the gut & other reflexes that are relayed to the gut from either the prevertebral ganglia or the basal regions of the brain. DIFFERENCES BETWEEN THE MYENTERIC AND SUBMUCOSAL PLEXUSES The myenteric plexus lies between the longitudinal and circular layers of intestinal smooth muscle, it is concerned mainly with controlling muscle activity along the length of the gut. When this plexus is stimulated, its principal effects are (1) Increased tonic contraction, or “tone,” of the gut wall (2) Increased intensity of the rhythmical contractions (3) Slightly increased rate of the rhythm of contraction (4) Increased velocity of conduction of excitatory waves along the gut wall, causing more rapid movement of the gut peristaltic waves. TYPES OF NEUROTRANSMITTERS SECRETED BY ENTERIC NEURONS Neurotransmitters that are released by the nerve endings of different types of enteric neurons, including: (1) acetylcholine, most often excites gastrointestinal activity (2) norepinephrine, almost always inhibits gastrointestinal activity (3) adenosine triphosphate, (4) serotonin, (5) dopamine, (6) cholecystokinin, (7) Substance P, (8) vasoactive intestinal polypeptide, (9) somatostatin, (10) leu-enkephalin, (11) met-enkephalin (12) bombesin AUTONOMIC CONTROL OF THE GASTROINTESTINAL TRACT In general, stimulation of the sympathetic nervous system inhibits activity of the gastrointestinal tract, causing many effects opposite to those of the parasympathetic system. It exerts its effects in two ways: (1)By direct effect of secreted norepinephrine to inhibit intestinal tract smooth muscle (except the mucosal muscle, which it excites) (2)By an inhibitory effect of norepinephrine on the neurons of the entire enteric nervous system. Strong stimulation of the sympathetic system can inhibit motor movements of the gut so greatly that this can literally block movement of food through the gastrointestinal tract. Autonomic Control of the Gastrointestinal Tract Afferent Sensory Nerve Fibers From the Gut Cell bodies may be in the Enteric Nervous System or in the dorsal root ganglia of the spinal cord; Nerves can be stimulated by 1. Irritation of the gut mucosa 2. Excessive distension of the gut 3. Presence of specific chemicals in the gut Autonomic Control of the Gastrointestinal Tract In addition, other sensory signals from the gut go all the way to multiple areas of the spinal cord and even to the brain stem. ❖For example, 80 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 I. Reflexes that are integrated entirely within the gut wall enteric nervous system Reflexes that control gastrointestinal secretion, peristalsis, mixing contractions, local inhibitory effects etc. GASTROINTESTINAL REFLEXES II. Reflexes from the gut to the prevertebral sympathetic ganglia and then back to the GI tract • Gastrocolic reflex: Signals from the stomach to cause evacuation of the colon • Enterogastric reflexes: Signals from the colon and small intestine to inhibit stomach motility and stomach secretion • Colonoileal reflex: Reflexes from the colon to inhibit emptying of ileal contents into the colon GASTROINTESTINAL REFLEXES III. Reflexes from the gut to the spinal cord or brain stem and then back to the gastrointestinal tract. These reflexes 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 to produce the 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. Most of the same hormones also affect motility in some parts of the gastrointestinal tract. 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 mixed at all times. which keep the intestinal contents thoroughly FUNCTIONAL TYPES OF MOVEMENTS IN THE GASTROINTESTINAL TRACT PROPULSIVE MOVEMENTS—PERISTALSIS The basic propulsive movement of the gastrointestinal tract is peristalsis ❖ A contractile ring appears around the gut and then moves forward ❖ Any material in front of the contractile ring is moved forward ❖ Stimulation at any point in the gut can cause a contractile ring to appear in the circular muscle, and this ring then spreads along the gut tube. FUNCTIONAL TYPES OF MOVEMENTS IN THE GASTROINTESTINAL TRACT Peristaltic Waves Move Toward the Anus With Downstream Receptive Relaxation—“Law of the Gut.” ❖Peristalsis, theoretically, can occur in either direction from a stimulated point, but it normally dies out rapidly in the orad (toward the mouth) direction while continuing for a considerable distance toward the anus. ❖This directional transmission of peristalsis probably results mainly from the fact that the myenteric plexus is “polarized” in the anal direction. Myenteric reflex (peristaltic reflex) ▪ the peristalsis begins on the orad (toward the mounth) side of the distended segment ▪ moves toward the distended segment, pushing the intestinal contents in the anal direction for 5 to 10 cm before dying out. ▪ at the same time, the gut sometimes relaxes several centimeters downstream toward the anus “receptive relaxation,” thus allowing the food to be propelled more easily anally than orad FUNCTIONAL TYPES OF MOVEMENTS IN THE GASTROINTESTINAL TRACT MIXING MOVEMENTS ❖Mixing movements differ in different parts of the alimentary tract. Peristaltic contractions cause most of the mixing When forward progression of the intestinal contents is blocked by a sphincter peristaltic wave churn the intestinal contents, rather than propelling them forward. ❖ At other times, local intermittent constrictive contractions occur every few centimeters in the gut wall. 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. GASTROINTESTINAL BLOOD FLOW— SPLANCHNIC CIRCULATION The blood vessels of the gastrointestinal system are part of a more extensive system called the splanchnic circulation ❖It includes the blood flow through the gut plus blood flows through the spleen, pancreas, and liver. ❖All the blood that courses through the gut, spleen, and pancreas then flows immediately into the liver by way of the portal vein. GASTROINTESTINAL BLOOD FLOW— SPLANCHNIC CIRCULATION ❖ In the liver, the blood passes through millions of minute liver sinusoids and finally leaves the liver by way of hepatic veins that empty into the vena cava of the general circulation ❖ This flow of blood through the liver, before it empties into the vena cava allows the reticuloendothelial cells that line the liver sinusoids to remove bacteria and other particulate matter that might enter the blood from the gastrointestinal tract thus preventing direct transport of potentially harmful agents into the remainder of the body. GASTROINTESTINAL BLOOD FLOW— SPLANCHNIC CIRCULATION ❖ The nonfat, water-soluble nutrients absorbed from the gut (such as carbohydrates and proteins) are transported in the portal venous blood to the same liver sinusoids. ❖ Here, both the reticuloendothelial cells and the principal parenchymal cells of the liver, the hepatic cells absorb and store temporarily of the nutrients. GASTROINTESTINAL BLOOD FLOW— SPLANCHNIC CIRCULATION Almost all of the fats absorbed from the intestinal tract are not carried in the portal blood but instead are absorbed into the intestinal lymphatics and then conducted to the systemic circulating blood by way of the thoracic duct ANATOMY OF THE GASTROINTESTINAL BLOOD SUPPLY • Special organization of the blood flow through an intestinal villus, including a small arteriole and venule that interconnect with a system of multiple looping capillaries. • The walls of the arterioles are highly muscular and highly active in controlling villus blood flow. Figure 63-8. Microvasculature of the villus, showing a countercurrent arrangement of blood flow in the arterioles and venules. EFFECT OF GUT ACTIVITY AND METABOLIC FACTORS ON GASTROINTESTINAL BLOOD FLOW “Countercurrent” Blood Flow in the Villi. ❖ The arterial flow into the villus and the venous flow out of the villus are in directions opposite to each other and that the vessels lie in close apposition to each other. ❖ Because of this vascular arrangement, much of the blood oxygen diffuses out of the arterioles directly into the adjacent venules without ever being carried in the blood to the tips of the villi. EFFECT OF GUT ACTIVITY AND METABOLIC FACTORS ON GASTROINTESTINAL BLOOD FLOW • The blood flow in each area of the gastrointestinal tract, as well as in each layer of the gut wall, is directly related to the level of local activity. • Blood flow in the muscle layers of the intestinal wall increases with increased motor activity in the gut. • For instance, after a meal, the motor activity, secretory activity, and absorptive activity all increase; likewise, the blood flow increases greatly but then decreases back to the resting level over another 2 to 4 hours. EFFECT OF GUT ACTIVITY AND METABOLIC FACTORS ON GASTROINTESTINAL BLOOD FLOW Possible Causes of the Increased Blood Flow During Gastrointestinal Activity. I. Several vasodilator substances are released from the mucosa of the intestinal tract during the digestive process. Most of these substances are peptide hormones, including • • • • cholecystokinin, vasoactive intestinal peptide, gastrin secretin. These same hormones control specific motor and secretory activities of the gut II. Some of the gastrointestinal glands release into the gut wall two kinins, kallidin and bradykinin, at the same time that they secrete other substances into the lumen. Cause much of the increased mucosal vasodilation that occurs along with secretion. EFFECT OF GUT ACTIVITY AND METABOLIC FACTORS ON GASTROINTESTINAL BLOOD FLOW III. Decreased oxygen concentration in the gut wall can increase blood flow The decrease in oxygen can also lead to as much as a fourfold increase of adenosine, a well-known vasodilator that could be responsible for much of the increased flow