Unit Twelve: Gastrointestinal Physiology Chapter 62: General Principles of GI Function—Motility, Nervous Control, and Blood Circulation Guyton and Hall, Textbook of Medical Physiology, 12th edition

Alimentary Tract Provides the Body with Water, Nutrients, Electrolytes, and Vitamins By: Movement of food through the alimentary tract Secretion of digestive juices and digestion of the food Absorption of water, various electrolytes, vitamins, and digestive products Circulation of blood through the GI organs to carry away the absorbed substances Control of all these functions by local, nervous, and hormonal systems

Alimentary Tract Fig. 62.1

General Principles of GI Motility Physiologic Anatomy of the GI Wall- Layers from the outer to inner Serosa Longitudinal smooth muscle layer Circular smooth muscle layer Submucosa Mucosa

General Principles of GI Motility Physiologic Anatomy of the GI Wall Fig. 62.2 Typical cross section of the gut

General Principles of GI Motility GI Smooth Muscle Functions As a Syncytium Individual smooth muscle fibers are 200-500 um in length, 2-10 um in diameter, and arranged in bundles containing as many as 1000 fibers Fibers are electrically connected through large numbers of gap junctions allowing rapid movement of electrical signals for contraction Muscle bundles fuse with each other at many points so in reality each layer is a branching latticework of smooth muscle bundles

General Principles of GI Motility GI Smooth Muscle Functions As a Syncytium When an AP is elicited anywhere within the muscle mass, it generally travels in all directions Electrical Activity of GI Smooth Muscle Slow waves-most GI contractions occur rhythmically, and this is determined mainly by the frequency of slow-waves of smooth muscle

General Principles of GI Motility Fig. 62.3 Membrane potentials in intestinal smooth muscle.

General Principles of GI Motility Slow Waves Not APs, but slow undulating changes in the resting membrane potential Appear to be caused by interactions between smooth muscle cells and the interstitial cells of Cajal (act as electrical pacemakers for smooth muscle cells Do not cause muscle contraction by themselves but excite the appearance of intermittent spike potentials, which then excite the muscle

General Principles of GI Motility Spike Potentials True action potentials Occur automatically when the resting membrane potential of the GI smooth muscle becomes more positive than -40 mV. Last 10-40X as long in GI smooth muscle as in large nerve fibers

General Principles of GI Motility Spike Potentials Channels responsible are calcium-sodium channels Channels are much slower to open and close than those of nerves

General Principles of GI Motility Changes in Voltage of the Resting Membrane Potential Under normal conditions the resting potential is -56 mV Factors that depolarize Stretching of the muscle Stimulation by AcH (parasympathetic) Stimulation by specific GI hormones

General Principles of GI Motility Changes in Voltage of the Resting Membrane Potential Factors that hyperpolarize Effect of epinephrine or norepinephrine Stimulation of sympathetic nerves that secrete mainly norepinephrine

General Principles of GI Motility Calcium Ions and Muscle Contraction Calcium ion, acting through a calmodulin mechanism activate the myosin fibers, causing interaction with the actin fibers to initiate contraction Slow waves do not cause calcium ions to enter the smooth muscle fiber (only sodium)-so no contraction Spike potentials allow significant calcium to enter and cause most of the contraction

General Principles of GI Motility Tonic Contraction of Some GI Smooth Muscle Tonic contraction is continuous and not associated with the basic electrical rhythm of the slow waves Sometimes caused by continuous repetitive spike potentials Can be caused by hormones Continuous entry of calcium in ways not associated with changes in membrane potential

Neural Control of GI Function-Enteric Nervous System Lies entirely within the wall of the gut Composed of 100 million neurons Composed of mainly two plexuses Myenteric plexus-outer plexus between the longitudinal and circular muscle layers 2. Submucosal pleuus-lies in the submucosa

Neural Control of GI Function-Enteric Nervous System Fig. 62.4

Neural Control of GI Function-Enteric Nervous System Sensory nerve endings that originate in the GI wall or epithelium send afferent fibers to both plexuses as well as Prevertebral ganglia of the sympathetic system To the spinal cord In the vagus nerves all the way to the brain stem

Neural Control of GI Function-Enteric Nervous System Differences Between the Myenteric and Submucosal Plexuses Stimulation of myenteric plexus causes Increased tonic contraction of the gut wall Increased intensity of rhythmical contractions Slightly increased rate of the rhythm of contraction Increased velocity of conduction of excitatory waves along the gut wall

Neural Control of GI Function-Enteric Nervous System Differences Between the Myenteric and Submucosal Plexuses Some neurons of the myenteric are inhibitory Submucosal plexus Mainly concerned with controlling function within the inner wall of each minute segment of the intestine

Neural Control of GI Function-Enteric Nervous System Types of Neurotransmitters Secreted by Enteric Neurons Acetylcholine-most often excitatory Norepinephrine and epinephrine-most often inhibitory ATP Serotonin Dopamine CCK Substance P Somatostatin Enkephalins

Neural Control of GI Function-Enteric Nervous System Autonomic Control of the GI Tract Parasympathetic stimulation increases activity of the Enteric Nervous System Sympathetic stimulation usually inhibits GI tract activity By the direct effect of norepinephrine of smooth muscle By the inhibitory effects of norepinephrine on the neurons of the Enteric Nervous System

Neural Control of GI Function-Enteric Nervous System 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; stimulated by Irritation of the gut mucosa Excessive distension of the gut Presence of specific chemicals in the gut

Neural Control of GI Function-Enteric Nervous System Gastrointestinal Reflexes Reflexes that are integrated entirely within the gut wall enteric nervous system Reflexes from the gut to the prevertebral sympathetic ganglia and then back to the GI tract Reflexes from the gut to the spinal cord or brain stem and back to the GI tract

Neural Control of GI Function-Enteric Nervous System Hormonal Control of GI Motility (Table 62.1) Hormone Stimulus for Secretion Site of Secretion Actions Gastrin Protein, Distension, Nerve (acid inhibits release) G cells of the antrum, duodenum, and jejunum Stimulates gastric acid secretion and mucosal growth CCK Protein, Fat, Acid I cells of the small intestine Stimulates pancreatic secretions, gallbladder contraction and growth of exocrine pancreas. Inhibits gastric emptying Secretin Acid, Fat S cells of the small intestine Stimulates pepsin secretion and bicarbonate secretion, growth of exocrine pancreas. Inhibits gastric acid secretion Gastric Inhibitory Peptide (GIP) Protein, Fat, Carbohydrate K cells of the duodenum and jejunum Stimulates insulin release and inhibits gastric acid secretion Motilin Fat, Acid, Nerve M cells of the duodenum and jejunum Stimulates gastric motility and intestinal motility

Functional Types of Movements in the GI Tract Propulsive Movements-Peristalsis Fig. 62.5 Peristalsis

Functional Types of Movements in the GI Tract Propulsive Movements-Peristalsis Usual stimulus is distension of the gut Other stimuli can include chemical or physical irritation of the gut or strong parasympathetic stimulation Function of the myenteric plexus-effectual peristalsis requires a functional myenteric plexus

Functional Types of Movements in the GI Tract Propulsive Movements-Peristalsis Directional movement of peristaltic waves is toward the anus Peristaltic Reflex and the “Law of the Gut”- alternating contraction and relaxation as peristalsis occurs; the peristaltic reflex plus the direction of movement is called the law of the gut

Functional Types of Movements in the GI Tract Mixing Movements Differ in different parts of the alimentary tract Other than typical peristalsis, there is local intermittent constrictive contractions also, if peristalsis is blocked by a sphincter then only churning occurs

Gastrointestinal Blood Flow- Splanchnic Circulation Mixing Movements Differ in different parts of the alimentary tract Other than typical peristalsis, there is local intermittent constrictive contractions Also, if peristalsis is blocked by a sphincter then only churning occurs

Gastrointestinal Blood Flow- Splanchnic Circulation Fig. 62.6 Splanchnic circulation

Gastrointestinal Blood Flow- Splanchnic Circulation Anatomy of the GI Blood Supply Fig. 62.7 Arterial blood supply to the intestines through the mesenteric web

Gastrointestinal Blood Flow- Splanchnic Circulation Effect of Gut Activity and Metabolic Factors on GI Blood Flow Blood flow in each area of the GI tract and layers of the gut wall is directly related to the level of local activity Causes of increased blood flow during GI activity Vasodilators released from the mucosa of the intestinal tract during digestion (CCK, gastrin, secretin, vasoactive intestinal peptide)

Gastrointestinal Blood Flow- Splanchnic Circulation Release of kallidin and bradykinin Decreased oxygen cocentration in the gut wall; decrease in oxygen can lead to a fourfold increase in adenosne (vasodilator)

Gastrointestinal Blood Flow- Splanchnic Circulation Countercurrent Blood Flow in the Villi Fig. 62.8

Gastrointestinal Blood Flow- Splanchnic Circulation Nervous Control of GI Blood Flow Parasympathetic nerves increase local blood flow and increases glandular secretion Sympathetic causes intense vasoconstriction of the arterioles and decreases blood flow Sympathetic vasoconstriction allows skeletal muscle and the heart to get extra flow when needed