General Principles of Gastrointestinal Function

Gastrointestinal Physiology General Principles of Gastrointestinal Function • Alimentary tract provides water, electrolytes, vitamins, and nutrients • 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 gastrointestinal organs to carry away the absorbed substances • Control of all these functions by local, nervous, and hormonal systems. General Principles of Gastrointestinal Function Physiological Anatomy Of The Gastrointestinal Wall Typical cross section of the intestinal wall, from the outer surface to inward: (1) Serosa (2) Longitudinal smooth muscle layer (3) Circular smooth muscle layer (4) Submucosa (5) Mucosa Sparse bundles of smooth muscle fibers, the mucosal muscle, lie in the deeper layers of the mucosa. General Principles of Gastrointestinal Function Gastrointestinal Smooth Muscle Functions as a Syncytium Within each bundle, the muscle fibers are connected with one another through gap junctions. Therefore, electrical signals that initiate muscle contractions can travel readily from one fiber to the next within each bundle but more rapidly along the length of the bundle than sideways. Each muscle layer functions as a syncytium. The distance that it travels depends on the excitability of the muscle. General Principles of Gastrointestinal Function Electrical Activity of Gastrointestinal Smooth Muscle Basic types of electrical waves: 1) slow waves 2) spikes Voltage of the RMP of the gastrointestinal smooth muscle can change to different levels. General Principles of Gastrointestinal Function Electrical Activity of Gastrointestinal Smooth Muscle slow waves Most gastrointestinal contractions occur rhythmically, and this rhythm is determined mainly by the frequency of “slow waves”. Frequency ranges in different parts of the human gastrointestinal tract from 3 to 12 per minute. • 3 in the body of the stomach • 12 in the duodenum • 8 or 9 in the terminal ileum Slow waves appear to be caused by complex interactions among the smooth muscle cells and the interstitial cells of Cajal, (act as electrical pacemakers) The slow waves usually do not by themselves cause muscle contraction in most parts of the GI tract, except perhaps in the stomach. General Principles of Gastrointestinal Function Electrical Activity of Gastrointestinal Smooth Muscle Spike Potentials The spike potentials are true action potentials. They occur automatically when the RMP of the gastrointestinal smooth muscle becomes more positive than about −40 millivolts. The higher the slow wave potential rises, the greater the frequency of the spike potentials, usually ranging between 1 and 10 spikes per second. General Principles of Gastrointestinal Function Changes in Voltage of the Resting Membrane Potential Factors that depolarize the membrane: • • • Stretching of the muscle Acetylcholine released from parasympathetic nerves Several specific gastrointestinal hormones Factors that hyperpolarize the membrane: • • Norepinephrine or epinephrine Stimulation of the sympathetic nerves Entry of Calcium Ions Causes Smooth Muscle Contraction.  Tonic Contraction Tonic contraction is continuous; it is not associated with the BER of the slow waves but often lasts several minutes or even hours. Tonic contraction often increases or decreases in intensity but continues. General Principles of Gastrointestinal Function Neural Control Of Gastrointestinal Function—enteric Nervous System • • Enteric nervous system is especially important in controlling gastrointestinal movements and secretion. Beginning in the esophagus and extending to the anus. 1) Myenteric plexus or Auerbach’s plexus 2) Submucosal plexus or Meissner’s plexus • • The myenteric plexus controls mainly the gastrointestinal movements. The submucosal plexus controls mainly gastrointestinal secretion and local blood flow. 1 (internal) 2 (external) General Principles of Gastrointestinal Function Neural Control Of Gastrointestinal Function—enteric Nervous System • Sensory nerves can elicit local reflexes within the gut wall itself and still other reflexes that are relayed to the gut from either the prevertebral ganglia or the basal regions of the brain. General Principles of Gastrointestinal Function Neural Control Of Gastrointestinal Function—enteric Nervous System • Differences Between The Myenteric And Submucosal Plexuses • Myenteric plexus consists mostly of a linear chain of many interconnecting neurons that extends the entire length of the gastrointestinal tract. Myenteric plexus is concerned mainly with controlling muscle activity: 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. The myenteric plexus should not be considered entirely excitatory because some of its neurons are inhibitory. The inhibitory signals are useful for inhibiting some of the intestinal sphincter muscles(pyloric sphincter, sphincter of the ileocecal valve). • • • Submucosal plexus is mainly concerned with controlling function within the inner wall of each minute segment of the intestine. General Principles of Gastrointestinal Function Autonomic Control of the Gastrointestinal Tract • • • Parasympathic Parasympathetic Stimulation Increases Activity of the Enteric Nervous System. Parasympathetic supply to the gut: cranial and sacral. Except for a few parasympathetic fibers to the mouth and pharyngeal regions of the alimentary tract, the cranial parasympathetic nerve fibers are in the vagus nerves. • Extensive innervation to the esophagus, stomach, and pancreas and somewhat less to the intestines down through the first half of the large intestine. Parasympathic General Principles of Gastrointestinal Function Autonomic Control of the Gastrointestinal Tract • The sacral parasympathetic originate in the S2-S4 segments of the spinal cord and pass through the pelvic nerves to the distal half of the large intestine and all the way to the anus. • The sigmoidal, rectal, and anal regions are considerably better supplied with parasympathetic fibers. Parasympathic General Principles of Gastrointestinal Function Autonomic Control of the Gastrointestinal Tract • • • • Sympathetic Stimulation Usually Inhibits Gastrointestinal Tract Activity. The sympathetic originate in the spinal cord between segments T5 and L2. The sympathetic innervate essentially all of the gastrointestinal tract. It exerts its effects in two ways: Sympathetic 1) Direct effect of NEP to inhibit intestinal tract smooth muscle (except the mucosal muscle, which it excites) 2) Inhibitory effect of NEP on the neurons of the ENS. Sympathetic General Principles of Gastrointestinal Function Afferent Sensory Nerve Fibers From the Gut • • Many afferent sensory nerve fibers innervate the gut. Some of the nerve fibers have their cell bodies in the enteric nervous system and some have them 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) Specific chemical substances in the gut • • Signals transmitted through the fibers can then cause excitation or inhibition of intestinal movements or intestinal secretion. Other sensory signals from the gut go all the way to multiple areas of the spinal cord and even to the brain stem. General Principles of Gastrointestinal Function Gastrointestinal Reflexes • 1. Reflexes that are integrated entirely within the gut wall ENS Gastrointestinal secretion Peristalsis Mixing contractions Local inhibitory effects • 2. Reflexes from the gut to the prevertebral sympathetic ganglia and then back to the gastrointestinal tract. Gastrocolic reflex: signals from the stomach to cause evacuation of the colon Enterogastric reflex: signals from the colon and small intestine to inhibit stomach motility and stomach secretion Colonoileal reflex: from the colon to inhibit emptying of ileal contents into the colon. • 3. Reflexes from the gut to the spinal cord or brain stem and then back to the gastrointestinal tract. 1) reflexes from the stomach and duodenum to the brain stem and back to the stomach— by vagus nerves—to control gastric motor and secretory activity 2) pain reflexes that cause general inhibition of the entire gastrointestinal tract 3) defecation reflexes