chapter 62: general principles of gi function—motility, nervous control, and blood circulation...
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Unit One: Introduction to Physiology: The Cell and General Physiology
Unit Twelve: Gastrointestinal PhysiologyChapter 62: General Principles of GI FunctionMotility, Nervous Control, and Blood CirculationGuyton and Hall, Textbook of Medical Physiology, 12th editionAlimentary Tract Provides the Body with Water, Nutrients, Electrolytes,and Vitamins By:
Movement of food through the alimentary tractSecretion of digestive juices and digestion of the foodAbsorption of water, various electrolytes, vitamins,and digestive productsCirculation of blood through the GI organs to carryaway the absorbed substancesControl of all these functions by local, nervous, andhormonal systemsAlimentary Tract
Fig. 62.1General Principles of GI Motility Physiologic Anatomy of the GI Wall- Layers fromthe outer to inner
SerosaLongitudinal smooth muscle layerCircular smooth muscle layerSubmucosaMucosaGeneral Principles of GI Motility Physiologic Anatomy of the GI Wall
Fig. 62.2 Typical cross section of the gutGeneral Principles of GI Motility GI Smooth Muscle Functions As a Syncytium
Individual smooth muscle fibers are 200-500 umin length, 2-10 um in diameter, and arranged inbundles containing as many as 1000 fibers
Fibers are electrically connected through largenumbers of gap junctions allowing rapid movementof electrical signals for contraction
Muscle bundles fuse with each other at many pointsso in reality each layer is a branching latticeworkof 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 ofslow-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 smoothmuscle cells
Do not cause muscle contraction by themselvesbut excite the appearance of intermittent spikepotentials, which then excite the muscle
General Principles of GI Motility Spike Potentials
True action potentials
Occur automatically when the restingmembrane potential of the GI smooth musclebecomes more positive than -40 mV.
Last 10-40X as long in GI smooth muscle as inlarge nerve fibers
General Principles of GI Motility Spike Potentials
Channels responsible are calcium-sodium channels
Channels are much slower to open and close thanthose of nerves
General Principles of GI Motility Changes in Voltage of the Resting MembranePotential
Under normal conditions the resting potential is-56 mV
Factors that depolarizeStretching of the muscleStimulation by AcH (parasympathetic)Stimulation by specific GI hormones
General Principles of GI Motility Changes in Voltage of the Resting MembranePotential
Factors that hyperpolarize
Effect of epinephrine or norepinephrineStimulation of sympathetic nerves thatsecrete mainly norepinephrine
General Principles of GI Motility Calcium Ions and Muscle Contraction
Calcium ion, acting through a calmodulin mechanismactivate the myosin fibers, causing interaction withthe actin fibers to initiate contraction
Slow waves do not cause calcium ions to enter thesmooth muscle fiber (only sodium)-so no contraction
Spike potentials allow significant calcium to enterand cause most of the contraction
General Principles of GI Motility Tonic Contraction of Some GI Smooth Muscle
Tonic contraction is continuous and notassociated with the basic electrical rhythm ofthe slow waves
Sometimes caused by continuous repetitivespike potentials
Can be caused by hormones
Continuous entry of calcium in ways not associatedwith changes in membrane potential
Neural Control of GI Function-Enteric Nervous System 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 layers2.Submucosal pleuus-lies in the submucosa
Neural Control of GI Function-Enteric Nervous System Enteric Nervous System
Fig. 62.4Neural Control of GI Function-Enteric Nervous SystemSensory nerve endings that originate in the GI wall orepithelium send afferent fibers to both plexuses as wellas
Prevertebral ganglia of the sympathetic systemTo the spinal cordIn the vagus nerves all the way to the brain stemNeural Control of GI Function-Enteric Nervous SystemDifferences Between the Myenteric and Submucosal Plexuses
Stimulation of myenteric plexus causes
Increased tonic contraction of the gut wallIncreased intensity of rhythmical contractionsSlightly increased rate of the rhythm of contractionIncreased velocity of conduction of excitatorywaves along the gut wallNeural Control of GI Function-Enteric Nervous SystemDifferences Between the Myenteric and Submucosal Plexuses
Some neurons of the myenteric are inhibitory
Submucosal plexus
Mainly concerned with controlling function withinthe inner wall of each minute segment of the intestine
Neural Control of GI Function-Enteric Nervous SystemTypes of Neurotransmitters Secreted by EntericNeurons
Acetylcholine-most often excitatoryNorepinephrine and epinephrine-most often inhibitoryATPSerotoninDopamineCCKSubstance PSomatostatinEnkephalins
Neural Control of GI Function-Enteric Nervous SystemAutonomic Control of the GI Tract
Parasympathetic stimulation increases activity ofthe Enteric Nervous System
Sympathetic stimulation usually inhibits GI tractactivity
By the direct effect of norepinephrine of smoothmuscleBy the inhibitory effects of norepinephrine on the neurons of the Enteric Nervous SystemNeural Control of GI Function-Enteric Nervous SystemAfferent Sensory Nerve Fibers From the Gut- cell bodies may be in the Enteric Nervous System orin the dorsal root ganglia of the spinal cord;stimulated by
Irritation of the gut mucosaExcessive distension of the gutPresence of specific chemicals in the gut
Neural Control of GI Function-Enteric Nervous SystemGastrointestinal Reflexes
Reflexes that are integrated entirely within the gutwall enteric nervous system
Reflexes from the gut to the prevertebral sympatheticganglia and then back to the GI tract
Reflexes from the gut to the spinal cord or brain stemand back to the GI tract
Neural Control of GI Function-Enteric Nervous SystemHormonal Control of GI Motility (Table 62.1)
Hormone Stimulus forSecretionSite of SecretionActionsGastrinProtein, Distension,Nerve (acid inhibits release)G cells of the antrum, duodenum, and jejunumStimulates gastric acid secretion and mucosal growthCCKProtein, Fat, AcidI cells of the small intestineStimulates pancreatic secretions, gallbladder contraction and growth of exocrine pancreas. Inhibits gastric emptyingSecretinAcid, FatS cells of the small intestineStimulates pepsin secretion and bicarbonate secretion, growth of exocrine pancreas. Inhibits gastric acid secretionGastric InhibitoryPeptide (GIP)Protein, Fat, CarbohydrateK cells of the duodenum and jejunumStimulates insulin release and inhibits gastric acid secretionMotilinFat, Acid, NerveM cells of the duodenum and jejunumStimulates gastric motility and intestinal motilityFunctional Types of Movements in the GI TractPropulsive Movements-Peristalsis
Fig. 62.5 PeristalsisFunctional Types of Movements in the GI TractPropulsive Movements-Peristalsis
Usual stimulus is distension of the gut
Other stimuli can include chemical or physicalirritation of the gut or strong parasympatheticstimulation
Function of the myenteric plexus-effectual peristalsis requires a functional myenteric plexus
Functional Types of Movements in the GI TractPropulsive Movements-Peristalsis
Directional movement of peristaltic waves is toward the anus
Peristaltic Reflex and the Law of the Gut-alternating contraction and relaxation as peristalsisoccurs; the peristaltic reflex plus the direction ofmovement is called the law of the gut
Functional Types of Movements in the GI TractMixing Movements
Differ in different parts of the alimentary tract
Other than typical peristalsis, there is localintermittent constrictive contractions
also, if peristalsis is blocked by a sphincter thenonly churning occursGastrointestinal Blood Flow- Splanchnic CirculationMixing Movements
Differ in different parts of the alimentary tract
Other than typical peristalsis, there is localintermittent constrictive contractions
Also, if peristalsis is blocked by a sphincter thenonly churning occursGastrointestinal Blood Flow- Splanchnic Circulation
Fig. 62.6 Splanchnic circulation31Gastrointestinal Blood Flow- Splanchnic CirculationFig. 62.7 Arterial blood supply to the intestines through the mesenteric web Anatomy of the GI Blood Supply
32Gastrointestinal Blood Flow- Splanchnic Circulation Effect of Gut Activity and Metabolic Factors on GIBlood 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 theintestinal tract during digestion (CCK, gastrin,secretin, vasoactive intestinal peptide)33Gastrointestinal Blood Flow- Splanchnic CirculationRelease of kallidin and bradykinin
Decreased oxygen cocentration in the gut wall; decrease in oxygen can lead to a fourfold increasein adenosne (vasodilator)34Gastrointestinal Blood Flow- Splanchnic CirculationCountercurrent Blood Flow in the Villi
Fig. 62.8 35Gastrointestinal Blood Flow- Splanchnic CirculationNervous Control of GI Blood Flow
Parasympathetic nerves increase local blood flowand increases glandular secretion
Sympathetic causes intense vasoconstriction ofthe arterioles and decreases blood flow
Sympathetic vasoconstriction allows skeletalmuscle and the heart to get extra flow when needed36
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