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