GI #12
Tuesday 2/18 2:00pm
Dr. Gwirtz
Kevin Stancoven for Kara Starnes
Page 1 of 6
Submitted for review
Gastrointestinal Motility – Part I
Response of gut smooth muscle to NE: NE binds to alpha & beta-receptors. Alpha-receptors cause IP3 to form, which opens Ca 2+ channels in sarcoplasmic reticulum & releases Ca 2+ , which causes muscle contraction. Beta-receptors generates intracellular cAMP, which cause the SR to take up Ca 2+ & causes
Ca 2+ pump to pump calcium out of cell, which causes muscle relaxtion. Predominant receptor in gut is beta-2 receptor.
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Objectives o Describe the basic patterns of GI motility: peristalsis, segmentation, & sphincter tone o Describe swallowing & the motor behavior of the lower esophageal sphincter o
Explain the process of mixing of stomach contents o Describe the control of stomach emptying o Explain the movement of material in the small & large intestines o Describe the interdigestive migrating motor complex o
Describe the defecation reflex o Explain disorders of the esophagus: dysphagia, achalasia, esophageal spasm, & heartburn o Describe the motor disorders of constipation, diarrhea, & gas o Explain the vomiting reflex
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Time a meal spends in the GI tract o Mouth to LES – 9 seconds o Digestion in the stomach – 3-5 hours
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Mixes with gastric secretions & ground into small particles o
Time to move through the pyloric sphincter – 1-5 minutes o Time spent in small intestines – 4-5 hours
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Digestion & absorption o In proximal colon – 6-7 hours o
In transverse colon – 9-10 hours
Spends most of the time in the o In distal colon – 12-24 hours
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Type of motility in the GI tract colon o Peristalsis
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Contractile ring which is proceeded by receptive relaxation
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Relaxation in front of chyme & contraction behind it
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Moves material slowly down the GI tract
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Reflex mediated by myenteric nerves. Myenteric nerves lead to weak contraction & reinforced by parasympathetic nerves.
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Peristalsis reflex
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Stimulus o Distention of the gut o Irritation of the epithelial lining
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Complex pattern which requires coordination by the myenteric nervous system
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Moves chyme analward (oral cavity towards anus) o Law of the gut
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Location: pharynx, esophagus, stomach, intestines, colon, & bile ducts o
Mixing movements
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Local constrictive contractions occurring every few cm in the gut wall
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Decreases particle size of food, increases surface area available for digestion by enzymes and for absorption

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Mixes with secretions & exposes to enzymes
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Local constrictive contractions of the gut wall
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Does not necessarily require myenteric nerves for coordination – not a reflex
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Attributed to activity of circular muscle layer
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Varies in rate from 12 (duodenum) to 3 (stomach) per minute in different parts of the gut
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Keeps gut contents thoroughly mixed at all times o Tonic contractions – at sphincters
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Tonic contractions with intermittent relaxation of sphincters
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Serves to regulate the movement of luminal contents
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Sphincters are rings of smooth muscle that remain in a continuous state of contraction, which produces a high pressure zone in the lumen
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Neurotransmitter constantly released to allow continuous Ca 2+ entry into cell
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Separates 2 specialized compartments along the GI tract
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Location of GI sphincters
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Upper esophageal sphincter o Skeletal muscle
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Lower esophageal sphincter o
Smooth muscle
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Gastroduodenal (pyloric) sphincter o Smooth muscle o Between stomach & duodenum
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Ileocecal sphincter o Smooth muscle o Between ileum & large intestines
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Internal anal sphincter o
Smooth muscle
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External anal sphincter o Skeletal muscle
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Sphincter of Oddi o Smooth muscle
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You have control over sphincter with skeletal muscle
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Chewing reflex – mastication o
Initiated by presence of food in mouth o Can be carried out voluntarily, but is more frequently a reflex behavior
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There is a specific part of brainstem that control chewing o Requires parasympathetic nerves & myenteric plexus: central reflex o
Controlled by signals from touch & pressure receptors in the mouth, stretch receptors in the muscles & joints o Aids in digestion of food by increasing the surface area available for enzymes to act o
Mixes food with saliva & makes easier to swallow
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Alpha-amylase begins starch digestion
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Swallowing reflex o Stages:
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Voluntary stage
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Initiates swallowing reflex
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Tongue pushes bolus of food back into pharynx, which stimulates nerve endings around pharynx
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Can start or stop at will – involves motor neuron activation of skeletal muscle o You can control this part of swallowing
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Pharyngeal stage
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Involuntary
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Controlled by brainstem swallowing center

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Initiated by stimulation of swallowing receptors in pharynx
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Results in the passage of food through pharynx into esophagus by pharyngeal muscle contraction
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Inhibition of respiratory center o
Very brief – not noticeable o Swallowing center located next to respiratory center in brainstem
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Trachea & nasopharynx closed by glottis – to prevent aspiration
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Esophageal stage
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Involuntary
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Promotes passage of food from pharynx to stomach
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Primary peristalsis o Initiated by swallowing reflex o Wave of contraction preceded & followed by wave of relaxation o
9 seconds in length o 1 wave carries food from pharynx to stomach o This can still occur when you stand on your head
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Secondary peristalsis o Initiated by irritation or distention of o Repeated peristalsis waves o Example: food getting stuck in esophagus
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Receptive relaxation of stomach
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Figures: o Food being chewed & tongue pushes back toward pharynx o Beginning of swallowing reflex, upper esophageal sphincter is relaxed so food can move through o Sphincter closed & contraction pushes bolus down esophagus o Receptive relaxation of LES & stomach so food moves completely into stomach
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Primary peristalsis o
Continuation of the peristaltic wave that begins in the pharynx & spreads into the esophagus during the pharyngeal stage of swallowing o Passes from pharynx to the stomach in 5-10 seconds o Mediated by vagal nerves & myenteric plexus
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Vagal nerves must be intact for swallowing reflex to occur normally
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Pressures during a swallow o Patient swallows balloon connected to manometer (to measure pressures) o Tracings on right show normal pressure in a swallow
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Pressure at rest is at zero in pharynx & esophagus
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High pressure (30 mm HG) recorded UES & LES at rest
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Get receptive relaxation at sphincters & stomach before bolus reaches sphincters & stomach o
Peristalsis causes an increase in pressure as the bolus is moved down the esophagus o Swallowing disorders cause abnormal pressure readings
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Secondary peristalsis o Stimulus is distension of smooth muscle wall of the esophagus, which may result if the primary peristaltic wave fails to move all of the food into the stomach o Initiated by intrinsic neural circuits in the myenteric plexus o Mediated by myenteric & reinforced by vagal nerve fibers
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LES – lower esophageal sphincter o
Thoracic cavity is at a negative pressure – so it keeps the esophagus open o Abdominal cavity is at a positive pressure – so it collapses the esophagus
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LES is in the abdominal cavity (below diaphragm) o Functional sphincter due to position below the level of the diaphragm
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Not anatomical – doesn’t have smooth muscle rings

o Tonically constricted o
Receptive relaxation of LES & stomach during swallowing o Prevents reflux of stomach contents (acid) into esophagus
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Factors increasing LES tone – increase muscle contraction o Chemical
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Muscarinic agents, alpha-receptor agonists
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ACh, cholenergic medications o Hormonal
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Gastrin (released during digestion in stomach), motilin (released between meals, controls motility in GI tract between meals), NPY (co-released with ACh) o Other
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Protein meal (amino acids causes gastrin secretion)
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Antacids & alkali agents
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Factors decreasing LES tone – promote gastric reflux (GERD) o Purinergic (ATP-type neurotransmitters), beta-agonists o CCK, secretin, VIP, glucagon, histamine (H
2
receptors), nitric oxide, progesterone
(pregnancy) o
Fats, carbohydrates, chocolate, cola drinks, peppermint o Anticholinergics, nitrates, nicotine, alcohol, caffeine o Increased abdominal pressure
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Advanced pregnancy, bending, squatting, heavy lifting, constrictive clothing, obesity o Hiatal hernia
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Esophageal hiatus (larger hole or weakening in diaphragm) causes LES to move into thoracic cavity – sphincter opens easier
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Disorders of the esophagus o Skeletal muscle – interfere with swallowing
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Myasthenia gravis, dystrophies, polio o Dysphagia
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Diffuse esophageal spasm – prolonged contraction of lower esophagus
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Achalasia – spasms of LES o GERD – gastroesophageal reflux
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Pregnancy
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Hiatal hernia
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Deep anesthesia
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Delayed gastric emptying
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Pressure figures o
1 st one has normal pressures in the esophagus with swallowing o 2 nd figure is a representation of a patient with diffuse esophageal spasm
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Motility disorder due to problems with myenteric nerve plexus
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Get high pressure contractions occurring at multiple places along esophagus at the same time
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UES & LES still functioning normally
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Scans show a normal esophagus & one from a patient with diffuse esophageal spasm
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This is a painful condition, pain radiates to neck & shoulder
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Mimics angina o 3 rd figure is achalasia
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Degeneration of myenteric plexus – fail to have peristalsis contractions in esophagus
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Affects primarily LES – LES fails to relax
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Food accumulates in the body of the esophagus
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Next pictures shows a enlarged esophagus o Patients are in pain, discomfort & may aspirate food o Get esophagitis or esophageal cancer
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GERD mechanism

o Table in power points o
Lists 4 different mechanisms of GERD & contributing factors o Read over & learn
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Ascites – increased vascular pressure causes backup in liver & increases liver pressure (portal hypertension). Liver weeps fluid & fluid accumulates in abdominal cavity
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Case study: GERD o F.J. is 44-year-old female with complaint of chest pain. She describes it as burning & pressure in middle of her chest. It radiates up into her neck & gets worse after eating o Possible causes of her GERD & their mechanisms
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Decreased tone of LES – pregnancy,
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Dietary
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Obesity
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Hiatal hernia
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Good history & physical is needed to identify exact cause of GERD
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Treatment depends on cause of GERD
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Diet will help, sleep with head elevated, don’t eat before going to sleep
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Anatomy of the stomach o Fundus – superior part o Body – primarily made of secretory glands
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Pacemaker zone – interstitial cells of Cajal
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Set slow wave rate (3 per minute) o Pyloric & antrum – inferior part
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Enters in duodenum
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Muscle thickness increases
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Contractions push food back into body of stomach until food is small enough to escape through pyloric sphincter
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Chyme needs to be 1-2 mm in diameter & be a liquid consistency in order to be squirted through pyloric sphincter
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Stomach only empties when pressure is higher in antrum than the pressure in duodenum
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Functions of the stomach o
Store large quantities of food – can hold 2 liters (normally)
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When stomach if filled with food, intragastric pressure stays the same because smooth muscle relaxes as volume increases
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If you loose vagus nerve, as you fill the stomach, pressure will increase o
Mix food with gastric secretions until it forms a semi-fluid mixture called chyme o
Slow emptying of the chyme into the small intestines at a rate suitable for proper digestion & absorption o Dilutes or concentrates fluids so they are the same concentration as the body fluids before reaching the intestines
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Makes chyme isoosmotic (relative to plasma) o Forms intrinsic factor which is necessary for vitamin B
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Receptive relaxation of the stomach
12
absorption o
Ability of the stomach to accommodate a large volume meal with only a small increase in intraluminal pressure o Vagal fibers innervate intrinsic nerves to cause receptive relaxation via a neurotransmitter
(VIP or NO) o
Vagotomy (vagus nerve is severed or removed) greatly diminishes receptive relaxation
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Motility in the stomach o Slow waves – 3 per minute
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Basic electrical rhythm
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Interstitial cells of Cajal
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Spontaneous fluctuations of membrane potential o Mixing wave
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Weak contractions which move toward antrum of stomach

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Mix food with gastric secretions
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You make about 2 liters of gastric secretions a day o Peristaltic constriction rings
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Force antral contents under pressure towards pylorus
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Emptying of the stomach o
Promoted by intense peristaltic contractions of the stomach antrum o Opposed by varying degrees of resistance (constriction) to the passage of chyme through o
Begins 30-60 minutes after eating
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Complete within 4-4 ½ hours after eating (depending on meal)
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Fatty meal – stays longer in stomach
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Liquid meal – passes through stomach quickly
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Figure the pyloric sphincter
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Influenced by nervous & humoral signals from the stomach & duodenum o Graphic showing different contractions of stomach o Also shows stomach emptying