GI Lecture 3 Flashcards
What is peristalsis?
Propulsive movements by which food moves forward along the tract. A ring of muscle contraction on the oral side of the bolus that moves toward the anus; as the ring moves, the muscle in front of the bolus relaxes, facilitating bolus movement. It is coordinated by the ENS.
How is peristalsis stimulated?
Distention of the gut (food is present). Also chemical irritation and strong parasympathetic stimulation.
Orad
Mouth side (behind bolus)
Caudad
Further along GI tract (ahead of bolus)
Describe ENS in terms of peristalisis
On the orad side, excitatory motor neuron with ACh for contraction of circular muscle. On the caudad side, inhibitory motor neuron with NO/VIP for relaxing of circular muscle, so the bolus can move down. Sensory neurons triggered by interneurons and distension (low pH) can cause either contraction or relaxation.
Significance of mixing
Aids digestion and absorption by allowing digestive enzymes to contact with the chyme and cause the chyme to come into contact with epithelial cells that absorb nutrients
Segmentation contractions
A common type of mixing motility seen especially in small intestine. A section contracts, sending the chyme both directions. Segment then relaxes, moving chyme back to segment = mixing without net movement.
Contractile tissue
Single-unit type smooth muscle
Product of depolarization of smooth muscle
Depolarization spreads via gap junctions to adjacent areas and results in a well coordinated contraction of smooth muscle.
What parts are skeletal muscle (voluntary control)
The pharynx, UES, upper esophagus and external anal sphincter.
Slow waves
The GI smooth muscle electrical activity that occurs in cyclic variations. Typical to each part of the gut. (3/min stomach 12/min duodenum)
What causes the depolarizing phase of a slow wave
Calcium influx
What causes the repolarizing phase of the slow wave
Potassium efflux
Is slow wave frequency influenced by neural or hormonal input?
No, but their amplitude is decreased by sympathetic stimulation and increased by parasympathetic stimulation.
What happens when a slow wave exceeds threshold
AP generated, and a muscle contraction occurs.
Does the GI muscle relax completely?
No, there is baseline tension called tonically contracted.
Interstitial cells
The different types form gap junctions with smooth muscle cells and enteric neurons, They generate pacemaker activity and probably transfer signals from enteric motor nerves to muscle cells. They set the smooth muscle membrane potential, and are responsible for generating slow waves, peristalsis and segmentation.
Mastication
Chewing, which breaks down and lubricates food, mixes it with salivary enzymes, allows better penetration of digestive secretions, and ultimately makes a bolus.
What reflex types are involved in mastication
Involuntary and voluntary
Oral phase
The voluntary phase whereby the tip of the tongue pushes food toward the pharnyx.
Pharyngeal phase
The involuntary phase whereby a series of protective reflexes initiated by stimulation of afferent fibers in the pharnyx and organized in the swallowing centre of the medulla and lower pons. Closes off nasal, oral, and laryngeal cavities to inhibit respiration.
Muscle type in UES and pharynx
Striated muscle
Muscle type in esophagus and LEs
Smooth muscle
Esophageal phase
- Food enters esophagus and UES closes.
- Primary peristaltic contraction moves down the esophagus and propels food bolus along.
- LES relaxes and food bolus enters the stomach.
- A secondary peristaltic contraction clears esophagus of remaining food.
When is there high pressure in pharynx
When the pharyngeal superior constrictor muscles contract
When is there low then high pressure in the UES
Reflex relaxation and constriction
When is there high pressure in esophagus
Peristaltic wave moves along
When is there a dip followed by a high point in LES pressure?
Dip (relaxation) upon swallowing, constrict after peristaltic wave goes through.
Dysphagia
Difficulty in swallowing. Can result from abnormalities in any of the components of the swallowing reflex or the anatomical structures.
Common problem especially in the elderly, associated with stroke, ALS, Parkinson’s disease.
Risk of malnutrition, aspiration, choking.
Achalasia
Failure to relax, LES does not relax and open.
Problem in the myenteric ganglion nerves.
Gastrointestinal reflux disease
(GERD)
Acidic gastric contents reflux to the distal esophagus.
Very common problem, severity varies.
May result into inflammation, ulcers.
What region of the stomach is mainly reception and storage
Orad
What region of the stomach is mainly mixing and propulsion
Caudad
What is special about stomach muscle
In addition to outer longitudinal and middle circular muscle layers, the stomach also has an inner oblique muscle layer.
Thin-walled part of somach
Body and Fundus
Thicker, more muscular, part of stomach
Antrum
Receptive relaxation
A vagovagal reflex that increases the intragastric volume without increase in pressure. Ex: swallowing induces relaxation of the LES and receptive relaxation to the orad part of the stomach.
Retropulsion
As the lower body and antrum contract, the pyloric sphincter closes and food bolus is propelled back to stomach to be mixed
What plays a role in mixing and digestion in the stomach
Muscle contractions, gastric secretions, digestion
Spurts
How the lower part of the stomach contracts to propel chyme into duodenum - in spurts.
Describe the relative time of gastric emptying.
Takes about 3 hours. Liquids faster than solids. Isotonic contents faster than hypo or hypertonic contents. Glucose faster than protein. Starts 1 hour after eating.
Why does the nature of duodenal contents affect gastral emptying?
To allow adequate time for neutralization of the chyme and digestion of fats.
Migrating myoelectric complex
How nondigestible material is emptied during the interdigestive period
Gastroparesis
Disorders in which gastric emptying is impaired or delayed, without evidence of obstruction.
Symptoms: early satiety, nausea, vomiting, bloating, and upper abdominal discomfort.
Varied etiology; systemic diseases that affect neuromuscular function (diabetes), medication, injury to vagus nerve.
Pyloric stenosis
A congenital condition usually presenting in infancy.
Pyloric muscle thickened and pylorus fails to relax after a meal, leading to regurgitation and vomiting.
Infants develop malnutrition and dehydration.
Cured by surgical myotomy (longitudinal incision to the muscles surrounding pyloric region)
How the small intestine aids in digestion and absorption of nutrients
- Mix contents with enzymes and pancreatic secretions
- Expose nutrients to intestinal epithelial cells for absorption
- Propel remaining contents to the colon.
Describe slow wave frequency gradient of small intestine
Gets slower the lower down the small intestine you go (duodenum =11-12, ileum=8)
Segmentation contractions
Contractions of the small intestinal muscle. Squirt luminal contents bidirectionally; since contractions also increase resistance to flow, and there are more contractions upstream, the net movement is aborad.
Peristaltic contractions of small intestine
Entry of chyme in the duodenum causes a
peristaltic wave. It travels only a few centimeters before dying out. The intestinal contents are slowly mixed and the chyme is steadily moved. This allows adequate time for digestion and absorption.
Ileocecal valve
Prevents backflow from colon to ileum. It is a sphincter that is normally constricted but strong peristaltic activity immediately after the meal relaxes it.
What happens when there is pressure of chemical irritation in the cecym of the colon
Peristalsis is inhibited in the ileum and the sphincter is excited -> contracts.
What are the functions of the colon
- Fluid and electrolyte absorption - mainly in proximal part
- Storage - mainly in distal part
- Movements to achieve these functions - slow.
Mixing of the colon
Localized segmental contractions: circular and longitudinal muscles contract simultaneously forming haustra on unstimulated areas. Haustrations move back and forth and slowly mix the contents.
Mass movements of the colon
1-3 times/day mass movements move the colonic contents over long distances
Final mass movement
Propels the feces to the rectum where they are stored until defecation.
Is defecation voluntary?
Yes, but also includes some reflexes.
How does the rectum permit entry of feces?
It relaxes the internal anal sphincter but the external sphincter is contracted until it is convenient to defecate, then it is relaxed voluntarily.
What ultimately forces feces out of the body?
Smooth muscle of the rectum contracts to force the feces out.
Does voluntary control have an excitatory or inhibitory effect on the external anal sphincter?
Inhibitory - we prevent defecation until it is convenient, and we let the reflex take over.
Hirschsprung’s Disease
Congenital megacolon, a developmental abnormality where the ENS fails to develop.
A segment from the internal anal sphincter upward, remains permanently contracted, causing obstruction.
The symptoms can be completely alleviated by surgical excision of the diseased segment.
Irritable Bowel Syndrome
Motility disorder caused by visceral hypersensitivity.
Very common, 10‐30% of population.
Causes cramping, abdominal pain, bloating, gas, diarrhea and constipation.
Partially due to dysmotility, although consistent motor abnormalities have not been found.
Patients with diarrhoea have shortened transit times through the intestines, and an increase in propulsive contractions in the colon.
Patients with constipation‐predominant disorders have slowed transit of intestinal contents.
Sequence of events in vomiting
- Hypersalivation
- Fundus becomes flaccid. Strong contraction of antrum and proximal duodenum.
- Soft palate rises. Glottis closes. Larynx moves forward. Esophagus dilates. LES relaxes and moves upwards.
- Diaphragm contracts. Abs contract. Gastric contents forced upwards.
What is the main control of vomiting response?
CNS - brain stem vomiting centre
What are the different areas involved in vomit stimulation?
Nucleus tractus solitarus, gastric mucosa, chemoreceptors in medulla, brain stem vomiting centre, higher centres, cerebellum, labyrinth
Whare are triggers of vomiting?
Pharyngeal stimulationenetic drugs, cytotoxic drugs, irritants, optiates, chemotherapy, hormones, motion vertigo, pain, sights, odors, tastes, anticipation
Nerves involved in vomiting
Glossopharyngeal nerve, vagus nerve
