Lecture 26 - Specfic Motility Patterns Flashcards
What is the function of Chewing / Mastication?
- mechanical digestion - ingestion
- mixing food with saliva - tasting
- stimulus for cephalic phase
What is the control of Chewing / Mastication?
Voluntary - skeletal muscle
Involuntary reflex - brainstem
- initiation – bolus in mouth
- stimulation of taste centres – rhythm
- strength – consistency of bolus
What is swallowing?
Rapid transfer of food from mouth to stomach
Complex
- Converts mouth and pharynx pathway from gas transfer function to food transfer function
- Prevention of reflux
What is the process of swallowing?
Oral events
Initiation of swallowing reflex
Cascade of sequential events in
- Pharynx
- Esophagous
- Stomach
What are the oral events of swallowing?
Initiation by Stretch Receptors
- Food forced into pharynx by tongue
- Stretch receptors
- Brain stem initiates swallowing
Soft Palate
- Soft palate pushes upward
- Prevents food from entering nasal passage
Epiglottis
- Moves down to cover the entrance to the trachea
- Prevents aspiration of food
What are the esophageal events of swallowing?
Upper Esophageal sphincter
- Relaxes (opens)
- Allows food to enter the esophagus
Lower Esophageal sphincter
- Opens (relaxes) at the start of swallowing
- Food enters stomach
- Closes after peristalsis wave
Esophagus (peristalsis)
- Smooth Muscle contracts
- Peristaltic wave pushes food to stomach
- Lasts about 9 seconds
What events are under voluntary control of swallowing?
Oral events are under voluntary control (we decide when to initiate a swallow)
What do voluntary events in the control of swallowing activate?
Voluntary events activate stretch receptors that activate an involuntary reflex.
- Swallowing centre in brainstem respond to stretch
What muscles are under involuntary control during the control of swallowing?
Pharyngeal muscles
- Striated muscle controlled directly by swallowing center
Oesophageal muscles
- Upper part is striated muscles controlled directly by swallowing center
- The Enteric nervous system which is modulated/co-ordinated by the swallowing center
What does the arrival of food in esophagus initiate?
Initiates a primary peristaltic wave
- Sweeps food down esophagus towards stomach
If food not cleared from esophagus by primary peristaltic wave a Secondary peristaltic wave is initiated
- Repeated until food cleared
Stretch reflux - single is around 9 seconds, can happen again
What events occur at the same time as the initiation of
swallowing?
Some events occur at the same time as the initiation of swallowing
Relaxation of upper and lower esophageal sphincters
- Allows passage of food
Relaxation of proximal stomach
- Receptive relaxation to reduce stomach pressure below esophageal pressure
What does peristalsis in the esophagus occur as?
Peristalsis in the esophagus occurs as a wave from proximal to distal.
What is the change in layout of muscle during oesophageal events in swallowing?
Striated muscle -controlled directly by swallowing center
Transition Zone
Smooth muscle- controlled indirectly (i.e. the enteric NS modulated by swallowing center)
What does the peristaltic wave do in swallowing?
Sweeps food down esophagus towards stomach
What does Relaxation of LES do in swallowing?
Allows passage of food
What is LES?
Lower esophageal sphincter
What does Receptive relaxation (Gastric
Motility) do in swallowing?
Reduction of gastric tone with swallowing. Reduces pressure below esophageal pressure.
What is GERD?
Gastroesophageal reflux disease
What happens during Gastroesophageal reflux disease (GERD)?
Reflux of acidic chyme into esophagus
Irritation to esophageal mucosa – “heartburn” is
a symptom
What are causes of GERD?
- Abnormal relaxation of lower esophageal sphincter (drops below stomach pressure)
- Lack of Receptive Relaxation or gastric accommodation in the stomach
- Conditions that increase gastric pressure
- Excessive gastric secretion
- Infection with H. pylori
What is the treatment of GERD?
- Antacids (neutralize gastric HCl)
- Antihistamines & proton pump inhibitors (stop HCl production)
- Lifestyle modifications (E.g. diet)
- Surgery (depends on cause)
What parts of GI tract have anti-reflux roles?
Upper and lower esophageal sphincters
Stomach
Describe the anti-reflux role of the Upper Esophageal Sphincter
Additional barrier to reflux
- Prevents air entering esophagus
Describe the anti-reflux role of the Lower Esophageal Sphincter
- Zone of high pressure
- 12-30 mmHg greater than gastric pressure
- Responds to changes in gastric pressure
- Acts with other structures e.g. diaphragm and lower esophageal sphincter. (Esophageal/Gastric junction: EGJ)
Describe the anti-reflux role of the Stomach
Storage functions maintain pressure as volume increases
- Receptive relaxation
- Gastric accommodation
Which of the following statements about the process of swallowing is CORRECT?
A. The stomach contracts during the pharyngeal stage of swallowing.
B. The lower esophageal sphincter muscles relax between swallows.
C. Gastric reflux is a normal event during swallowing.
D. Both upper esophageal sphincter and lower esophageal sphincter muscles relax during swallowing.
D. Both upper esophageal sphincter and lower esophageal sphincter muscles relax during swallowing.
A - Relaxes
B - Contracts
C - Not normal
What are the three types of gastric motility?
Storage
Retropulsion
Controlled delivery to the duodenum
What is the function of storage in gastric motility?
Allowing food entry during swallowing
Accommodating increased volume during a meal
What is the function of retropulsion in gastric motility?
Combines peristaltic contraction (pushes food forward) and pyloric sphincter contraction (pushes food back)
Mixing function
What is the function of controlled delivery to the duodenum in gastric motility?
Changing contraction/ relaxation of the pyloric sphincter
Initially closed – allows mixing to occur
Short periods of opening – allows the entry of food to match capacity (volume and secretion rate)
What are the functional sections of the stomach?
Proximal stomach
Transitional area
Distal stomach
What structure is found in the proximal stomach and what is its function?
Fundus
Site of storage
What structure is found in the transitional area of stomach and what is its function?
Body
Some storage
Some movement
What structure is found in the distal stomach and what is its function?
Pylorus / antrum
Movement
Mechanical breakdown
Controlled release to duodenum
What types of motility occur in the proximal stomach?
Receptive relaxation
Gastric accommodation
What happens during receptive relaxation in the proximal stomach?
Reduction of gastric tone with swallowing
What happens during gastric accommodation in the proximal stomach?
- Reflex relaxation of proximal stomach with gastric distention
- local ENS reaction and…
- Vago/vagal reflex initiated by the long reflex loop (stretch information goes to the brain and back again).
How long after gastric distension does peristalsis in distal stomach begin?
5 - 10mins
Describe peristalsis in the distal stomach
Contractions spread from greater curvature towards the pylorus
- First 60 minutes - gentle ripples (small amplitude contractions)
- 60 minutes to completion - strong contractions
Coordinated with pyloric sphincter
What is the coordination between peristalsis and the pyloric sphincter?
Closed = Retropulsion for mixing and mechanical digestion
Open = Controlled delivery to duodenum
What is regulation of motility in the distal stomach based off?
Regulation based on phases of gastric digestion
What is (and is not) regulated in gastric motility?
Regulated: Contraction force
Not regulated: Frequency of contraction
Frequency of contraction doesn’t change (for a given section of GI tract)
Force of contraction depends on the amount of time of each wave is above threshold - duration of depolarisation
What type of regulation is the force of contraction under in gastric motility?
Nervous and hormonal
How is regulation of motility and the rate of gastric emptying changes by phase achieved?
Achieved by modulation of motility in proximal and distal stomach
What is the regulation of motility and the rate of gastric emptying during the cephalic and gastric phases?
- Proximal stomach relaxed (storage)
- Pyloric sphincter closed (retropulsion)
- Distal motility increased but most material returns to proximal region
What is the regulation of motility and the rate of gastric emptying during the intestinal phase?
-Tone of muscle in proximal region increased (feeds food to the distal region)
- Pyloric sphincter opens more frequently
- Controlled release of material from stomach to duodenum
- Duodenal contractions are coordinated with stomach and inhibited as chyme is released into the duodenum.
What is the regulation of motility and the rate of gastric emptying during the intestinal phase?
-Tone of muscle in proximal region increased (feeds food to the distal region)
- Pyloric sphincter opens more frequently
- Controlled release of material from stomach to duodenum
- Duodenal contractions are coordinated with stomach and inhibited as chyme is released into the duodenum.
What does the CNS do in response to the presence of food in the mouth, chewing and swallowing?
Inhibits muscles in body of the stomach by modulating ENS
Low level stimulation of pyloric contractions by modulation of ENS to:
- Activate muscular contractions in preparation for food
- Stimulate the release of gastrin from G Cells
Pyloric sphincter mainly closed
What % of secretion associated with a meal occurs during the cephalic phase?
20%
What % of secretion associated with a meal occurs during the gastric phase?
70%
What is the gastric phase mediated by?
Mediated by receptors in the stomach wall
- Mechanoreceptors respond to distention of stomach wall by food
- Chemoreceptors respond to presence of products of digestion (amino acids and short peptides)
What do the local and long reflex pathways do during the gastric phase?
Inhibit tone in proximal stomach to accommodate food
- Gastric accommodation
Stimulate contraction in distal stomach through
- Release of Acetylcholine and substance P
- Stimulation of G cells to release gastrin
Why do we have extrinsic (long) reflexes during the gastric phase?
Because the stomach is a large organ with distinct functional regions
What neurotransmitter is released by the ENS during the gastric phase?
Acetylcholine
What hormone is released during gastric phase?
Gastrin
What motility does the proximal stomach do during gastric phase?
Gastric accommodation
What motility does the distal stomach do during gastric phase?
↑Peristalsis for retropulsion (ACh)
What % of secretion associated with a meal occurs during the intestinal phase?
10%
What happens during the intestinal phase?
Controlled delivery of food from stomach to duodenum
Describe the Controlled delivery of food from
stomach to duodenum
- Ensures food is processed correctly in stomach
- Matches delivery of food to small intestine to its capacity
- Matched to duodenal enzyme and bicarbonate release (hormones)
- Pyloric sphincter opens more during peristaltic waves
- Duodenal contractions inhibited when sphincter is open
When are duodenal contraction inhibited?
When pyloric sphincter opens
What does the gastric epithelium do during the cephalic phase?
Start release of gastrin from G cells
What stimulates contraction in distal stomach?
- Release of Acetylcholine and substance P
- Stimulation of G cells to release gastrin
What products of digestion does the chemoreceptors respond to during gastric phase?
Amino acids and short peptides
How is the rate of delivery of chyme into the
duodenum controlled?
By negative feedback inhibition of gastric muscle
What hormones are released during the intestinal phase?
Secretin, CCK, GIP
What is the main control factor of the intestinal phase?
Composition of chyme
What can effect the composition of chyme?
Energy content of meal
PH of meal
Liquid meals
Osmolarity
How does the energy content of a meal effect the composition of chyme/intestinal phase?
Energy content of meal, higher energy (e.g. fat) is slower
How does the PH of meal effect the composition of chyme/intestinal phase?
PH of meal – acid = slower
What is released in response of fat /nutrients in the duodenum?
Cholecystokinin CCK
What is released in response of acid in the duodenum?
Secretin
How do liquid meals effect the composition of chyme/intestinal phase?
Liquid meals delivered to SI faster than solid
Faster delivery since usually less nutrients
How does osmolarity effect the composition of chyme/intestinal phase?
Isosmotic faster than hyperosmotic or hypo-osmotic
When can the stomach over-ride signals from the composition of chyme in the intestinal phase?
Size of meal
Larger is faster – over stretching of stomach wall
What can over-ride the composition of chyme as main control factor of the intestinal phase?
The stomach
Which is not a major function of gastric motility:
1. Mixing food / chyme with digestive enzymes.
2. Propelling food / chyme.
3. Mechanical digestion of food / chyme.
4. Exposing food / chyme to absorptive epithelia.
- Exposing food / chyme to absorptive epithelia
- not a lot of absorption
What determines frequency of contraction in the SI?
Frequency of contraction determined by the interstitial cells of Cajal (not regulated)
What does segmentation in SI aid?
Aids digestion by mixing secretions
Aids contact digestion and absorption by exposing
contents to small intestinal epithelial layer
Describe peristalsis in SI
Periodic for propulsion / movement
- Ascending circular muscle contraction
- Descending circular muscle relaxation
- Longitudinal muscle shortening (contraction)
What regulated the magnitude of contraction in SI?
Magnitude of contractions regulated by ENS reflexes (with input from CNS)
What type of motility occurs in SI?
Segmentation (most)
Peristalsis
What type of motility occurs in the LI?
Storage
Mass movement - peristalsis
Elimination (defecation reflex)
Describe storage in LI
- large intestinal transit is slow (24-48 hrs)
- This inactivity enhances water absorption
- Some segmentation (Mixing) occurs during storage
- Short duration contractions – circular muscle
- Turnover of contents aids regulated reabsorption
Describe mass movement in LI
- high intensity contractions (around 2x per day / after main meals)
- propel contents into rectum
Describe Elimination (Defecation reflex) in LI
- Relaxation of internal anal sphincter – smooth muscle.
- The rectoanal inhibitory reflex prevents spontaneous opening of external anal sphincter.
- External anal sphincter is under conscious control - skeletal muscle – relaxes when defecation is appropriate.
- Valsalva maneuver – closing the airway while exhaling to increase force for elimination
