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Physiology > Lecture 42: GI Tract Motility > Flashcards

Lecture 42: GI Tract Motility Flashcards

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1
Q

How are GI smooth muscle cells connected?

A
  • Electrically via gap junctions
  • Physically via adherens junctions
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2
Q

What do the connections between GI smooth muscle cells allow for?

A

Cells act together as a single unit - unitary smooth muscle

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3
Q

What are the 2 contraction patterns in GIT smooth muscle?

A
  1. Phasic contractions: contract/relax over seconds e.g peristalsis
  2. Tonic contractions: sustained from minutes to hours e.g. sphincters
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4
Q

What are phasic contractions called and what are they triggered by?

A

Slow wave contractions triggered by depolarisation and repolarisation of smooth muscle

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5
Q

What is the basic electrical rhythm (BER) and what is it caused by?

A
  • Cyclic depolarisation and repolarisation in GI tract
    smooth muscle
  • Caused by Pacemaker cells called Interstitial cells
    of Cajal
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6
Q

How does the BER cause slow wave contractions?

A
  • When membrane potentials of muscle cells depolarize and reach threshold APs occur
  • APs trigger contractions (excitation contraction
    coupling)
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7
Q

What cyclic changes do interstitial cells of Cajal cause?

A
  • The activity of Na+/K+-ATPase changes cyclically
  • Membrane K+ conductance (opening and closing K+ channels)
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8
Q

Where are interstitial cells of Cajal located?

A
  • Stomach and Small Intestine - boundaries between the smooth muscle layers and the myenteric plexus
  • Large intestine - boundary between the muscularus and submucosal layer
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9
Q

How are the interstitial cells of Cajal linked to muscle cells?

A

They have multiple branching processes which are electrically linked to muscle cells by gap junctions

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10
Q

How do motility patterns in the different regions of the GIT differ? Use the SI and stomach as examples.

A

Different regions have different functions
e.g. digestion, in the proximal SI needs faster contractions vs absorption in the distal SI which need slower contractions
e.g. stomach has regions for receiving food and regions for digesting food

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11
Q

Why do motility patterns differ during different phases of digestion?

A
  • Fasting vs fed individuals need different motility patterns
  • Different volumes or nutrient composition of chyme in the GI tract will require faster or slower transit
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12
Q

Describe the migrating motor complex process during fasting:

A
  • Motility pattern that clears the GI tract
  • Starts 4-5 hours post meal absorption
  • Duration: 2 hours from the stomach to the end of the LI
  • 3 phases of motility: 1. intense, 2. inactive, 3. intermittent
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13
Q

What are the functions of the migrating motor complex?

A
  1. Clears undigested material & secretions
  2. Regulates intestinal microflora
  3. Stimulates epithelial cell turnover
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14
Q

What regulates the migrating motor complex?

A
  • Hormonal: Motilin is released by intestinal m-cells
  • Neuronal: Motilin stimulates both the ENS and ANS
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15
Q

In the fed state, name the 3 functions of changing motility patterns in the GIT:

A
  1. Storage
  2. Moving
  3. Mixing/exposure to surfaces
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16
Q

How do GI tract motility patterns change for storage?

A
  • Alterations in muscle tone to control movement of food from one part of the GIT to the next
  • Occurs in the proximal stomach, large intestines, and rectum
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17
Q

How do GI tract motility patterns change for movement?

A
  • Phasic contractions to move food along the tract
    e.g. Peristalsis - occurs in the esophagus, stomach and intestines
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18
Q

How do GI tract motility patterns change for mixing/exposure to surfaces?

A
  • Phasic contractions for mixing food and exposure to absorptive epithelia
    e.g. segmentation - occurs in the small and large intestine only
    e.g. retropulsion - occurs in the stomach only
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19
Q

What motility patterns occur in the mouth, pharynx and esophagus and what are the functions of this?

A

Chewing and swallowing
- Controls the entry of food into the GIT
- Prevents food from entering the respiratory tract
- Prevents food and acid from the stomach from damaging the esophagus (i.e. prevents reflux)

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20
Q

What motility pattern occurs in the stomach and what are the functions of this?

A

Gastric motility
- Prevents acid reflux
- Mixing food with acid and digestive enzymes
- Controlling gastric emptying

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21
Q

What motility pattern occurs in the intestines and what are the functions of this?

A

Intestinal motility
- Mixing food and enzymes
- Increasing contact with intestine wall
- Moving food along the tract
- Store and eliminate waste at the end of the large intestine

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22
Q

Describe the mechanism behind the intestinal smooth muscle contraction:

A
  • Depolarization triggers the release of Ca2+
  • Ca2+ binds to calmodulin and activates myosin light chain kinase
    (MLCK) to phosphorylate myosin
  • Frequency doesn’t
    change as the ICC cells are the main trigger for depolarisation to smooth muscle cells
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23
Q

What does the force/size of intestinal smooth muscle contraction depend on?

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A

How long the muscle cell is above threshold - determines how long Ca2+ is present in the cytoplasm of smooth muscle cells

24
Q

How can you alter the strength of contraction in the GIT?

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A
  1. Increase or decrease depolarization via stimulus gated ion channels
    - Results in more voltage gated Ca2+ channels opening
  2. Increase or decrease Ca2+ independent of voltage via Gαq signalling opening sarcoplasmic reticulum Ca2+ channels
25
How do hormones and neurotransmitters regulate the strength of contraction?
* Increase: Depolarize or increase Ca2+ * Decrease: - Hyperpolarize or decrease Ca2+ - Increase amount of Myosin Light Chain Phosphatase to de-phosphorylate myosin and reduce cross bridge cycling - Block excitatory neurotransmission
26
Describe how opioids inhibit excitatory ENS neurotransmission:
1. Opioids bind to opioid receptors in the myenteric plexus 2. These are Gαi receptors which inhibit neurotransmission by ENS neurons 3. Excitatory signals are blocked and don’t get to the GI smooth muscle
27
Describe how stress decreases the size of contraction in the GIT:
1. Stress is detected by the CNS 2. CNS sends message to ENS via the SNS which uses adrenaline as a neurotransmitter 3. ENS has α adrenergic receptors 4. ENS sends inhibitory signal to smooth muscles using different inhibitory neurotransmitters e.g. Nitric Oxide NO 5. NO increases amount of Myosin light chain Phosphatase, de-phosphorylates myosin and reduce cross bridge cycling
28
What is the function and control of chewing?
* Function: Mechanical reduction of food Mixing with saliva * Control: Skeletal muscle under voluntary control and some reflex control of chewing rhythm and strength
29
What are the functions of swallowing?
1. Rapid transfer of food from mouth to stomach 2. Converts path from gas to food transfer 3. Stops acid exiting the stomach
30
What parts of swallowing are under voluntary control?
Oral events - skeletal muscle
31
What is the involuntary swallowing reflex?
* Pharynx and upper Esophagus have striated muscle controlled directly by swallowing center * Rest of esophagus has smooth muscle controlled by the ENS which is coordinated by the swallowing center * The stomach has receptive relaxation
32
What are the oral events of swallowing?
1. Tongue pushes food into the pharynx and activates stretch receptors 2. Swallowing center in brain stem responds activating an involuntary reflex 3. Soft palate and epiglottis prevent food entering the nasal cavity and lower respiratory tract
33
What are the esophageal events of swallowing?
1. Upper and lower esophageal sphincters open 2. Wave of peristalsis pushes food along the esophagus and into the stomach 3. Stomach relaxes so that food can enter (pressure gradients)
34
Name the 3 main types of gastric motility:
1. Storage (strategic relaxation) 2. Mixing - retropulsion 3. Propulsion and controlled delivery to the duodenum
35
Describe storage as a type of gastric motility:
* Allows food entry during swallowing * Accommodating increased volume during a meal * Preventing reflux
36
Describe propulsion as a type of gastric motility:
* Periodic opening of the pyloric sphincter * Peristalsis propels chyme into the duodenum in small amounts to match capacity of the duodenum + accessory organs
36
What is gastric accommodation?
* Food activates stretch receptors * Muscle in the stomach relax * Pressure drops and the stomach can accommodate more food
36
Describe mixing as a type of gastric motility:
Combines peristaltic contraction (pushes food forward) and pyloric sphincter being closed/contracted (pushes food back)
37
What is receptive relaxation?
* Swallowing reduces muscle tone in the stomach * Allows food to enter - lower pressure compared with esophagus low resistance to food entering
38
What is gastroesophageal reflux disease (GERD)?
* Reflux of acidic chyme into esophagus causing irritation to esophageal mucosa (heartburn)
39
When is retropulsion initiated?
5-10 mins after gastric distension
39
What 5 factors can causes gastroesophageal reflux disease (GERD)?
1. Changes in the tone of the lower esophageal sphincter 2. Conditions that increase gastric pressure 3. Failed gastric accommodation 4. Failed receptive relaxation 5. Excessive gastric acid secretion 6. Infection with H. pylori
40
What 2 motility patterns are involved in retropulsion?
1. Phasic peristaltic contractions: spread of contractions to push food from body to the pyloric sphincter 2. Contraction of pyloric sphincter - bounces food back to the body of the stomach
40
What is the treatment for gastroesophageal reflux disease (GERD)?
1. Antacids (neutralize gastric HCl) 2. Antihistamines & proton pump inhibitors 3. Lifestyle modifications 4. Surgery (fundoplication)
41
What is the function and control of retropulsion?
* Function: - Mix gastric contents with secretions to produce chyme - Mechanically reduce size of food particles * Control: - Gastric phase - chemo and stretch receptors initiate local and long reflex responses to stomach contents
42
What is the role of the duodenum?
Deliver digestive enzymes to chyme and neutralise acid to make chyme safe for intestinal epithelia
43
Describe the gastric delivery of chyme to the duodenum at the start of the gastric phase:
* Delivery is restricted * Contracted pyloric sphincter (retropulsion) * Ensures effective gastric digestion
43
Describe the gastric delivery of chyme to the duodenum at the start of the intestinal phase:
* Chyme moves from the stomach to match capacity of duodenum * Controlled periodic relaxation of the pyloric sphincter * Tonic proximal stomach contraction * Phasic distal stomach contraction to move chyme along
44
Describe how gastric delivery to the intestine is controlled:
1. Receptors in the Duodenum detect stretch, acid pH, osmolarity, and nutrients 2. CNS (long - enterogastric reflex) and, ENS (Local reflex) responses 3. Hormonal responses: CCK - slows emptying in response to peptides and Secretin slows emptying in response to acid pH
45
Name the 2 motility mechanisms in the small intestine:
1. Segmentation 2. Peristalsis
45
What is the function and mechanism of segmentation in the small intestine?
Main fed motility pattern * Function : mixing - aids digestion by mixing secretions - aids contact digestion and absorption by exposing contents to small intestinal epithelial layer * Mechanism: - Circular muscle contraction in segments - Magnitude of contractions controlled by ENS reflexes
46
What is the function and mechanism of peristalsis in the small intestine?
Occurs periodically * Function : movement/propulsion - Transport of chyme - Transfer of waste to large intestine * Mechanism: - Circular muscle contracts behind chyme and relaxes ahead of chyme - Longitudinal muscle shortens
46
Name the 4 motility mechanisms in the large intestine:
1. Storage 2. Segmentation 3. Peristalsis 4. Elimination
47
What is the function of storage in the large intestine?
Large intestinal transit is slow (24-48 hrs) - inactivity enhances water absorption
48
Describe the process of elimination in the large intestine:
1. Reflex relaxation of internal anal sphincter in response to pressure in the rectum – smooth muscle 2. Initial contraction of external anal sphincter until conscious control determines release is appropriate - skeletal muscle 3. Valsalva maneuver – closing the airway while exhaling to increase force for elimination
48
What is the function of peristalsis in the large intestine?
Mass movement occurs periodically - high intensity contractions propel contents into rectum
49
What is the function of segmentation in the large intestine?
Mixing occurs periodically- turnover of contents aids absorption

Physiology (45 decks)

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