10 GI tract motility: chewing and swallowing

Question 1

Which areas of contractile tissue in GI tract are not smooth muscle?

Answer 1

• Pharynx
• Upper 1/3 of oesophagus
• External anal sphincter

Question 2

How does GI motility work?

Answer 2

• Involves contration and relaxation of GI wall and sphincters
• Grinds, mixes and fragments ingested food:
-prepares food for digestion
-propels food along GI tract

Question 3

What are the layers of GI tract?

Answer 3

1. Mucosa:
• Epithelium
• Lamina propria
• Muscularis mucosa
2. Submucosa
3. Submucosal plexus
4. Circular muscle
5. Myenteric plexus
6. Longitudinal muscle
7. Serosa

Question 4

What are the stages of calcium activation of myosin in smooth muscle?

Answer 4

• Calmodulin + Ca2 binds to inactive myosin light chain kinase, activating it
• Active myosin light chain kinase + Pi activates and phosphorylates myosin

Question 5

What is the latch state in smooth muscle contraction?

Answer 5

High force – low energy consumption

Latch state - maintains high tension:
• Decrease in detachment rate of myosin heads
– Maintains cross bridge binding with reduced ATP consumption

Question 6

What are the stages of smooth muscle relaxation?

Answer 6

1. Decrease in Ca2+
2. Ca2+-Calmodulin –> Calmodulin
3. Active –> inactive myosin light chain kinase
4. Active phosphorylated –> inactive myosin via myosin phosphatase

Question 7

What are the important things about the origin and frequency of slow waves in smooth muscle?

Answer 7

1. Origin and initiation:
   • Interstitial Cells of Cajal (ICC)
   • These are pacemaker cells found in myenteric plexus that generate slow waves
2. Frequency:
   • Intrinsic rate varies from 3-12 per minute
   • Stomach- 3 waves per min
   • Duodenum- 12 waves per min

Question 8

What frequencies are affected/ unaffected by neural or hormonal input?

Answer 8

• Slow wave frequency unaffected

* Action potential frequency (strength of contraction) influenced

Question 9

What are the steps in slow wave generation in the GI tract muscle?

Answer 9

Depolarisation due to the cyclical opening on voltage gated Ca2+ channels

1. Increase in the intracellular Ca2+ concentration
2. Opening of Ca2+ dependent K+ channels
3. Increased K+ permeability leads to slow hyperpolarisation
4. Voltage gated Ca2+ channels close and intracellular Ca2+ concentration falls
5. Ca2+ dependent K+ channels close
6. Voltage gated Ca2+ channels open

Question 10

What is the consequence of stimulation by stretch, ACh and parasympathetics on the GI smooth muscle electrical activity?

Answer 10

Spikes and depolarisation

Question 11

What is the consequence of the stimulation by NE and sympathetics on the GI smooth muscle electrical activity?

Answer 11

Hyperpolarisation

Question 12

Functions of chewing?

Answer 12

1. Mixes food with salvia
   • Lubrication - mucin (glycoprotein)
2. Reduces size of food particles
   • Facilitates swallowing
3. Mixes food components with digestive enzymes
   • Carbohydrates with salivary amylase
   • Fat with lipase

Question 13

How is an involuntary reflex initiated during chewing?

Answer 13

• Involuntary reflexes initiated by food in mouth
• Sensory information relayed from mechanoreceptors to the brain stem initiates a reflex oscillatory pattern of activity in chewing muscles

Question 14

What are the 3 phases of swallowing?

Answer 14

1. Oral
2. Pharyngeal
3. Oesophageal

Question 15

What is the pathway of the swallowing reflex?

Answer 15

1. Afferent pathway:
   • Sensory information from touch receptors near the pharynx
   • Swallowing centre in the medulla and lower pons
2. Efferent motor pathway: • Cranial innervation of pharynx and upper oesophagus
   • Vagal motor innervation of the lower oesophagus

Question 16

Where is the swallowing centre located?

Answer 16

• Medulla

* Lower pons

Question 17

What is the sequence of sphincter opening and closing in the oesophagus; along with oesophageal motility and swallowing reflex?

Answer 17

1. Upper oesophageal sphincter opens to allow bolus of food to enter oesophagus
2. Upper oesophageal sphincter closes
3. Primary peristaltic contraction mediated by swallowing reflex involves a series of coordinated contractions creating a region of high pressure behind the bolus (accelerated by gravity)
4. Lower oesophageal sphincter opens mediated by peptidergic nerves in the vagus releasing VIP (vasoactive intestinal peptide)
5. Receptive relaxation of the orad region of the stomach
6. Lower oesophageal sphincter closes

Question 18

What is achalasia (absence of relaxation)?
Leads to…
Due to..

Answer 18

• Difficulty in swallowing (i.e. dysphagia)

• Failure of the lower oesophageal sphincter to relax
• Impaired peristalsis in distal regions

• Selective loss of neurones which regulate the lower oesophageal sphincter by releasing VIP (Vasoactive intestinal peptide)