9. GI tract motility/ Chewing and swallowing

Question 1

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

Answer 1

Calmodulin + Ca2 binds to inactive myosin light chain kinase, activating it,

Active myosin light chain kinase + Pi activates and phosphorylates myosin

Question 2

What are the stages of smooth muscle relaxation?

Answer 2

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

Question 3

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

Answer 3

Origin and initiation: Interstitial Cells of Cajal (ICC) These are pacemaker cells found in the myenteric plexus that generate slow waves

Frequency:
Intrinsic rate varies from 3-12 per minute.
Stomach- 3 waves per min
Duodenum- 12 waves per min

Question 4

Smooth muscle slow wave frequency unaffected by neural or hormonal input but….

Answer 4

Neural and hormonal input influences action potential frequency (strength of contraction)

Question 5

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

Answer 5

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 6

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

Answer 6

Spikes and depolarisation

Question 7

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

Answer 7

Hyperpolarisation

Question 8

Functions of chewing?

Answer 8

1. Mixes food with salvia, mucin glycoprotein acts as a lubricant
2. Reduces size of food particles, facilitates swallowing
3. Mixes food components with digestive enzymes, carbohydrates with salivary amylase and fat with lipase

Question 9

How is an involuntary reflex initiated during chewing?

Answer 9

Sensory information relayed from mechanoreceptors to the brain stem initiates a reflex oscillatory pattern of activity in chewing muscles

Question 10

What are the 3 phases of swallowing?

Answer 10

Oral
Pharyngeal
Oesophageal

Question 11

What is the pathway of the swallowing reflex?

Answer 11

Afferent pathway: Sensory information from touch receptors near the pharynx

Swallowing centre in the medulla and lower pons

Efferent motor pathway: involves cranial innervation of pharynx and upper oesophagus and vagal motor innervation of the lower oesophagus

Question 12

What is the sequence of sphincter opening and closing in the oesophagus?

Answer 12

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 13

What is achalasia?
Leads to…
Due to..

Answer 13

Means: Absence of relaxation

Leads to difficulty in swallowing (i.e. dysphagia)

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

Due to selective loss of neurones which regulate the lower oesophageal sphincter by released VIP (Vasoactive intestinal peptide)