Control of Smooth Muscle

Question 1

What are the types of muscle in the GI tract and where are they found?

Answer 1

• smooth muscle in most regions

- skeletal muscle in pharynx and top third of oesophagus as well as anal sphincter

Question 2

What is motility in the GI tract supported by?

Answer 2

intestinal smooth muscle

Question 3

What are the two types of smooth muscle in the GI tract?

Answer 3

• phasic (rapid contraction and relaxation) in oesophagus, stomach antrum and intestines
• tonic (sustained contractions) sphincters and orad (upper stomach)

Question 4

What cells are important in pace making the contractions and how?

Answer 4

interstitial cells of CAJAL, these cells are electrically coupled to the smooth muscle through gap junctions allowing the spread of the slow wave and a have spontaneous oscillating membrane potential

Question 5

What is depolarisation (contraction) of the smooth muscle fibres stimulated by?

Answer 5

• stretch
• acetylcholine
• parasympathetics

Question 6

What is hyper polarisation (relaxation) of the smooth muscle fibres stimulated by?

Answer 6

• noradrenaline

- sympathetics

Question 7

What is the connection between electrical activity and contractile activity?

Answer 7

The longer the electrical activity is over the threshold, the greater the contractile event is

Question 8

What is the difference between phasic and tonic electrical activity?

Answer 8

• phasic contraction the electrical activity is on and off

- tonic contraction electric activity constantly on and above threshold

Question 9

How does the smooth muscle contract?

Answer 9

1. calcium binds calmodulin which activates a myosin light chain kinase which phosphorylates myosin
2. myosin can interact with actin which causes a contractile event

Question 10

In terms of calcium, how is relaxation enabled to occur?

Answer 10

Ca2+ reuptake into the sarcoplasmic reticulum is mediated by a pump and the calcium itself binds to molecules to prevent it being free and binding to calmodulin

Question 11

Why is reducing Ca2+ conc alone not enough to cause relaxation?

Answer 11

because MLC required dephosphorylation to prevent interaction with actin, this occurs using myosin light chain phosphatase

Question 12

What are the two types of innervation in the GI tract?

Answer 12

• intrinsic innervation of the enteric nervous system

- extrinsic innervation of the autonomic nervous system

Question 13

What is the enteric nervous system made up of?

Answer 13

• submucosal plexus (adjacent to mucosa)

- myenteric plexus (between circular muscle and longitudinal muscle)

Question 14

What does activation of the myenteric plexus cause?

Answer 14

• increases tonic contraction
• increases intensity of rhythmic contractions
• increases the rate of rhythmic contractions
• increases velocity of contractions

Question 15

What does the activation of the submucosal plexus cause?

Answer 15

• increases secretory activity

- modulates intestinal absorption

Question 16

What are the types of movement in the GIT?

Answer 16

• propulsive movements (peristalsis)
• non-propulsive movements (segmentation)
• interdigestive (migrating motor complex)

Question 17

What is the intrinsic pathway?

Answer 17

• motor neurone
• interneurone
• sensory neurone

Question 18

What nerve is important in the extrinsic pathway?

Answer 18

-vagus nerve innervates most structures of the GI tract down to the top of the large intestine where it is then innervated by the pelvic nerve

Question 19

What are three diseases that occur as a result of lack of GIT motility?

Answer 19

• Hirschsprung disease (congenital lack of neuronal ganglionic cells in the ENS plexus) (megacolon usually results)
• Chagas disease (infectious disease of a parasitic nature resulting in reduction in ganglionic cells in ENS)
• Achalasia (reduction in no of neuronal cells in lower oesophageal segment)

Question 20

Symptoms of HD?

Answer 20

vomiting, constipation, distention of abdomen, intestinal obstruction

Question 21

How do you treat HD?

Answer 21

remove the ganglionic segment

Question 22

What causes achalasia?

Answer 22

failure of the LOS to relax due to lack of inhibitory neurones

Question 23

How are changes in wall tension and intraluminal pressure in the stomach minimised?

Answer 23

stomach is highly folded and upon filling the folds flatten out

Question 24

What causes relaxation in the fungus?

Answer 24

the vago-vagl reflex, receptive relaxation

Question 25

What are the phases of gastric motility?

Answer 25

• propulsion
• grinding
• retropulsion (of large particles and clearing of terminal antrum)

Question 26

Functions of pylorus?

Answer 26

• carefully regulates emptying of gastric contents

- prevents regurgitation of duodenal contents into stomach

Question 27

What are the three phases of gastric emptying?

Answer 27

• cephalic (inhibitory phase which relaxes stomach which allows it to store large volumes)
• gastric (excitatory phase where stomach empties at rate proportional to volume)
• intestinal

Question 28

What causes the gastric phase?

Answer 28

• myogenic reflex (stomach stretching)
• activation of pressure receptors (send signals to vagus nerve)
• gastrin released in response to peptides

Question 29

How does the intestinal phase occur?

Answer 29

• low PH activates secretion of secretin
• high levels of lipids increase secretion of CCK
• high levels of amino acids increase secretion of gastrin
• high levels of carbs increase secretion of GIP

Question 30

What is the benefit of the intestinal phase?

Answer 30

allows more time for digestion

Question 31

What happens in the intestinal phase?

Answer 31

an inhibitory event that prevents the stomach from emptying contents when there is chyme in the duodenum