Smooth Muscle

Question 1

What muscle is found in the GI tract and where?

Answer 1

Smooth muscle (involuntary) found in most regions
Skeletal muscle (voluntary) found in the pharynx, upper oesophageal sphincter, external anal sphincter

Question 2

What two types of smooth muscle are there and where are they found?

Answer 2

Phasic - rapid contraction and relaxation in the body of the oesophagus, stomach antrum, small and large intestines.
Tonic - sustained contraction in the sphincters (upper third of the oesophageal, ileocaecal, internal anal), orad (upper) stomach

Question 3

How are electrical impulses passed between smooth muscle cells?

Answer 3

Through gap junctions.

Question 4

How are smooth muscle cells stimulated?

Answer 4

1. Stretch
2. Acetylcholine
3. Parasympathetic
   These cause depolarisation if the threshold is met.

Question 5

What is the correlation between electrical and contractile activity in smooth muscle?

Answer 5

When the electrical threshold is met it causes an increase in contraction. The more the electrical threshold is met, the stronger and more numerous the contraction. The lag is a time gap between the time the electrical threshold is reached and the contraction occurs.

Question 6

How does calcium cause contraction and relaxation in smooth muscle?

Answer 6

When the actin light chain kinase is activated by calmodulin, it phosphorylates myosin using ATP to produce myosin-P. This then binds to actin to produce a contraction with the dephosphorylating of ATP.
Myosin light chain phosphatase dephosphorlates myosin-P to myosin and this causes relaxation.

Question 7

How is Ca2+ contraction controlled?

Answer 7

The cell extrudes Ca2+ by Na-Ca exchanger or Ca2+ pumps at the plasma membrane, however will eventually deplete. Therefore, Ca2+ re-uptake into the sarcoplasmic reticulum (SERCA pump) needed to return the cell to its resting Ca2+ levels.

Question 8

What initiates smooth muscle cells to contract?

Answer 8

Ach - excitatory neurotransmitter
VIP and NO - inhibitory neurotransmitter
Hormones release from endocrine cells into the blood can also act via the circulation.
Regulation by the enteric nervous system and some input by the autonomic NS.

Question 9

What does activation of the myenteric plexus cause?

Answer 9

Increase in tonic contraction, intensity of rhythmic contractions, rate of rhythmic contractions and velocity of conductance.

Question 10

What does activation of the submucosal plexus cause?

Answer 10

Increase in secretory activity and modulates absorption.

Question 11

What are the types of movement in the GIT.

Answer 11

1. Propulsive movements = peristalsis
2. Non-propulsive movements = segmentation
3. Interdigestive = Migration Motor Complex (MMC)

Question 12

What is Hirschsprung Disease?

Answer 12

Congenital lack of neuronal ganglionic cells in the submucosal and myenteric plexus. The aganglionic, bowel segment effectively prevents the propulsion of the faecal stream, resulting in a megacolon. Nerves are missing and there is hypertrophy (increase in volume in an organ) of the normal proximal colon.

Question 13

What are the symptoms of Hirchsprung Disease?

Answer 13

Vomiting, constipation, distension of the abdomen and intestinal obstruction.
Male predominance 4 to 1 in HD.
Treatment involves the removal of the aganglionic segment and subsequent anastomosis.

Question 14

What is Chagas Disease?

Answer 14

Infectious disease with the protozoan parasite Trypanosome cruzi. Results in the significant reduction in the number of ganglionic cells in the ENS. It can effect any region of the gut and cause a decrease in the number of intrinsic myenteric neurones, decrease in smooth muscle cells and ganglion damage by T lymphocytes.

Question 15

What is Achalasia?

Answer 15

Disease of the oesophagus. Dramatic reduction in the number of inhibitory neurones in the lower oesophageal and the less lower oesophageal sphincter.

Question 16

What are the effects and diagnosis of achalasia?

Answer 16

Effects: 
Dysphagia (difficult to swallow)
Regurgitation
Chest pain
Weight loss

Diagnosis: Barium esophargram
Endoscopy

Question 17

Treatment of achalasia

Answer 17

Nitrates and Ca2+ channel blockers
Botulinum toxin
Pneumatic Dilation
Surgical myotomy

Question 18

How is the stomach structure suited to its function?

Answer 18

Empty stomach has a approx. volume of 50ml. Highly folded and upon filling the folds flatten out, therefore wall tension and intraluminal pressure change is very small. At 500ml all fold are fully open and beyond here the intraluminal pressure increases further.
Relaxation in the fundus is regulated by vago-vagal reflex termed ‘receptive relaxation’. If vagal innervation is interrupted then intra-gastric pressure increases.

Question 19

What is the first phase of gastric motility?

Answer 19

Phase of propulsion:
Movement of solid particles towards the antrum, by gastric contractions and occlusion of the pylorus, causing propulsion. Small particles are allowed through by contraction of the pyloric musculature and large are left behind.

Question 20

What is the second phase of gastric motility?

Answer 20

Phase of Grinding:
Emptying of liquids with small particles whereas large particles are retained in the buldge of the terminal antrium and subjected to grinding. Only particles smaller than 2mm are propelled through the pylorus to the duodenum.

Question 21

What is the third phase of gastric motility?

Answer 21

Phase of retropulsion:
Most of the gastric contents are returned to the body of the stomach for pulverisation and shearing of solid particles, this is retropulsion.

Question 22

How many times is the gastric motility cycle repeated?

Answer 22

This cycle is repeated until all particles are ground down to <2mm in size and the gastric contents are emptied.

Question 23

What is the function of the Pylorus and why is it important?

Answer 23

The pylorus regulates what goes into the small intestine to ensure the small intestine isn’t overwhelmed as an increase in acid secreted due to large quantities of food can cause the lining of the small intestine to be damaged by acid.

Question 24

What is the first phase of gastric emptying?

Answer 24

Cephalic:
Sight, smell, taste, chewing
It is an inhibitory phase - inhibitory nerve fibres in the vagus nerve, resulting in the relaxation of the stomach enabling it to store large volumes,

Question 25

What is the second phase of gastric emptying?

Answer 25

Gastric:
Food is present in the stomach.
Excitatory phase in which the stomach empties at a rate proportional to its volume.
High acid content for digestion.
Myogenic reflex = stretching smooth muscle causes reflex contraction
Activation of pressure receptors which sends impulses to local nerve plexi in the vagus nerve.
Gastrin (excitatory) released in response to peptide etc

Question 26

What is the third phase of gastric emptying?

Answer 26

Mainly an inhibitory phase, through which the duodenum adapts the “work-load”.
Controlled by hormonal and paracrine mechanisms.
Low pH = activates secretion of Secretin
High fats/lipids = increases secretion of CCK
High aa level = increase secretion of gastrin
High carbohydrate level = increases secretion of GIP

Some motor reflexes. Pressure receptors delays gastric emptying via nerves in plexi, this allows more times for digestion.