*Mechanisms of GI Motility

Question 1

What are the two plexuses locally control the muscularis externa?

Answer 1

Myenteric/Auerbach’s plexus

Submucosal/Meissner’s plexus

Question 2

What is the function of the myenteric plexus?

Answer 2

Mainly involved in coordination of muscle contraction

Influenced by ANS

Question 3

What is the function of the submucosal plexus?

Answer 3

Mainly involved in sensory functions and local responses to stimulation of sensory nerve endings in mucosa

Question 4

Where are the locally innervating plexuses located?

Answer 4

Myenteric - between muscle layers of muscularis externa

Submucosal - submucosa/closer to lumen than circular layer of muscle

Question 5

What do the motor neurons of the enteric nervous system supply?

Answer 5

Smooth muscle (excitatory and inhibitory)

Vasomotor neurons for intrinsic arterioles to control blood flow in gut

Secretomotor neurons for cells regulating acid secretion

Epithelium to sense what is occurring in lumen (enteroendocrine cells)

Question 6

Why does the enteric nervous system have interneurons?

Answer 6

Coordinate reflexes

Question 7

What are the two types of sensory neurons of the enternic nervous system?

Answer 7

Chemosensitive

Mechanoreceptors

Question 8

What is the function of sphincters/valves at certain points?

Answer 8

Ensure one-way traffic

Appropriate delaying

Question 9

Describe the phases of swallowing.

Answer 9

Oral - tongue pushes bolus against palate and back of mouth (detected by back of palate) to trigger swallowing reflex

Pharyngeal - upper oesophageal sphincter relaxes while epiglottis closes (prevent aspiration)

Oesophageal - bolus moves downwards into oesophagus propelled by peristalsis and gravity into stomach (seconds)

Question 10

What controls swallowing?

Answer 10

Oesophageal muscle and its innervation

Question 11

What controls the striated muscle of the oesophagus and therefore the swallowing reflex?

Answer 11

Brainstem motor neurons

Question 12

What controls the smooth muscle of the oesophagus?

Answer 12

Parasympathetic nervous system via the vagus nerve modulate activity of ENS

Question 13

What is peristalsis?

Answer 13

Progressive contraction behind bolus to propel it forwards

Question 14

Why is the lower oesophageal sphincter important?

Answer 14

Prevents reflux of acid into oesophagus/mouth

Question 15

Why does the food enter the stomach at an angle (rather than at the top)?

Answer 15

Allows intragastric pressure to close the end of the oesophagus/lower oesophageal sphincter by contracting one wall

Question 16

What helps shut the lower oesophageal sphincter?

Answer 16

Surrounding diaphragm

Tonic contraction of circular smooth muscle in abdominal oesophagus

Question 17

When does the lower oesophageal sphincter shut?

Answer 17

Inspiration

When intra-abdominal pressure rises

Question 18

Why do pregnant women tend to experience excess gastric reflux?

Answer 18

Greater pressures below diaphragm/in abdomen alter closing of lower oesophageal sphincter

Question 19

How does the capacity of the stomach change when food arrives?

Answer 19

50ml - 1.5L

Internal rugae flatten

Muscle wall relaxes (reflex)

Question 20

What initiates stomach contractions?

Answer 20

Pacemaker cells

Question 21

Describe the contractions of the stomach wall.

Answer 21

Pacemaker cells cause waves of contractions (3/min) along the wall angles towards the pyloric region

Waves get stronger as they approach pyloric region to force chyme into duodenum

Question 22

What happens to food which is too large to pass into the duodenum?

Answer 22

Returned to body of stomach for further breakdown

Question 23

How is gastric emptying controlled?

Answer 23

Duodenum detects increased acidity, fat, amino acids, hypertonicity, distension

Increases secretion of enterogastrones

Stimulates:

• Short neural reflexes via enteric neurons
• Long neural reflexes to CNS (increase SNS and decrease PNS)

To decrease gastric emptying

Question 24

What is the main form of motility in the small intestine?

Answer 24

Segmentation

Question 25

What is the difference between segmentation and peristalsis?

Answer 25

Segmentation = no forward movement, for mixing

Question 26

What is segmentation?

Answer 26

Simultaneous contraction of different segments

Question 27

What is the transit rate in the small intestine?

Answer 27

2 hours

Question 28

Why is peristalsis less frequent in the small intestine?

Answer 28

Allow time for digestion and absorption

Question 29

What helps forward movement in the small and large intestines?

Answer 29

Small pressure gradient (higher pressure in proximal regions)

Question 30

Which valve separates the small and large intestines?

Answer 30

Ileo-caecal valve

Question 31

What causes the ileo-caecal valve to contract/relax?

Answer 31

Local control: - relaxes in response to upstream pressure (mechanoreceptors) - contracts in response to downstream pressure AND sympathetic stimulation

Question 32

What are haustrations?

Answer 32

Short segments where circular and longitudinal muscle in wall constrict

Question 33

How is the contents propelled forward in the large intestine?

Answer 33

Mass/bulk movements Contraction of longer segments of circular muscle every 25-30mins

Question 34

How does the contents of the large intestine change as it passes through?

Answer 34

Fluid --> mush --> solid

Question 35

What type of muscle are the internal and external anal sphincters?

Answer 35

External - voluntary/skeletal Internal - smooth (parasympathetic)

Question 36

What aids in defecation other than sphincters?

Answer 36

Levator ani pull up Increased abdominal pressure

Question 37

What is the process of defecation?

Answer 37

1. Filling of rectum detected by afferents results in the desire to defecate 2. Rectal circular muscle contracts while internal anal sphincter relaxes 3. External anal sphincter relaxes and levator ani contract (voluntary), rectal smooth muscle also contracts

Question 38

Stimulation by what part of the ANS will usually increase gastric motility?

Answer 38

Parasympathetic

Question 39

What is the interdigestive period?

Answer 39

Fasting period between meals

Question 40

What does motilin do?

Answer 40

During interdigestive period, regulates 'background' motility (migrating contractile activity starting stomach and propagating along small intestine)

Question 41

What is the post-prandial period?

Answer 41

When digesting a meal

Question 42

What hormonal factors are involved during the post-prandial period?

Answer 42

Initially gastrin Then cholecystokinin and GIP and GL1-P

Question 43

What effect does gastrin have on gastric motility?

Answer 43

Increases

Question 44

What effect does cholecystokinin have on gastric motility?

Answer 44

Decreases

Question 45

What stimulates the release of cholecystokinin?

Answer 45

Lipids

Question 46

What stimulates the release of GIP and GL1-P?

Answer 46

Glucose in duodenum

Question 47

What effects do GIP and GL1-P have?

Answer 47

Decrease gastric motility Increase insulin secretion

Question 48

Where is cholecystokinin released?

Answer 48

Duodenum

Question 49

How can diet composition affect digestion?

Answer 49

Influence water retention by gut contents = stool consistency Influences microbiota in gut lumen which may affect local ENS neurons

Question 50

Give an example of how microbiota in the gut lumen can affect digestion.

Answer 50

Some produce short chain fatty acids which aid water reabsorption

Question 51

What physiological factors can influence motility?

Answer 51

ANS Hormones

Question 52

What extrinsic factors can influence motility?

Answer 52

Diet composition Medication Age

Question 53

Why might the elderly experience constipation?

Answer 53

Degeneration of ENS neurons results in decreased reflexes

Question 54

What can abnormal motility cause?

Answer 54

Regurgitation Malabsorption Bacterial overgrowth in intestine Diarrhoea Constipation

Question 55

What can cause acid reflux/regurgitation?

Answer 55

Diaphragmatic hernia (open lower oesophageal sphincter) Obesity Bulimia Stroke (affects central reflexes like swallowing) Medications

Question 56

Why is diarrhoea bad?

Answer 56

Dehydration Acid-base issues

Question 57

Why is constipation bad?

Answer 57

Discomfort Dangerous straining

Question 58

What is the danger of the Valsalva manouvre?

Answer 58

Large changes in blood pressure and heart rate due to straining with increased abdominal pressure Death if you have a weak heart