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PHSL233 > Lecture 25 - GI Tract Regulation > Flashcards

Lecture 25 - GI Tract Regulation Flashcards

1
Q

What is the distance ENS regulation can occur over in the GI tract?

A

Very short (cm)

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2
Q

What is the distance CNS regulation can occur over in the GI tract?

A

Long distances
(Via the autonomic nervous system)

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3
Q

What distance can hormonal regulation occur over in the GI tract?

A

Long distances

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4
Q

What are the ENS regulation action sites in the GI tract?

A

Very local, generally only effectors within one organ?

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5
Q

What are the CNS regulation action sites in the GI tract?

A

Coordinates fast and complex actions involving several sites of action (e.g. swallowing)

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6
Q

What are the hormonal action sites in the GI tract?

A

Each hormone regulates more than one cell type in more than one organ

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7
Q

What is the speed and duration of ENS response in the GI tract?

A

Fast and short duration

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8
Q

What is the speed and duration of CNS response in the GI tract?

A

Fast and short duration

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9
Q

What is the speed and duration of hormonal response in the GI tract?

A

Slow and sustained

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10
Q

What are the stimuli for ENS response in the GI tract?

A

Intrinsic - chemo/ osmo /mechano receptors within the GI tract

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11
Q

What are the stimuli for CNS response in the GI tract?

A

Extrinsic - Special senses, emotional responses.
Long reflex pathway - receptors within the GI tract

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12
Q

What are the stimuli for hormonal response in the GI tract?

A

Stimuli include both the GI tract luminal content and external / extrinsic stimuli

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13
Q

Can the ENS act independently of other systems in the GI tract?

A

Yes – ENS is sometimes called the brain of the gut

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14
Q

Can the CNS act independently of other systems in the GI tract?

A

No, only ENS directly innervates GI tract structures so must act via the ENS

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15
Q

Can hormones act independently of other systems in the GI tract?

A

Yes – some hormones released from the GI tract in response to GI tract luminal conditions act on the GI tract.

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16
Q

What are the ENS control signals/signal molecules in the GI tract?

A

Neural / action potentials
Neurotransmitters:
- Ach - stimulates
- NOS- inhibits

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17
Q

What are the CNS control signals/signal molecules in the GI tract?

A

Neural / AP’s
Neurotransmitters:
Parasympathetic: Ach - stimulates
Sympathetic: adrenaline - inhibits

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18
Q

What is the hormonal signalling pathway in GI tract?

A

Enteroendocrine cells
-> Peptide hormones
-> extracellular receptors.
Endocrine (Blood) and paracrine (diffusion)

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19
Q

What is the advantage of ENS regulation in the GI tract?

A

Fast local responses to conditions within a region of the GI tract

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20
Q

What is the advantage of CNS regulation in the GI tract?

A

Fast coordination of complex movements of short duration over long distances

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21
Q

What is the advantage of hormonal regulation in the GI tract?

A

Slow but sustained coordinated response over long distances / several organs of the GI tract

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22
Q

What does motility/motility pattern refer to?

A

Any pattern of contraction or relaxation of GI tract smooth muscle for a specific function.

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23
Q

What can motility patterns be?

A
  • Simple
  • Combination of two or more simpler patterns
  • Complex series of events
  • Relaxation
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24
Q

What motility patterns specifically move or propel GI contents along the tract?

A

Movement or Propulsion

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25
Q

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

A

Phasic
Tonic

26
Q

Where do phasic contractions occur in GI tract?

A

Esophagus, gastric antrum (stomach), small intestine

27
Q

Where do tonic contractions occur in GI tract?

A

Lower esophageal sphincter, oral (near mouth) stomach, ileocecal and internal anal sphincters

28
Q

How long are phasic contractions?

A

Several seconds

29
Q

How long are tonic contractions?

A

Minutes-hours

30
Q

What are the two main stages of motility patterns in the GI tract?

A

Fasting and fed

31
Q

What is the function of fasting motility patterns?

A
  • Clearing undigested material & secretions
  • Regulating intestinal microflora
32
Q

What is the function of fed motility patterns?

A
  • Storage
  • Propulsion and Movement at a controlled rate
  • Mixing (and exposure to absorptive surfaces)
33
Q

In what stage does MMC occur?

A

Fasting

34
Q

What is the MMC?

A

Migrating Motor Complex

35
Q

When does MMC start?

A

Starts 4-5 hours post meal absorption

36
Q

What is the duration of MMC?

A

Duration – 2 hours from the stomach to the end of the large intestine

37
Q

What are the 3 phases of the MMC?

A

(1) intense
(2) inactive
(3) intermittent

38
Q

Where is the MMC coordinated?

A

Coordinated in stomach then small intestine

39
Q

What is the function of the MMC?

A

Clears undigested material & secretions
Regulates intestinal microflora
Epithelial cell turnover

40
Q

What is the regulation of the MMC?

A

Hormonal - Motilin released by intestinal m-cells
Neuronal - Motilin stimulates both the enteric and autonomic NS

41
Q

What does storage involve in the GI tract?

A

Relaxation of smooth muscle allows the volume of luminal contents to increase without change in pressure

42
Q

Where does storage occur in the GI tract?

A

Occurs in the stomach and the colon

43
Q

What is receptive relaxation in the stomach?

A

Swallowing triggers reduced muscle tone as food is moved down the esophagus.
Allows food to enter the stomach without a change in pressure in the stomach
- Prevents gastric reflux in the stomach

44
Q

What is the accommodation in the stomach?

A

Progressive relaxation in response to a volume change

45
Q

Where does propulsion occur?

A

Occurs in the oesophagus, stomach, small intestine and large intestine

46
Q

What are the steps of propulsion in the GI tract?

A

1) Arrival of bolus
2) Ascending circular muscle contraction behind bolus. Descending circular muscle relaxation ahead of bolus.
3)Longitudinal muscle shortening (contraction)

47
Q

What is segmentation?

A

Circular muscle contraction in alternating segments.
Mixing function.

48
Q

Where does segmentation occur?

A

Small intestine

49
Q

Describe the GI tract smooth muscle

A

Single unit or unitary smooth muscle
Act as a syncytium (cells contract together)
Extensive intercellular communication - Gap junctions (electrical)
Cells physically connected - Adherens junctions
Arranged in layers with a neural plexus between

50
Q

What is the arrangement of intestinal smooth muscle?

A

Cells are 5 – 20 µM diameter and ~ 500 µM long
Individual muscles interact with ~10 surrounding cells
- Gap junctions and Adherens junctions
Contain actin and myosin filaments
- irregular arrangement (not striated as in SKM)

51
Q

Describe the MP of intestinal smooth muscle

A

The membrane potential of intestinal smooth muscle is not stable - fluctuates cyclically
Varies between -40 and -80 mV
Fluctuates in a pattern of Slow Waves or Basic Electrical Rhythm
- Generated by Interstitial Cells of Cajal

52
Q

What do the Interstitial cells of Cajal (ICC) do?

A

Generate slow waves
Function as pacemakers
Rhythmic changes in:
- Activity of Na+/K+-ATPase
- Membrane K+ conductance

53
Q

Where are the Interstitial cells of Cajal (ICC) located?

A

Stomach and small intestine
- In smooth muscle layer close to myenteric plexus
Colon
- Boundary of muscle layer and submucosal layer

54
Q

What is the arrangement of Interstitial cells of Cajal (ICC)?

A

Send multiple branching processes to surrounding smooth muscle cells
Electrically linked to other ICC & muscle cells by gap junctions

55
Q

What determines frequency of contraction in GI tract?

A

Slow waves ALWAYS determine the frequency of contraction
Frequency of slow waves is a property of the ICC in each region

56
Q

What is the frequency of contraction in different areas of the GI tract?

A
  • Faster in the proximal regions of the GI tract (digestion)
  • Duodenum 12 – 20 per minute
  • Slower in the distal regions of the GI tract (absorption)
  • Colon 6-8 per minute
57
Q

What determines the force of contraction in the GI tract?

A

Action potentials
Small / no contractions occur in absence of action potentials
Greater the number of action potentials the greater the force of contraction

58
Q

What is mechanism 1 of contraction in GI tract smooth muscle?

A

AP and membrane depolarisation
Opening of voltage gated calcium channels
Increased cytosolic Ca2+
Ca2+ binds to calmodulin
Ca2+ /calmodulin complex activates myosin light chain (MLCK)
MLCK uses ATP to phosphorylate myosin
Actin and myosin interact and cross bridge cycles produce tension

59
Q

What is mechanism 2 of contraction in GI tract smooth muscle?

A

Pharmomechanical contraction.
Hormones & neurotransmitters bind to a receptor to induce Ca2+ release.
Gαq - Signalling pathway activated
- Receptor activation of phospholipase C
- Production of IP3
- IP3 induces release of Ca2+ from sarcoplasmic reticulum

60
Q

What is the down-regulation of smooth muscle contractions in the GI tract?

A
  1. Hyperpolarisation of smooth muscle cell membrane (or decrease Ca2+)
  2. Increased activity of Myosin Light Chain Phosphatase
  3. Inhibition of excitatory ENS neurotransmission
61
Q

How does increased activity of Myosin Light Chain Phosphatase down-regulate GIT smooth muscle contraction?

A

Dephosphorylates myosin so reduced force of contraction – Nitric Oxide inhibitory neurotransmitter

PHSL233 (32 decks)

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