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imed3004 > Coordination of GI function > Flashcards

Coordination of GI function Flashcards

1
Q

What are the 2 plexuses that innervate the GI tissue?

A

The Myenteric plexus (plexus of auerbach) which controls muscle contraction

The submucosal plexus (plexus of meissner) which controls secretion

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2
Q

What happens to osmotic gradient when the stomach secretes acid?

A

Stomach acid creates a lot of osmotic drag (osmosis into the GI tract)

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3
Q

What does the acidic pH of the stomach do?

A

It acts as a sterilizer and a reservoir for food. Also breaks down proteins

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4
Q

What does the pyloric sphincter do?

A

It releases little bits of food stochastically into the duodenum. Feedback control and metering.

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5
Q

What produces enzymes for the duodenum? What else does it produce?

A

The pancreas and neutralizing compounds (bicarbonate) which is released to neutralize stomach acid.

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6
Q

How do water soluble lipases access fatty acids to break them down?

A

Detergents are released into the duodenum

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7
Q

What do chemosensitive cells of the small intestine do?

A

They behave as endocrine cells which perceive chemical changes and release endocrine hormones that regulate pH of duodenum.

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8
Q

Where is detergent produced for the GI tract?

A

In the liver and the gall bladder (bile)

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9
Q

What is produced by salivary glands?

A

Mucus

Ptylin

Lysozyme

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10
Q

What is produced in the stomach?

A

Mucus

HCl

Pepsin(ogen)

Intrinsic factor

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11
Q

What is produced by the pancreas into the duodenum?

A

Bicarbonate and lots of digestive enzymes

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12
Q

What happens in the duodenum?

A

Equilibration

Digestion

Absorption

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13
Q

What happens in the ileum?

A

Absorption reserve

Specific absorption

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14
Q

What happens in the jejunum?

A

Digestion

Mixing

Absorption

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15
Q

How do muscles contract during mechanical digestion?

A

Mixing waves - gentle, rippling peristaltic movements - creates chyme.

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16
Q

How is food degraded during chemical digestion?

A

Different enzymes require different pH environments

Many molecules require stepwise digestion

All require specific conditions

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17
Q

What are the types of enzymes that can be released?

A

Secreted enzymes

Brush-border enzymes

Cytosolic enzymes

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18
Q

What breaks down the major macromolecules?

A

Carbohydrates - carbohydrases (pancreatic amylase, brush border alpha-dextrinase, sucrase, lactase, and maltase, ends with monosaccharides which can be absorbed)

Proteins - proteases (trypsin, chymotrypsin, carboxypeptidase, and elastase from pancreas, aminopeptidase and dipeptidase in brush border)

Lipids - lipases (pancreatic lipase and emulsification by bile salts.)

Nucleic acids - nucleases (ribonuclease and deoxyribonuclease in pancreatic juice, nucleosidases and phosphatases in brush border)

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19
Q

How is chemical digestion regulated?

A

Coordinated by both nervous and endocrine mechanisms.

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20
Q

How does mechanical digestion work?

A

Provides motility to mix and deliver food to appropriate site at appropriate rate.

Propulsive to deliver (This is via MMC and peristalsis)

Non-propulsive to mix food with enzymes (Segmentation, quivers) this retards anterograde movement to ensure adequate mixture of food with digestive enzymes

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21
Q

What controls mechanical digestion?

A

Contraction + relaxation of enteric smooth muscle

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22
Q

How is rhythm created in enteric smooth muscle?

A

There are gap junctions between enteric muscle cells. The rhythm starts in one cell and slowly moves through these gap junctions to the other muscle cells.

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23
Q

What stimulates contraction in smooth muscle?

A

Stimulated by Ca2+ via calmodulin -> MLCK

Ca2+ channels are slow to open/close = Long AP duration/contraction (10-20ms)

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24
Q

What does the phasic pacemaker of the enteric smooth muscle do?

A

Displays continual slow intrinsic electrical activity - Basal electrical rhythm (BER)

2 types:

Slow waves

Spikes

25
Q

What are slow waves?

A

They are not APs and they do not cause muscle contraction and oscillating changes in membrane potential.

When slow waves pass threshold then an AP is generated leading to spike in polarity.

26
Q

What is the migrating motor complex?

A

Lasts 90 - 120 minutes and stops with feeding. It is characterised by spikes that transition from stomach all the way through the GI tract.

27
Q

What triggers MMC to stop acting?

A

Feeding. It resumes automatically after food leaves..

28
Q

What controls motility after feeding?

A

Peristalsis. Gut wall reflex which initiates retrograde circular relaxation and moves anterograde at a rate of 2 - 25 cm/s

29
Q

How is peristalsis controlled?

A

Purely reflexive. Occurence is independent of extrinsic innervation.

Increased rate in response to ParaSNS and decreased rate in response to SNS

30
Q

How is peristalsis regulated by the enteric nervous system?

A

2 signals created in response to stretch. Retrograde signal is excitatory to smooth muscle behind bolus and anterograde signal is inhibitory to smooth muscle in front of the bolus pushing the bolus forward.

31
Q

What neurotransmitters are secreted by retrograde motor neurons?

A

Excitatory motor neurons release ACh or substance P (only in severe distension reflexes)

32
Q

What type of interneurons are present between myenteric plexus sensory neurons and motor neurons?

A

Cholinergic interneurons.

33
Q

What neurotransmitters are secreted by anterograde motor neurons?

A

Inhibitory motor neurons release NO or VIP or ATP

34
Q

How does the intestine contract for absorption?

A

Segmentation/mixing phase (stationary contraction/relaxation cycles resulting in subdivision and mixing of chyme),

Slow wave rhythm decreases along length of intestine (Duodenum 12 cycles/min and ileum 8 cycles/min)

Slow migration towards large intestine. Eventually peristaltic activity replaces segmenation

35
Q

How does the intestine contract after absorption is completed?

A

Migrating Myoelectric Complex (MMC) which consists of repeated waves which migrates down small intestine taking about 2 hours to reach large intestine.

36
Q

What kind of motility occurs in the colon?

A

It is really slow to absorb Na+ and water. 2L isotonic chyme results in 200 ml semisolid faeces

37
Q

What does the ileocaecal valve do?

A

Restricts reflux bug movement. (bacteria would enter bloodstream if it doesn’t do that)

Increase in ileum pressure opens this valve and then closes when colon pressure rises above ileocaecal valve.

This results in rhythmic opening with each peristaltic wave and closing when fluid has moved across.

38
Q

How long does food take to get to caecum, colon, and anus?

A

4h to get to caecum

8h to get to colon

12 hours to leave the body through the anus.

39
Q

How are involuntary Gi sphincters controlled?

A

By both myogenic tone and relative pressures:

Resting state is closed

When pressure in front of sphincter is higher than after the sphincter allows forward flow.

When pressure after the sphincter is higher than in front then the sphincter closes to prevent backward flow.

40
Q

How are sphincters controlled?

A

Direct results in constriction. This prevents flow and is controlled by acetylcholine.

Relaxation is indirectly controlled. Interneurons are excited resulting in NO/VIP release at the sphincter which facilitates flow.

41
Q

How is defecation controlled?

A

Voluntary control via skeletal motor nerve.

Stretch receptors feed into spinal cord and control parasympathetic and sympathetic reflexes which relax the INTERNAL anal sphincter.

The afferent nerves that control the internal anal sphincter also go to the somatosensory motor cortex which allows control of the skeletal muscle associated with EXTERNAL anal sphincter.

42
Q

What 2 nerve plexuses are important for gut motility?

A

Submucosal plexus (Meissner’s): Between mucosa and circular muscle layer.

Myenteric plexus (Auerbach’s): Between circular and longitudinal muscle layer.

43
Q

What neurotransmitters control enteric nervous system produced by the 2 nerve plexuses?

A

Acetylcholine and nor/adrenaline

Peptides

CO and NO

44
Q

What kind of digestion is controlled by the myenteric plexus?

A

Mechanical digestion

45
Q

What kind of digestion is controlled by submucosal plexus?

A

Chemical digestion.

46
Q

What do messages from the autonomic nervous system tell the GI tract?

A

Parasympathetic signals increase secretion and activity

Sympathetic stimulation decreases them.

47
Q

What pathways of activation do stimuli follow when activated?

A

Chemoreceptors, mechanoreceptors, and osmoreceptors detect stimulus and then signal along 2 pathways; long reflex pathway involves signalling to central nervous system and then via the autonomic nervous system to the enteric nervous system to act on effector cells. Short reflex pathway signals directly to enteric nervous system to create response.

48
Q

How does the enteric nervous system act on effector cells?

A

Via both direct nervous system stimulation or via endocrine system

49
Q

What do short reflexes of the GIT control?

A

Secretion, peristalsis, mixing, and local effects

Enterogastric reflex which acts between small intestine and stomach. Stomach acid motility/secretion is inhibited when small intestine has too much food in it.

50
Q

What are the types of extrinsic reflexes that can affect the GIT?

A

GIT -> Prevertebral ganglia -> GIT

Important examples:

Gastrocolic reflex: Stomach increases colon motility and evacuation.

Gastroileal reflex: Stomach opens ileocaecal valve to allow intestine to empty before stomach contents are emptied into the duodenum

51
Q

What do long reflexes of the GIT control and how?

A

Gastric motility/secretion: Stomach/duodenum -> Brainstem -> GIT

Pain reflexes: General inhibition of entire GIT

Defecation reflex: Colon/rectum -> Spinal cord -> Powerful colon/rectum/abdominal contractions.

52
Q

What hormones are important for GI motility?

A

Secretin: Duodenum S cells (in response to acid) -> Mildly inhibits GIT motility

Cholecystokinin (CCK): Duodenum/jejunum I cells (In response to FAs/sugars) -> Moderate inhibitors of GIT motility.

Motilin: Small intestine M cells (In response to high pH) -> Initiates MMCs (Interdigestive clearing) and is suppressed by feeding.

53
Q

What are the phases of digestion and what happens in each phase?

A

Interdigestive phase: MMCs go through at this stage.

Cephalic phase: Largely neural control and responds to smell, sight, thought, or initial taste of food preparing mouth for food to be eaten.

Gastric phase: Neural and hormonal control when food enters the stomach

Intestinal phase: Largely hormonal control and begins when food enters small intestine.

54
Q

How is the cephalic phase of digestion coordinated?

A

Nervous reflex arc (vagal): Stimulatory, reflex from taste buds stimulate vagal center in the CNS

Cranial nerves from medulla stimulate secretion from salivary glands, stomach, and pancreas. Vagus also increases motility of stomach and intestine.

55
Q

How is the gastric phase coordinated?

A

Nerves and hormones: It is stimulatory

Food in stomach stimulates vago-vagal reflexes and release of hormones.

Increase in vagal activity stimulates secretion from pancreas and stomach of digestive enzymes.

Hormones further stimulate gastric secretion

Increased vagal activity stimulates gastric motility.

56
Q

How is the intestinal phase coordinated?

A

Food releases stimulatory hormones from gut mucosa.

Hormones stimulate secretion of exocrine pancreatic enzymes and bicarbonate.

Hormones stimulate contraction of the gallbladder delivering bile into the intestine in high concentration.

Inhibitory hormones also act to inhibit acid secretion and gastric emptying to prevent overload

57
Q

How is the gastric phase coordinated?

A

Between meals the automatic rhythmic contractions of the stomach start to take over in the form of migrating motor complex.

58
Q

What is gut control most similar to?

A

Heart pacemaker rhythm.

imed3004 (50 decks)

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