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Alimentary > S1: Alimentary Transport 1 > Flashcards

S1: Alimentary Transport 1 Flashcards

1
Q

Name the regions of the stomach

A

Fundus
Body
Antrum
Pylorus

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2
Q

What are the 2 functional regions of the stomach and what do they do?

A
  1. Gastric Reservoir

This allows tonic contractions from the top of the stomach where muscle is thin allowing relaxation

  1. Gastric Pump

This allows phasic contractions at the bottom of the stomach which has thicker muscle

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3
Q

What are the three types of reflex’s that mediate the gastric reservoir?

A

Receptive (mechanical stimulation of the pharynx- mechanoreceptors, sight)

Adaptive (vagal innervation (NO/VIP), tension of the stomach)

Feedback (nutrients, CCK, from receptors past the pyloric sphincter)

The receptive, adaptive and feedback-relaxation of the stomach are mediated by non-adrenergic, non-cholinergic (NANC) mechanisms (i.e., inhibition involving NO, VIP, etc.) as well as by reflex chains involving release of noradrenaline. When the stomach is ready to receive the food, very early on, noradrenaline is released from the sympathetic nerve fibres which helps the stomach to relax.

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4
Q

What functions does motility allow the stomach to carry out?

A
  • Accommodation and storage
  • Mechanical and enzymatic breakdown
  • Slow delivery of chyme (partially digested foods and digestion juices) to the aboral regions of the gut (to allows digestion/absorption)
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5
Q

Explain gastric accommodation

A

In normal fasting state the empty stomach is very small

The vagovagal reflex, chewing/swallowing, mechanoreceptors and vagal nerve innervation can make the fundic area relax

The relaxation is caused by a reduction of ongoing cholinergic activity and activation of NANC inhibitory (VIP/NO) system

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6
Q

What area of the stomach is involved in emptying?

A

The antral area is involved in emptying and controlled by increased cholinergic acitivity

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7
Q

Explain emptying of the gastric reservoir

A

The transport of digested material from the gastric reservoir into antral pump is caused by 2 mechanisms:

  • Tonic contractions
  • Peristaltic waves in the region of the gastric corpus (body)

Tonic contractions are caused by pacemaker cells in the proximal stomach which is then taken over by peristaltic waves in the corpus
This genererates propulsive forces

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8
Q

Explain how the type of meal ingested affects gastric emptying

A
  • Fatty, hypertonic, acidic chyme in the duodenum are slow to be emptied. They float on top of a liquid later and are inhibitors of gastric motor events.
  • Large indigestible materials remain
  • Gastric emptying of fibre rich or solid materials start after a lag phase
  • Gastric emptying of liquids is in spurts and quite rapid (without lag time) and nutrients containing liquid is retained longer.
  • Solids are broken down into smaller sizes by trituration before passing and there are two phases:
    1. lag phase (related to size of particle)
    2. linear phase (rapid)
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9
Q

What is the function of the pyloric sphincter?

A

The pyloric sphincter regulates passage of material and performs a sieving function.

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10
Q

What are the three phases in gastric emptying?

A

Propulsion (pushes food out)

Emptying (small particles exit through the pyloric sphincter)

Retropulsion (large particles can’t get through as pyloric sphincter is closed and are retained in proximal part of stomach to be grinded up through contraction and digestive juices)

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11
Q

Why is gastric accommodation and emptying so important?

A

Storage: The proximal stomach relaxes to store food at low pressure whilst it is acted upon by acid, enzymes and mechanically. VIP/NO/ATP are involved in producing this relaxation.

Emptying: This is carefully regulated to ensure adequate acidification, action of enzymes, mechanical breakdown and to avoid swamping of the duodenum (allowing digestion/absorption to be carefully controlled). Some key controllers of this are the nutrient content of the food (e.g. fatty food stays longer), as well as various hormones like CCK, GLP-1, PYY.

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12
Q

What is gastric stasis?

A

Chronic long term condition in which stomach cannot empty properly causing food to pass through slowly

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13
Q

Explain the reciprocal vagal control of gastric motility

A

During accommodation, there is Ach release to the stomach (which causes contraction) as well as there being the VIP/NO release from NANC nerves (which causes relaxation). However, as it is accommodation when the individual is eating, the VIP/NO nerves override the Ach release.

Once the food is accommodated, we start to see the effects of Ach as the cholinergic fibres increase in activity and the NANC activity decreases.

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14
Q

What are gastro-gastric reflexes?

A

They provide balance between the gastric reservoir and antral pump.
Distention (enlargement) of the reservoir stimulates antral contractions.
Distention of the antrum enhances and prolongs relaxation of the reservoir (inhibitory reflex)

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15
Q

How does the small intestine regulate gastric motility and emptying?

A

Pyloric activity is modulated by antral inhibitory fibres and duodenual excitatory fibres.

  • Descending inhibitory reflex (from the antrum) causes pyloric relaxation through NO/VIP
  • Ascending excitatory reflex (from duodenum) causes pyloric contractions and increases tone to prevent food moving back up into the stomach.
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16
Q

List the factors that affect the rate of gastric motility

A
  • Types of food eaten : carbs>proteins>fatty foods>indigestible solids
  • Osmotic pressure of duodenal contents: Hyperosmolar chyme (high) reduces gastric emptying
  • Vagal innervation upon over distension reduces gastric motility (NO/VIP), Ach mediates increased motility
  • Hormones can inhibit emptying (somatostatin, secretin, CCK, GIP)
  • Injury to intestinal wall and bacterial infections decrease motility
  • Pain and fear decrease gastric motility
17
Q

Explain myogenic control of gastric emptying

A

The gut has an intrinsic basic electrical rhythm especially in the fundic area which is under vagal excitatory control

Stomach muscle cells can produce electrical depolarisation from RMP. The ripples then move towards the antrum and there is also slow waves from interstitial cells of cajal (ICC) which produce regular migrating ripples (BER basic electrical rhythm)

  • BER (and Ca2+ influx) allows smooth muscle cell to depolarise and contract rhythmically when exposed to hormone signals and determine frequency of GI contractions
18
Q

What are intestinal cells of cajal?

A

Specialised pacemaker cells located in the wall of the stomach, small intestine, and large intestine.
They produce BER

19
Q

Explain neurohormonal control of gastric motility

A 
Stimulation of mechanoreceptors will cause relaxation, factors that have a relaxing effect on fundic motor activity include—

Gastrin
Cholecystokinin (CCK)
Secretin 
VIP
Somatostatin
Glucagon
Duodneal distension, duodenal acid, fat

The following increase fundic contractions: Thyrotropin releasing hormone (TRH), Motilin
20
Q

How is movement of chryme controlled through the small intestine?

A
  • Hormonal and nervous factors initiate and maintain peristalsis and mixing. There is localised distension of the duodenum.
    (CCK, gastrin, serotonin, insulin and motilin can increase motility)

Secretin decreases the activity and glucagon can decrease the motility within the sI

21
Q

What is feedback control of gastric emptying?

A

Gastric emptying is regulated by negative feedback systems e.g. antral over-distension and vago-vagal reflex which allows fundic area to be controlled.

Duodenal over-distension and chemical stimulation will cause vago-vagal reflex causing hormones to be released.

The entire small intestine is involved in the feedback regulation of gastric emptying.

22
Q

Describe motility of the intestine

A

In the small intestine with regards to motility you have segmentation occurring, peristalsis occurring and later on migrating motor complex and then near the end mass movements (evacuation).

Segmentation is mixing contractions, that are stationary contraction and relaxation. Segmenting contractions primarily churn the food but also propel it towards the anus.

Peristalsis is circular propulsive contractions propagating aborally. Peristaltic (propulsive) contractions spread the food out allowing digestive enzymes to mix with it but primarily push food towards the anus. Peristalsis is global in nature.

23
Q

Explain segmentation in the small intestine

A
  • Originates in the pacemaker cells (ICCs)
  • Segmentation causes divisions and subdivisions of chyme (breaking it up) and brings the chyme in contact with intestinal wall allowing some absorption to occur
  • It causes slow migration of chyme towards the lumen
24
Q

Explain peristalsis in the small intestine

A

The regulation of peristalsis requires neural reflexes. Peristalsis is a propagating contraction of successive circular smooth muscle preceded by dilation (i.e. the part of the gut where the bolus is moving into will relax, VIP/NO are important in causing this).

So peristalsis is produced by coordinated contraction and relaxation of muscle cells

  • mediated by ENS
  • can be enhance/suppresses by para/symp NS
25
Q

Difference between segmentation and peristalsis

A

• Peristaltic (propulsive) contractions spread the food out allowing digestive enzymes to mix with it, but primarily push the food towards the anus (global movement)
Segmenting (mixing) contractions primarily churn the food, but also propel it towards the anus - some localisation

26
Q

What are the three phases of gastric motor activity?

A

Phase I – Quiescence/quiet period
Phase II – Irregular propulsive contractions
Phase III – Burst of uninterrupted phasic contractions (peristaltic rush)

27
Q

Explain the migrating motor complex (MMC)

A

Phase III allows clearance of the gut completely through MMC

The MMC is highly organised motor activity and occurs between means when the stomach/intestine is ‘empty’ allowing the stomach to be cleansed.

It consists of high frequency, large amplitude contractions starting at the lower portion and the stomach and then migrating the length of the intestine and the die out. The successive peristaltic waves start more aboral each time and propagate slightly further than when the previous one stopped.

28
Q

What are the functions of MMC?

A
  • Intestinal housekeeper and indigestible residues are moved out of the stomach by large contraction
  • Removes dead epithelial cells by abrasion as well as preventing bacterial overgrowth in the small intestine
29
Q

What controls the MMC?

A

Smooth muscle cells of stomach can produce “slow waves”, contractions are coordinated by the enteric nervous system pacemaker cells (interstitial cells of Cajal)
There is also innervation by the vagus nerve in the upper tract. There is also some evidence for cylical secretion of the hormone motilin from stomach and duodenum.
Feeding inhibits the release of motilin.

30
Q

Functions of the large intestine

A
  • Intensive mixing and slow movement of waste and indigestible aborally
  • Fermenting chambers on the sides where there is hydrolysis of fibre and indigestible nutrients
  • Large intestine produces faeces by the absorption of water
31
Q

Is peristalsis in colon slower than small intestine?

A

Yes

Peristalsis moves the contents towards the anus and distension initiates contraction.

32
Q

Describe diarrhoea and constipation

A

Disorders of motility, fluid secretion and absorption and important in the pathogenesis of diarrhoea and constipation.

These occur due to the transit time spent in the gut

Diarrrhoea is frequent (over 3x a day) discharge of liquid faeces
Constipation is difficulty/some constraint in opening/emptying bowels

Alimentary (18 decks)

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