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Introduction to medicine (Year 1) > Alimentary Transport 1: Along the GI tract > Flashcards

Alimentary Transport 1: Along the GI tract Flashcards

1
Q

How is food moved along the oesophagus into the stomach?

A
  • Waves of muscle contractions (peristalsis) - forces food down through oesophagus to stomach
  • Bolus enters oesophagus, causes distension of muscle
  • Receptors on circular muscle send signals to vagus nerve, vagus nerve secretes Ach, causes peristalsis of bolus, pushes food down.
  • Longitudinal muscle must relax, and this is mediated by NO/VIP secreted by the myenteric plexus. Does this to prevent reflux
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2
Q

What are the 3 anatomical regions of the stomach?

A
  • Fundus (top)
  • Body (middle)
  • Antrum (bottom)
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3
Q

What are the 2 functional regions of the stomach and what are their roles?

A
  • Gastric reservoir - allow tonic contractions where the muscle is thin (at top) to allow for relaxation
  • Gastric pump - allows the phasic contractions at the bottom where the muscle is thicker (peristalsis contractions)
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4
Q

How does motility of the stomach contribute to its digestive functions?

A
  • Accommodation and storage
  • Mechanical and enzymatic breakdown
  • Slow delivery of chyme to aboral regions of gut allowing digestion and absorption
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5
Q

List the mechanisms which allows the fundic area of the stomach to relax and accommodate more stomach volume

A
  • Vago-vagal reflex
  • Chewing/swallowing
  • Mechanoreceptors
  • Vagal nerve innervation
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6
Q

Which nervous mechanisms influence the relaxation of the fundic area?

A
  • Reduced cholinergic activity
  • Activation of NANC inhibitory systems - mediated by NO/VIP
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7
Q

What is reciprocal control of gastric motility?

A
  • During accommodation Ach released as well as VIP/NO- during eating accommodation occurs so VIP/NO overrides Ach
  • Once accommodated, the effects of Ach become more prominent = Increased cholinergic acitivity, decrease NANC activity
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8
Q

How is gastric motility myogenically controlled?

A
  • Intrinsic basic electric rhythm (BER)-Produced by ICC cells (these are the pacemaker cells of the gut)
  • Smooth muscle cells produce electric depolarisations from resting potential
  • Ripples move towards the antrum
  • Fundus is under vagal excitatory control
    Slow wave from ICC (Interstitial cells of Cajal) -regular recurring migrating ripples (3 waves/min) known as BER (rhythm of depolarisation-repolarisation)
  • BER allows the smooth muscle cell to depolarise and contract rhythmically when exposed to hormonal signals
  • Depolarisation of the GI smooth muscle is caused by calcium-sodium entry
  • Repolarisation of the GI smooth muscle is caused by K+ efflux
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9
Q

What are ICC cells?

A

Mesenchymal cells located within the muscle layers of the alimentary tract that mediate communication between the autonomic nervous system and smooth muscle

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10
Q

Why must gastric emptying be regulated?

A

To ensure adequate:

  • Neutralisation of acidic chyme
  • Emulsification of fats
  • Appropriate functioning of pancreatic enzymes
  • Mechanical breakdown
  • Too much volume is not handled by the duodenum to avoid swamping it
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11
Q

What factors determine the rate of gastric motility?

A
  • Type of food eaten: carbohydrates > proteins > fatty foods
  • Osmotic pressure of duodenal contents: hyperosmolar chyme causes decreased gastric emptying
  • Vagal innervation upon over-distension of duodenum/duodenal acid decreases gastric motility
  • Hormones (somatostatin, secretin, CCK) - inhibit emptying
  • Motilin increases fundic contractions
  • Injury to intestinal wall decreases motility, as does injury to the GI tract
  • Bacterial infections can increase gastric motility
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12
Q

How is regulation of gastric motility and emptying elicited from the stomach?

A
  • Gastro-gastric reflexes provide balance between gastric reservoir and antral pump.
  • Distension of the reservoir stimulate antral contractions (excitatory reflex) - the astral pump is switched on and intensified as food enters the stomach and distension of the gastric reservoir increases
  • Distension of the antrum enhances and prolongs relaxation of the reservoir- But distension of the reservoir induces inhibitory reflexes via prolonged relaxation of the gastric reservoir
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13
Q

What are the 3 parts of the small intestine?

A
  • Duodenum
  • Jejunum
  • Ileum
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14
Q

What part of the small intestine contains the pyloric sphincter?

A

Duodenum - Pyloric sphincter modulates passage of food from bottom of antrum into the duodenum

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15
Q

How is pyloric activity modulated?

A
  • By antral inhibitory and duodenal excitatory reflexes
  • Contraction of the middle antrum induces a descending inhibitory reflex causing pyloric relaxation (mediated by NO/VIP)
  • Duodenal stimuli (e.g. presence of acidic chyme (HCL) or lipids/fatty acids) induces an ascending excitatory reflex causing pyloric contractions and increasing smooth muscle tone- this prevents duodeno-gastric reflux
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16
Q

How is gastric emptying mediated by a negative feedback system?

A
  • Antral over-distension- Vago-vagal reflex
  • Duodenal over-distension and chemical stimulation - Vago-vagal reflex and hormones
  • The pyloric sphincter contracts in response to antral or duodenal rhythm; fatty acids in duodenum cause contraction of pylorus
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17
Q

What hormones and nervous factors initiate and maintain peristalsis and mixing in the small intestine?

A

Hormones:
- Motilin - Causes the cyclical bursts of gastro-duodenal contractions during fasting

Nervous factors

  • Ach
  • Substance P
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18
Q

What is the emptying of the gastric reservoir caused by?

A
  • Transport of digester from the gastric reservoir is caused by 2 mechanisms: tonic contraction and peristaltic waves in the region of the gastric corpus
  • Tonic contractions - contractions that are maintained from minutes up to hours at a time
  • Can occur in stomach and sphincter of GIT
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19
Q

Describe storage and gastric emptying

A

Storage- proximal stomach relaxes to store food at low pressure, whilst it’s acted on by acid, enzyme and mechanically

Emptying- carefully regulated to ensure adequate acidification/neutralisation, action of enzymes, mechanical breakdown and to avoid swamping of the duodenum

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20
Q

What is gastric emptying dependent on?

A
  • Propulsive force generated by the tonic contractions of the proximal stomach
  • Stomach’s ability to differentiate types of meals ingested and their components
  • Fatty, hypertonic, acidic chyme in the duodenum decreases the force and rate of gastric emptying
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21
Q

Summarise the emptying of different food
components (liquids, solids, fatty foods and
indigestible solids).

A
  • Liquids - rapidly disperse, empty without lag time, rate of emptying influenced by nutrient content, greater nutrients, longer retention
  • Solids - 2 phases (lag + linear phase), duration of lag time related to size of particle
    Liquid part emptied, solid component is retained in proximal stomach
    Trituration of larger particles to smaller ones (trituration is a form of reducing particle size, or creating a homogenous solution through thorough mixing)
    Pylorus regulates passage of material
  • Fatty foods: Liquefy at body temperature, flat on top of liquid layer and empty slowly, fasts are potent inhibitors of gastric motor events and gastric emptying
  • Indigestible solids: Don’t empty in immediate post-prandial period, need to be emptied by migrating motor complex (MMC)
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22
Q

Describe MMC

A
  • Highly organised motor activity
  • Cyclically recurring sequence of events
  • Occurs between meals when the stomach/intestine are ‘empty’
  • Bursts of high frequency, large amplitude contractions that migrate along the length of intestine and die out
  • Begins at antrum -> duodenum -> jejunum -> ileum
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23
Q

What are the 3 phases of MMC?

A
  • Phase I = quiescence
  • Phase II = Irregular propulsive contractions
  • Phase III = uninterrupted peristaltic rush
24
Q

What are the functions of MMC?

A
  • Indigestible residues moved out of stomach by large contractions and wide opening of the pyloric sphincter during phase III
  • Removes dead epithelial cells by abrasion
  • Prevents bacterial overgrowth
  • Prevents colonic bacteria from entering small intestine
  • Occurs following digestion and absorption of a meal (empty stomach)
  • Causes mass movement of intestinal contents, leading to evacuation of faeces
25
Q

How is MMC controlled?

A
  • Not fully known
  • Smooth muscle cells of stomach can produce ‘slow waves’
  • Contractions coordinated by the ENS by interstitial cells of Cajal (pacemaker cells)
  • Initiated by the vagus nerve in the upper tract
  • Some evidence for cyclical secretion of the hormone motilin from stomach and duodenum
  • Feeding inhibits release of motilin
26
Q

Describe segmentation (mixing contractions)

A
  • Stationary contractions and relaxation
  • Originates in the pacemaker cells (ICC)
  • Segmentation -> divisions and subdivisions of chyme, bringing chyme in contact with intestinal walls
  • Segmentation causes the slow migration of chyme towards ileum
  • Dudoenum/jejunum- 10-12 contractions/min
  • Ileum- 8-9 contractions/min
27
Q

Describe peristalsis (propulsive)

A
  • In stomach (3 waves/min) - migrating motor complex, mass movements (evacuation)
  • Is a propagating contraction of successive sections of circular smooth muscle preceded by a dilatation
28
Q

Describe the 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
  • Segmenting (mixing) contractions primarily churn the food, but also propel it towards the anus
29
Q

Outline the pathway for the small intestinal peristaltic reflex

A
  • Sensory neurones detect luminal contents
  • They relay action potentials to the vagal centre of the medulla
  • The medulla relays vasovagal reflexes to integrating and programme circuits within the ENS
  • These circuits stimulate motor neurones, which stimulate contraction or relaxation in the small intestine
30
Q

Describe Haustral/segmental contraction in the colon (large intestine)

A

Haustral contractions are a type of segmentation contraction, play significant role in absorption of water, electrolytes

Triggered by distension of smooth muscle by faecal matter

Occurs primarily in ascending + transverse colon
- Haustra = sacs of colon caused by band-like arrangement of the taenia coli smooth muscle

Taenia coli are 3 longitudinal smooth muscle bands in the, they’re parallel, equally distributed and form a triple helix structure from the appendix to the sigmoid colon

  • Faecal residue enters cecum, triggers stretch reflex
    • Stretch receptors activated, send signals to myenteric plexus
    • Myenteric plexus initiates contration of smooth muscle
  • This contraction brings the faecal matter in close contact with the mucosa of the large intestines and GIT secretions, enhances absorption of electrolytes, water

Contraction of taenia coli causes haustra to bunch up, mixing contents of colon

31
Q

Describe the peristalsis contractions in the colon (large intestine)

A
  • Propulsive contractile waves moving the contents of the colon towards the anus
  • They are initiated by distension of the intestinal wall activating mechanoreceptors
  • Peristalsis in the colon is much slower than the small intestine
32
Q

Describe Mass movement in the colon

A

Type of peristaltic contraction

Powerful contractions in the mid-transverse colon that sweep contents to the rectum

  • Occur during or after meals, usually 3 to 5 times a day
  • Triggered by:
    • Distension
    • Irritants
    • Gastrocolic reflex
      • Parasympathetic fibres send signals to transverse + descending colon, releases acetylcholine to initiate contractions
      • G cells release gastrin, this also stimulates contractions
33
Q

What is the sympathetic innervation of the GI system?

A
  • Splanchnic nerves - Celiac plexus, superior mesenteric plexus, inferior mesenteric plexus

The greater splanchnic nerve provides sympathetic innervation to the foregut, and inhibits motility and secretions

The lesser splanchnic nerve provides sympathetic innervation to the midgut

The lumbar splanchnic nerve provides sympathetic innervation to the hindgut

34
Q

What is the parasympathetic innervation of the GI system?

A
  • Vagus nerve
35
Q

What organs does the celiac trunk supply?

A
  • Stomach
  • Liver
  • Spleen
36
Q

What organ does the superior mesenteric artery supply?

A
  • Small intestine (duodenum, jujenum, ileum)
37
Q

What organ does the inferior mesenteric artery supply?

A
  • Large intestine (colon, cecum, rectum, alimentary canal)
38
Q

How does activation of n.a.n.c systems allow the stomach to accommodate food?

A
  • Efferent fibres of Vagus nerve stimulate release of NO and VIP, causes fundus to relax
  • This is receptive relaxation, causes dilation of fundus, preparing it for accommodation of food.
  • As food enters stomach, stomach walls begin to stretch (distension), this triggers further release of NO and VIP, further increasing dilation of fundus, so fundus can accommodate more food (this is adaptive relaxation)
  • Both of these allow for accommodation of food
39
Q

Describe segmentation in the small intestine

A
  • Occurs whenever we eat
  • Short, weaker contractions
  • Mixing chyme with bile (allows for emulsification of fats) and pancreatic secretions (proteases, lipases, amylases - break down proteins, carbs, lipids) as well as intestinal and pancreatic fluids (rich in bicarbonate and mucus) - All of this enhances absorption
  • Moves chyme toward anal side (anterograde) and also moves chyme toward oral side (retrograde) as this is what allows mixing
  • Slows down contraction by increasing contact time with chyme and mucosal surface (this is where absorption occurs) - enhances absorption
40
Q

What is activated when chyme is in the small intestine?

A
  • Chyme causes distension of small intestine
  • This triggers stretch receptors which trigger the myenteric plexus to induce constriction of the small intestine
41
Q

What factors play a role in segmentation within the small intestine?

A
  • Distension of small intestine (stimulatory)
  • Sympathetic nervous system (inhibitory)
  • Parasympathetic nervous system (stimulatory)
  • Irritants (stimulatory)
42
Q

Describe peristalsis in the small intestine

A
  • Peristalsis occurs whenever there’s already food in the small intestine, whenever we eat
  • Bolus causes distension of small intestine, causes stretch receptors to signal myenteric plexus
  • Auerbach’s plexus = between inner circular layer of musuclaris externa and outer longitudinal layer of muscularis externa
  • Stretch receptors regulate activity of myenteric plexus, circular layer stimulated, releases acetylcholine to contract around bolus
  • Stretch receptors inhibit contraction of longitudinal muscle via n.a.n.c by releasing NO and VIP, keeps lumen closed preventing food from going backwards
  • After circular layer surrounding bolus contracts, circular layer underneath must be relaxed (by NO and VIP) so bolus can be pushed through. Acetylcholine would be released onto the longitudinal muscle to cause it to contract so lumen opens, allowing bolus through . This repeats back and forth until food goes through to the large intestine
43
Q

What stimulates peristalsis?

A
  • Parasympathetic nervous system
  • Distension of walls
  • Irritants
44
Q

What inhibits peristalsis?

A

Sympathetic nervous system

45
Q

Describe MMC in the small intestine

A
  • Migrating motor complex - this is specialised peristalsis, but normal peristalsis also occurs in the small intestine
  • Occurs between meals (Fasting state - not eating)
  • MMC = peristaltic wave, begins at dudoenum, goes all the way to the ileum (Takes approx 2 hours)
  • Pulls any food residue, dead cells, bacteria, undigested substances (coin etc.), moves into large intestine
46
Q

What hormone initiates MMC?

A

Motillin

47
Q

How is food moved along the oesophagus?

A
  • Peristalsis - alternating wave of contraction and relaxation to move food (bolus) through
  • Food causes distension of oesophageal walls, this is detected by mechanoreceptors
  • Sphincters contract to close and prevent reflux
  • Acetylcholine is released, causes circular smooth muscle contraction, pushes food forward
  • NO and VIP causes longitudinal muscle to relax, by stimulation of inhibitory neurones, this will ensure the lumen of the oesophageal sphincter remains closed, prevent reflux
  • This propulsive pattern repeats until the bolus is passed into the stomach
48
Q

How does the muscularis externa produce both segmentation and peristalsis contractions?

A
  • Muscularis externa consists of both longitudinal outer muscular layer and an inner circular layer, and between these layers is the myenteric plexus
  • The longitudinal layer is responsible for pushing the food along the digestive tract
  • The circular layer is responsible for segmentation (this begins in duodenum) and this is because the circular muscle contractions churn the food, and mix it with gastric and digestive juices, allowing for nutrients to be absorbed
49
Q

How does activation of n.a.n.c systems allow the stomach to accommodate food?

A
  • Efferent fibres of Vagus nerve stimulate release of NO and VIP, causes fundus to relax
  • This is receptive relaxation, causes dilation of fundus, preparing it for accommodation of food.
  • As food enters stomach, stomach walls begin to stretch (distension), this triggers further release of NO and VIP, further increasing dilation of fundus, so fundus can accommodate more food (this is adaptive relaxation)
  • Both of these allow for accommodation of food
50
Q

What are the 3 fundamental modes of motility in the GI tract?

A
  • Segmentation - Mixes chyme with digestive juices, increases absorption of nutrients (rhythmic contraction - wave like, depolarisation, repolarisation)
  • Propulsion (peristalsis) - Move GI content along GI tract (rhythmic contraction wave like, depolarisation, repolarisation)
  • Reservoir function (storage function) - Carried out by sphincters (tonic contraction - these are sustained)
51
Q

Where are the ICC cells most concentrated?

A

Between outer longitudinal layer and inner circular layer of muscularis externa

52
Q

What do splanchnic nerves do?

A
  • Innervate internal organs, described as paired visceral nerves
  • Paired nerves send messages to spinal cord, which is connected to brain
  • They carry fibres of the ANS (efferent fibres) as well as sensory fibres form the organs (afferent fibres)

All carry sympathetic fibres except for the pelvic splanchnic nerves, which carry parasympathetic fibres and these are the parasympathetic nerves that innervate the hindgut

Vagus afferent = gut to brain
Vagus efferent = brain to enteric

Splanchnic nerve afferent = gut to spinal cord

Splanchnic nerve efferent = spinal cord to enteric

53
Q

What hormones and neurotransmitters decrease GI fundic motility and secretions?

A

Hormones:
- CCK
- Secretin
- Gastrin
- Somatostatin - This inhibits the release of gastrin to decrease acidity in the stomach one food has entered the duodenum

NTs:
- NO
- VIP

54
Q

What hormones increase small intestine motility and secretions?

A

Hormones:
- Motilin - Causes cyclic bursts of contractions during fasts
- CCK
- Gastrin
(CCK and gastrin usually cause contractions in the fed state)

55
Q

What hormones and neurotransmitters decrease small intestine motility and secretions?

A

Secretin
VIP
Glucagon

Introduction to medicine (Year 1) (59 decks)

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