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104 Theme 1 - The Alimentary System > Motility of the GI tract > Flashcards

Motility of the GI tract Flashcards

1
Q

What is the role of the digestive system?

A

•Role: to extract chemical energy, vitamins, minerals and water from ingested products

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2
Q

What are the 6 processes of the digestive system?

A

•Six processes: ingestion, secretion,motility, mechanical and chemical digestion, absorption and elimination of waste

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3
Q

What is the 4 layer structure of the GI tract?

A
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4
Q

Where in the digestive tract is this epithelium found?

A
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5
Q

What is the function of the

Oesophagus

Stomach

Small intestine

Large intestine

A
  • Oesophagus - transport
  • Stomach - storage, secretion, mixing, digestion
  • Small intestine - secretion, mixing, majority of digestion, absorption
  • Large intestine - limited absorption (water, ions), faeces formation, gut microbiota
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6
Q

What is motility governed by and what is the exception?

A
  • Motility is governed by involuntary contraction of smooth muscle with pacemaker interstitial cells of Cajal (ICC)
  • Except upper oesophagus and external anal sphincter (striated skeletal muscle/voluntary)
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7
Q

What is a single unit?

A

•Smooth muscle is single unit- gap junctions allow electrical coupling and contraction as a functional syncytium

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8
Q

How is smooth muscle organised?

A

•Smooth muscle organised into connected bundles of outer longitudinal and inner circular smooth muscle in muscularis layer

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9
Q

What does the intrinsic enteric nervous system control?

A

•The intrinsic enteric nervous system (ENS) controls GI motility and secretion independently

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10
Q

What are the 2 interconnected plexuses in the gut wall?

A
  • Myenteric plexus
  • Submucosal plexus
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11
Q

What is the role of the autonomic nervous system?

A

Extrinsic autonomic sympathetic and parasympathetic innervation allows central modification

ANS modifies basal activity of the ENS

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12
Q

What causes a reflex contraction in the intrinsic enteric innervation?

A

Reflex contraction in response to local stimuli (stretch, nutrients, irritation, hormones)

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13
Q

Where are the plexuses located and what do they control?

A
  • Myenteric plexus (Auerbach’s) in muscularis layer
  • motility
  • Submucosal plexus (Meissner’s) in submucosal layer
  • secretion and local blood flow
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14
Q

What is the parasympathetic innervation?

A

excitatory to motility and secretion (via Vagus and pelvic splanchnic nerves)

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15
Q

What is the sympathetic innervation?

A

Inhibitory to motility and secretion (via thoraco-lumbar innervation)

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16
Q

Complete the table on hormonal secretions

A
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17
Q

How are hormones secreted into the GI tract?

A

Endocrine hormones are secreted by entero-endocrine cells in the epithelial layer of the GI mucosa and enter portal blood circulation

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18
Q

What mechanisms cause contraction?

A

Like all excitable cells smooth muscle cells have a fluctuating negative electrical potential difference (-50-60mV) across the membrane

•Results in two types of electrical activity

ØSlow waves

ØSpike potentials

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19
Q

What are slow waves?

A

Øcyclical oscillations of membrane potential spontaneously initiated by pacemaker ICCs

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20
Q

What are spike potentials?

A

Øgenerated once threshold is reached resulting in Ca2+ influx and smooth muscle contraction

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21
Q

What does this show?

A

Electrophysiology trace showing cyclical slow waves followed by spike potentials resulting in muscle contraction

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22
Q

What is the role of slow waves?

A

•Slow waves provide a basic electrical rhythm (BER) (ICC)(3-12 /min)

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23
Q

What is the role of spike potentials?

A

•spike potential causes contraction by further depolarisation to threshold levels

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24
Q

What is depolarisation stimulated by?

A

•Depolarisation stimulated by stretch, hormones (motilin), excitatory neurotransmitter acetylcholine release from ENS excitatory motor neurons or P/S

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25
Q

What is depolarisation inhibited by?

A

•Inhibition by hyperpolarisation caused by inhibitory ENS, sympathetic NT norepinephrine or hormones (secretin)

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26
Q

What are the 2 types of contraction?

A
  • Segmentation for mixing
  • Peristalsis for propulsion
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27
Q

How does segmentation for mixing occur?

A

ØBursts of circular muscle contraction and relaxation

ØBack and forth pendular movements also occur

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28
Q

How does peristalsis for propulsion occur?

A

Ølocal distension triggers contraction behind bolus and relaxation in-front

ØWave of contraction

ØRequires functional myenteric plexus

Ølaw of the intestines: movement aborally

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29
Q

What is Hirshsprungs Disease?

A
  • A rare congenital absence of the myenteric plexus, usually involving a portion of the distal colon
  • The pathologic aganglionic section of colon lacks peristalsis and undergoes continuous spasm, leading to functional obstruction and severe constipation
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30
Q

What are the 3 stages of swallowing?

A
  • Oral - voluntary initiation of swallowing in the oral cavity
  • Pharyngeal – involuntary passage of food through pharynx into oesophagus
  • Oesophageal – involuntary passage of food from pharynx to stomach
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31
Q

How does the oral phase of swallowing occur?

A
  • tongue pushes up against hard palate and contracts to force lubricated bolus into the pharynx
  • The pharynx (throat) consists of the oropharynx, the nasopharynx and larygopharynx
  • Bolus enters the oropharynx initiating the pharyngeal stage through stimulation of sensory receptors
32
Q

Is the oral phase of swallowing under voluntary or involuntary control?

A

Voluntary

33
Q

How does the pharyngeal phase of swallowing occur?

A
  • swallowing centre in the medulla oblongata and pons in the brain stem (reflex)
  • Motor efferents in trigeminal, glossopharyngeal and vagal nerves cause series of muscle contractions moving bolus through oropharynx into laryngopharynx and into oesophagus
  • Soft palette elevates over posterior nares to close nasal pharynx
  • Epiglottis closes larynx
  • Respiration is inhibited
  • Upper oesophageal sphincter relaxes
  • Pharyngeal muscle contraction propels bolus into oesophagus
34
Q

How does the oesophageal phase of swallowing occur?

A
  • Primary peristalsis moves bolus downwards
  • Circular muscle contracts behind bolus, longitudinal muscle contracts in front to shorten fibres and push wall outward
  • Mucus lubricates and reduces friction
  • Relaxation of the lower oesophagus and lower oesophageal sphincter (LOS) occurs
  • Secondary peristalsis stimulated by stretch
35
Q

What is the coordination of the oesophageal phase of swallowing via?

A

Coordination is via intrinsic myenteric and extrinsic vagal innervation

36
Q

What is achalsia?

A
  • Lower oesophageal sphincter fails to relax causing food to remain in oesophagus
  • Cause may be vagal or myenteric defect
  • Distention, inflammation, Infection and ulceration
37
Q

What is gastro-oesophageal reflux?

A
  • Lower Oesophageal Sphincter tone lost leading to flow of acidic gastric contents into oesophagus
  • Inflammation, ulceration
  • May be linked to hiatus hernia where portion of stomach protrudes through diaphragm into thorax causing gastric reflux
38
Q

What are the 3 primary motor functions of the stomach?

A
  • Storage
  • The vagovagal reflex mediates receptive relaxation reducing muscle tone and allowing reservoir function
  • Mixing
  • Fragmentation of food and mixing with secreted gastric juice for digestion
  • Emptying contents into the duodenum at a controlled rate
39
Q

Where is the 3rd oblique muscle layer?

A

Third oblique muscle layer primarily in body of stomach

40
Q

What is the role of the rugae?

A

Rugal folds allow stomach to stretch out

41
Q

Which part of the stomach functions primarily as a reservoir for storage of stomach contents?

A

Fundus

42
Q

What is the part of the stomach expanding?

A

Fundus

43
Q

How does mixing occur in the stomach?

A

Slow peristaltic waves are initiated in the body of the stomach moving stomach contents towards pyloric antrum. Food is forced back for further mixing and digestion. This process of propulsion and retropulsion occurs in cycles to produce chyme

44
Q

What is this process?

A

Mixing

45
Q

How does stomach emptying occur?

A
  • Highly regulated with primary inhibitory feedback signals from small intestine
  • More powerful peristaltic contractions build to force chyme into the duodenum
46
Q

What is the excitory and inhibitory regulation of stomach emptying?

A
  • Excitatory
  • ENS/ANS neuronal stimulation and hormones eg motilin
  • Inhibitory
  • ANS regulation, duodenal enterogastric reflexes and hormones eg CCK, secretin
47
Q

Complete the diagram on ANS/ENS reduction in gastric emptying

A
48
Q

Complete the diagram on Hormonal reduction in gastric emptying

A
49
Q

What is dumping syndrome?

A
  • Rapid emptying of gastric contents into the small intestine
  • Occurs following ingestion of large meal after gastrectomy
  • May be caused by hypertonic duodenal contents causing rapid entrance of fluid
50
Q

What are the symptoms of dumping syndrome?

A

•characterized by nausea, pallor, sweating, cramps, vertigo, and sometimes fainting within minutes

51
Q

What is gastroparesis?

A
  • Stomach fails to empty
  • prevents proper digestion
  • May be caused by gastric cancer or peptic ulcers
  • occasionally observed through impaired vagal stimulation to the stomach in severely diabetic patients who develop autonomic neuropathy
52
Q

What are the symptoms of gastroparesis?

A

•Causes bloating and nausea

53
Q

What is the role of the small intestine?

A

•Motility patterns allow the majority of digestion and absorption of nutrients here over 3-5 hrs

54
Q

What are the histological features of small intestine?

A

•Large surface area for absorption provided by circular folds (plicae circulares), villi projections of the mucosa and ‘brush border’ microvilli on the epithelial cell apical surface

55
Q

What are the 2 types of motility occuring in the small intestine?

A
  • Mixing and circulation for maximum exposure to absorptive epithelium
  • Propulsion of chyme aborally
56
Q

What is motility in the small intestine controlled by?

A

•Motility controlled by intrinsic motor patterns modified by hormonal and ANS neural stimuli

57
Q

What are the stimuli for segmentation for mixing?

A
  • stretch receptors trigger myenteric stimulation of muscle contraction
  • No net movement
58
Q

What are the stimuli for propulsive peristalsis?

A
  • Stretch
  • hormones:
  • Excitation: gastrin, CCK, insulin, motilin, serotonin
  • Inhibition: secretin and glucagon
59
Q

What are the 3 propulsive peristaltic reflexes?

A
  • Gastroenteric reflex
  • Gastroileal reflex

Migrating motor complex (MMC)

60
Q

What is the gastroenteric reflex?

A

•Gastroenteric reflex: gastric distention activates myenteric plexus to promote SI peristalsis

61
Q

What is the gastroileal reflex?

A

•Gastroileal reflex: gastric distention promotes peristalsis in the ileum to force chyme through ileocecal valve into caecum

62
Q

What is the migrating motor complex?

A

•Series of peristaltic contractions, between meals, every 90 mins sweeps contents into colon

63
Q

What is the migrating motor complex controlled by?

A

•Intrinsic enteric control, hormone motilin

64
Q

What can the absence of the migrating motor complex lead to?

A

Bacterial overgrowth

65
Q

What is the role of the ileocaecul valve?

A

Ileocecal valve controls emptying of chyme into colon

66
Q

What is the peristaltic rush?

A

•Peristaltic rush –mucosal irritation, ENS and ANS neural reflexes rapidly sweep contents of SI into colon

67
Q

What is the paralytic ileus?

A

•Paralytic ileus – loss of peristalsis following mechanical trauma

68
Q

How does vomiting disrupt peristalsis?

A

•Vomiting – reverse peristalsis initiated in distal small intestine to expel intestinal and gastric contents

69
Q

Why is motility more sluggish in the large intestine?

A
  • Motility more sluggish to allow optimal:
  • Absorption of water and electrolytes (proximal)
  • Formation and storage of faeces (distal)
  • Commensal microbiome aids digestion, synthesises B and K vitamins
70
Q

What is the structure of the longitudinal muscle in the large intestine?

A

•Longitudinal muscle in muscularis thickened to form three bands, taniae coli, which tonically contract to form haustral bulges

71
Q

Label the diagram

A
72
Q

How does motility occur in the large intestine?

A
  • Motility is via mixing and propulsion under intrinsic enteric control modified by neural and hormonal stimuli
  • mixing contractions via haustral churning
73
Q

What peristalsis occurs in the large intestine?

A
  • mass movements occur 2-3x per day (forceful peristaltic contractions force contents into sigmoid colon and rectum)
  • gastro-colic and duodeno-colic reflexes: mass movements occur after meals on stretching via ANS
74
Q

How does the defecation reflex work?

A
  • mass movements push faecal matter into the normally empty rectum
  • stretch receptors are stimulated and activate the ENS and parasympathetic ANS
  • Involuntary contraction of longitudinal muscle in the rectum opens the internal anal sphincter
  • The constricted external anal sphincter is voluntarily relaxed to allow defecation
75
Q

What is the role of the defecation reflex?

A

•Initiates defecation to expel faeces containing residues of digestion, bacteria, bile pigment, mucosal debris

104 Theme 1 - The Alimentary System (17 decks)

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