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Smooth Muscle Flashcards

1
Q

Where are smooth and skeletal muscle found in the GIT?

A
  • Smooth = most regions
  • Skeletal = pharynx, top 1/3 oesophagus, external anal sphincter
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2
Q

2 types of smooth muscle in the GIT, and where each is found

A
  • Phasic, rapid contraction + relaxation
    > Body of oesophagus, stomach antrum, small + large intestines
  • Tonic, sustained contractions
    > Sphincters (LOS, ileocecal, internal anal), orad (upper) stomach
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3
Q

What does activation of the myenteric plexus do?

A
  • Increase tonic contraction
  • Increase intensity of rhythmic contractions
  • Increase rate of rhythmic contractions + velocity of conduction
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4
Q

What does activation of the submucosal plexus do?

A
  • Increase secretory activity
  • Modulate intestinal absorption
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5
Q

Inhibitory long-range intestinal reflexes

A
  • Ileo-gastric
  • Intestino-intestinal
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6
Q

Excitatory long-range intestinal reflexes

A
  • Gastro-enteric
  • Gastro-ileal
  • Gastro-colic
  • Duodeno-colic
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7
Q

Pathologies of GIT motility

A
  • Hirschsprung disease
  • Chagas disease
  • Achalasia
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8
Q

What is Hirschsprung disease?

A
  • Congenital lack of neuronal ganglionic cells in the ENS plexi
  • Aperistaltic bowel segment prevents propulsion of faecal stream, resulting in megacolon above point where nerves are missing + hypertrophy of normal proximal colon
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9
Q

What is Chagas disease?

A

Infectious disease of parasitic nature, resulting in significant reduction in number of ganglionic cells in ENS

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10
Q

What is achalasia?

A

Dramatic reduction in number of neuronal cells in lower oesophagus

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11
Q

What is the characteristic radiological feature of achalasia?

A

Bird’s beak

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12
Q

Clinical presentation of achalasia

A

Solid dysphagia 90-100% (75% dysphagia to liquids)

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13
Q

Diagnosis of achalasia

A
  • Plain film (air-fluid level, wide mediastinum, absent gastric bubble, pulmonary infiltrates)
  • Barium oesophagram (dilated oesophagus with taper at LOS)
  • Endoscopy (rule out GE junction tumours)
  • Oesophageal manometry (absent peristalsis, decreased LOS relaxation + resting LOS >45mmHg)
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14
Q

Treatment of achalasia

A
  • Reduce LOS pressure + increase emptying:
    > Nitrates + calcium channel blockers
    > Botulinum toxin
    > Pneumatic dilation
    > Surgical myotomy
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15
Q

What is the role of the highly-folded nature of the stomach?

A
  • Folds flatten out upon filling
  • Wall tension + intraluminal pressure change only very slightly
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16
Q

What is receptive relaxation?

A

Relaxation of the fundus is regulated by the vago-vagal reflex (if interrupted intra-gastric pressure increases)

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17
Q

What is the sieving function of the stomach?

A

Liquids + small particles leave more rapidly than larger ones

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18
Q

3 phases of the stomachs sieving function

A
  • Phase of propulsion
  • Phase of grinding
  • Phase of retropulsion
19
Q

What is the phase of propulsion?

A

Rapid flow of liquids with suspended small particles + delayed flow of large particles towards pylorus

20
Q

What is the phase of grinding?

A

Emptying of liquids with small particles whereas large particles retained in bulge of terminal antrum + subject to grinding

21
Q

What is the phase of retropulsion?

A

Retropulsion of large particles + clearing of terminal antrum

22
Q

Functions of the pylorus

A
  • Allow carefully regulated emptying of gastric contents
  • Prevent regurgitation of duodenal contents into stomach
23
Q

3 phases of gastric emptying

A
  • Cephalic
  • Gastric
  • Intestinal
24
Q

What is the cephalic phase of gastric emptying?

A
  • Inhibitory phase (vagus)
  • Sight, smell (conditioned reflex)
  • Taste, chewing (unconditioned reflex)
  • Relaxation of stomach to enable storage of large volumes
25
What is the gastric phase of gastric emptying?
- Excitatory phase - Stomach empties at rate proportional to volume due to: > Myogenic reflex (distension causes contraction) > Activation of pressure receptors > Gastrin release (response to peptides etc.)
26
What is the intestinal phase of gastric emptying?
- Inhibitory phase - Duodenum adapts work load as function of state of digestive process - Allows more time for digestion
27
General scheme of the intestinal phase of gastric emptying
- Hormonal + paracrine mechanisms activated by duodenal chemoreception: > Low pH --> secretin secreted > High fats/lipids --> CCK secreted > High AAs --> gastrin secreted > High carbs --> GIP secreted
28
3 functions of motor activity of the GIT
- Produce segmental contractions associated with non-propulsive movement of luminal contents (increase churning) - Produce peristaltic contractions - Allow hollow organs (eg. stomach, colon) to act as reservoirs
29
Functions of motility of the small intestine
- Mixing (chyme with pancreatic, hepatic + intestinal secretions) - Propulsion - Release of chyme into colon
30
Types rhythmic phasic contractions of the small intestine in the absorptive state
- Segmentation - Peristalsis
31
What is segmentation?
- Rings of circular muscles at intervals contract + relax - Then adjacent rings contract + relax - Overall result = mixing
32
What is peristalsis?
- Sequential contractions of rings of circular muscle followed by sequential relaxation - Overall result = propulsion
33
What is the migrating motor complex?
- In fasting state small bowel is relatively quiescent - But does exhibit synchronised, rhythmic changes in electrical + motor activity
34
Time interval of migrating motor complexes
90-120 minutes
35
4 distinct phases of migrating motor complexes
- Prolonged quiescent period - Period of increasing action potential frequency + contractility - Period of peak electrical + mechanical activity (few mins) - Period of declining activity merging into next quiescent period
36
Role of migrating motor complexes
- Propel particles greater than 2mm from stomach to duodenum - Clear small intestine of residual content - Stop colonic bacteria entering terminal ileum
37
Functions of the colon
- Absorb large quantities of fluid + electrolytes - Absorb SCFAs - Storage/reservoir - Regulate release of faeces - Provide environment of beneficial bacteria which synthesise vitamins - Secrete mucous + ions
38
Function of colonic rhythmic phasic contractions
Combat fluid contents of ascending colon becoming semi-solid to solid in sigmoid colon
39
Types of colonic rhythmic phasic contraction, and how long each lasts
- Short-duration RPCs (2-3s) - Long-duration RPCs (15-20s)
40
What are colonic giant migrating contractions?
Large-amplitude, lumen-occluding contractions that propagate very rapidly (1cm/s) in distal direction to produce mass movements
41
What provides parasympathetic innervation to the cecum + ascending/transverse colon?
Branches of vagus nerve
42
What provides parasympathetic innervation to the descending + sigmoid colon, rectum + anal canal?
Pelvic splanchnic nerves
43
Action of nerves when the rectum is empty
L1/2 sympathetic nerves inhibit contraction of rectum + cause contraction of internal anal sphincter
44
Mechanism of the defaecation reflex
- Faeces enter rectum - ENS stimulated - Triggers contraction of rectal wall reinforced by parasympathetic nerves S2-4 - Internal relaxation of internal anal sphincter - Voluntary relaxation of external anal sphincter allows defaecation

DIG (19 decks)

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