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Z-GI > Introduction to structure > Flashcards

Introduction to structure Flashcards

1
Q

what is the structure of the alimentary canal?

A

Series of hollow organs running from mouth to anus (oral to aboral) that are separated by sphincters, controlling movement

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2
Q

what is the purpose of the mouth and oropharynx?

A

chops and lubricates food, starts carbohydrate digestion, propels food to oesophagus

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3
Q

what is the purpose of the oesophagus?

A

muscular tube - propels food to the stomach

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4
Q

what is the purpose of the stomach?

A

stores/churns food, continues carbohydrate and initiates protein digestion, regulates delivery of chyme to duodenum

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5
Q

what is the purpose of the small intestine?

A

principal site of digestion and absorption of nutrients

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6
Q

what is the purpose of the large intestine?

A

Colon reabsorbs fluids and electrolytes, stores faecal matter before delivery to rectum

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7
Q

what is the purpose of the rectum and anus?

A

storage and regulated expulsion of faeces

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8
Q

what are the accessory structures of the alimentary canal?

A

(i) salivary glands,
(ii) the pancreas,
(iii) the liver and gall bladder (hepatobiliary system)

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9
Q

what is it called when food moves towards the anus?

A

aboral direction

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10
Q

what is it called when food moves towards the mouth?

A

oral direction

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11
Q

what is the modality?

A

Mechanical activity mostly involving smooth muscle (skeletal at mouth, pharynx, upper oesophagus and external anal sphincter)

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12
Q

what is secretion?

A

Into the lumen of the digestive tract occurs from itself and accessory structures in response to the presence of food, hormonal and neural signals

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13
Q

what is secretion required for?

A

(i) digestion,
(ii) protection
(iii) lubrication

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14
Q

what is digestion?

A

Chemical breakdown by enzymatic hydrolysis of complex foodstuffs to smaller, absorbable, units (note physical digestion in the mouth, stomach and small intestine contributes)

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15
Q

what is absorption?

A

Transfer of the absorbable products of digestion (with water, electrolytes and vitamins) from the digestive tract to the blood, or lymph – largely mediated by numerous transport mechanisms

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16
Q

what is the inner most layer of the generalised digestive tract wall?

A

Mucosa containing:

  • epithelial cells
  • exocrine cells
  • endocrine gland cells
  • lamina propria (capillaries, enteric neurones, immune cells)
  • muscularis mucosae
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17
Q

what is the 2nd layer from the inside of the digestive tract?

A 
Submucosa containing:
-connective tissue
-larger blood and lymph vessels
-glands
-nerve network
(submucous plexus)
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18
Q

what are the two layers of the 3rd layer from the inside of the tract wall :muscularis externa?

A
  • circular muscle layer

- longitudinal muscle layer

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19
Q

what other structure is in the muscularis externa layer?

A

nerve netwrok

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20
Q

what is the outer layer of the digestive tract wall?

A

serosa containing:

connective tissue

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21
Q

what is gastrointestinal motility?

A

Mostly due to the activity of smooth muscle (circular, longitudinal layers and the muscularis mucosae), but skeletal muscle is important in the mouth, pharynx, upper oesophagus and external anal sphincter

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22
Q

what happens to the lumen when circular muscle contraction occurs?

A

lumen becomes narrower and longer

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23
Q

what happens to the intestine when longitudinal muscle contraction occurs?

A

intestine becomes shorter and fatter

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24
Q

what happens when muscarlis mucosae contraction occurs?

A

change in absorptive and secretory area of mucosa (folding), mixing activity

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25
Q

what are adjacent smooth muscle cells coupled by and what does this allow?

A

Gap junctions -allow spread of electrical currents from cell to cell forming a functional syncytium

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26
Q

what is a synchronous wave?

A

when hundreds of cells are depolarised and contract at the same time i.e single unit smooth muscle

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27
Q

what drives spontaneous activity across the syncytium ?

A

specialised pacemaker cells

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28
Q

what modulates the spontaneous activity produced by pacemaker cells?

A
  • Intrinsic (enteric) and extrinsic (autonomic) nerves

- Numerous hormones

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29
Q

what spontaneous electrical activity occurs in the stomach, small intestine and large intestine?

A

occurs as slow waves - rhythmic patterns of membrane depolarization and repolarization that spread from cell to cell via gap junctions

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30
Q

what does the slow wave electrical activity determine?

A

max. frequency, direction and velocity of rhythmic contractions

31
Q

what drives slow wave electrical activity?

A

the interstitial cells of Cajal (ICCs) - pacemaker cells interspersed between the far more numerous smooth muscle cells (SMCs)

32
Q

when does contraction occur?

A

Contraction only occurs if the slow wave amplitude is sufficient to trigger SMC action potentials (upstroke mediated by voltage-activated Ca2+ channels, downstroke by voltage-activated K+ channels)

33
Q

what is the force of contractions related to?

A

number of action potentials discharged

34
Q

where are ICCs located?

A

between the longitudinal and circular muscle layers and in the submucosa

35
Q

what is the basic electrical rhythm determined by?

A

the slow waves

36
Q

what determines if the slow wave amplitude reaches threshold?

A
  • Neuronal stimuli
  • Hormonal stimuli
  • Mechanical stimuli
37
Q

what do these 3 stimulus do?

A

Generally act to depolarize SMCs rather than influence slow waves directly – depolarization brings slow wave peak to threshold for opening of L-type voltage-activated Ca2+ channels

38
Q

what is the BER frequency in the stomach?

A

3 slow waves per minute

39
Q

what is the BER frequency in the small intestine?

A

approximately 12 waves per minute in the duodenum; approximately 8 waves per minute in the terminal ileum – tends to drive luminal contents in the aboral direction

40
Q

what is the BER frequency in the large intestine?

A

approximately 8 waves per minute in the proximal colon, approximately 16 waves per minute in the distal (sigmoid) colon – favours retention of luminal contents facilitating reabsorption of water and electrolytes

41
Q

what is the enteric nervous system?

A

The ‘little brain of the gut’ - about 100 million neurones, cell bodies mostly located in ganglia connected by fibre tracts within:

  • Myenteric plexus
  • Submucous plexus
42
Q

what does myenteric plexus regulate?

A

motility and sphincters

43
Q

what does submucous plexus regulate?

A

epithelia and blood vessels

44
Q

where is the enteric nervous system found?

A

Is intrinsic to G.I. tissue

45
Q

how does the enteric nervous system operate?

A

Can operate independently, but hormones and extrinsic nerves exert a strong regulatory influence

46
Q

how does it co-ordinate muscular , secretive and absorptive activities?

A
  • Sensory neurones (mechanoreceptors, chemoreceptors, thermoreceptors)
  • Interneurones (the majority, co-ordinating reflexes and motor programs)
  • Effector neurones (excitatory and inhibitory motor neurones supplying both smooth muscle layers, secretory epithelium, endocrine cells and blood vessels)
47
Q

How does the parasympathetic innervate the GI tract?

A

Preganglionic fibres (releasing ACh) synapse with ganglion cells (in essence post-ganglionic neurones) within the ENS

48
Q

what are the excitatory influences of the parasympathetic innervation?

A

Increased gastric, pancreatic and small intestinal secretion, blood flow and smooth muscle contraction

49
Q

what are the inhibitory influences of the parasympathetic innervation?

A

Relaxation of some sphincters, receptive relaxation of stomach

50
Q

How does the sympathetic innervate the GI tract?

A

Preganglionic fibres (releasing ACh) synapse in the prevertebral ganglia. Postganglionic fibres (releasing NA) innervate mainly enteric neurones, but also other structures

51
Q

what are the excitatory influences of the sympathetic innervation?

A

Increased sphincter tone

52
Q

what are the inhibitory influences of the sympathetic innervation?

A

Decreased motility, secretion and blood flow

53
Q

what 3 neurones are involved in nerve reflexes?

A
  • sensory neurone
  • interneurone
  • effector neurone
  • see slide for photo
54
Q

what is an example of a local reflex?

A

peristalsis

55
Q

what is an example of a short reflex?

A

intestino-intestinal inhibitory reflex (local distension activates sensory neurones exciting sympathetic pre-ganglionic fibres that cause inhibition of muscle activity in adjacent areas)

56
Q

what is an example of a long reflex?

A

gastroileal reflex (increase in gastric activity causes increased propulsive activity in the terminal ileum)

57
Q

what is peristalsis?

A

a wave of relaxation, followed by contraction, that normally proceeds along the gut in an aboral direction – triggered by distension of the gut wall

58
Q

when GI tract walls detect distension from an abolus , what occurs on the oral side (propulsive segment)?

A

Distension activates sensory neurones > altered activity of interneurones > altered activity of motor neurones > longitudinal muscles relax (release NO/VIP) > circular muscles contracts (release of ACh)

59
Q

when GI tract walls detect distension from an abolus , what occurs on the aboral side (receiving segment)?

A

Distension activates sensory neurones > altered activity of interneurones > altered activity of motor neurones > longitudinal muscles contract (release of ACh) > circular muscles relaxes (release of NO/VIP)

60
Q

How is both NO/VIP and ACh released?

A
  • release of VIP and NO from inhibitory motoneurone

- release of ACh and substance P from excitatory motoneurone

61
Q

what is segmentation?

A

rhythmic contractions of the circular muscle layer that mix and divide luminal contents

62
Q

where does segmentation occur?

A
  • small intestine (in fed state)

- large intesine ( haustration)

63
Q

what is colonic mass movement?

A

powerful sweeping contraction that forces faeces into the rectum – occurs a few times a day

64
Q

what is migrating motor complex (MMC)?

A

powerful sweeping contraction from stomach to terminal ileum

65
Q

what are the 2 tonic contractions?

A

Low pressure - organs with a major storage function (e.g. stomach)
High pressure - sphincters

66
Q

what are sphincters and how many are there?

A
  • composed of specialised circular, generally smooth muscle

- 6 in total ( not sphincter of Oddi)

67
Q

how to sphincters work?

A

Act as essentially as one way valves by maintaining a positive resting pressure relative to two adjacent structures (e.g. oesophagus and stomach)

68
Q

what causes opening and closing of sphincters?

A

opening- proximal stimuli

closing - distal stimuli

69
Q

what are the 6 sphincters?

A
  • upper oesophageal sphincter
  • lower oesophageal sphncter
  • Pyloric sphincter
  • Ileocaecal valve
  • internal anal sphincters
  • external anal sphincters
70
Q

what function does the upper oesophageal sphincter have?

A

i) relaxes to allow swallowing

(ii) closes during inspiration

71
Q

what function does the lower oesophageal sphincter have?

A

(i) relaxes to permit entry of food to the stomach

(ii) closes to prevent reflux of gastric contents to the oesophagus

72
Q

what function does the pyloric sphincter have?

A

(i) regulates gastric emptying

(ii) usually prevents duodenal gastric reflux

73
Q

what function does the Ileocacecal valve have?

A

regulates flow from ileum to caecum (i) distension of ileum opens, distension of proximal colon closes

74
Q

what regulates internal and external anal sphincters?

A

defaecation reflex

Z-GI (5 decks)

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