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Introduction to structure Flashcards

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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
what are adjacent smooth muscle cells coupled by and what does this allow?
Gap junctions -allow spread of electrical currents from cell to cell forming a functional syncytium
26
what is a synchronous wave?
when hundreds of cells are depolarised and contract at the same time i.e single unit smooth muscle
27
what drives spontaneous activity across the syncytium ?
specialised pacemaker cells
28
what modulates the spontaneous activity produced by pacemaker cells?
- Intrinsic (enteric) and extrinsic (autonomic) nerves | - Numerous hormones
29
what spontaneous electrical activity occurs in the stomach, small intestine and large intestine?
occurs as slow waves - rhythmic patterns of membrane depolarization and repolarization that spread from cell to cell via gap junctions
30
what does the slow wave electrical activity determine?
max. frequency, direction and velocity of rhythmic contractions
31
what drives slow wave electrical activity?
the interstitial cells of Cajal (ICCs) - pacemaker cells interspersed between the far more numerous smooth muscle cells (SMCs)
32
when does contraction occur?
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
what is the force of contractions related to?
number of action potentials discharged
34
where are ICCs located?
between the longitudinal and circular muscle layers and in the submucosa
35
what is the basic electrical rhythm determined by?
the slow waves
36
what determines if the slow wave amplitude reaches threshold?
- Neuronal stimuli - Hormonal stimuli - Mechanical stimuli
37
what do these 3 stimulus do?
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
what is the BER frequency in the stomach?
3 slow waves per minute
39
what is the BER frequency in the small intestine?
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
what is the BER frequency in the large intestine?
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
what is the enteric nervous system?
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
what does myenteric plexus regulate?
motility and sphincters
43
what does submucous plexus regulate?
epithelia and blood vessels
44
where is the enteric nervous system found?
Is intrinsic to G.I. tissue
45
how does the enteric nervous system operate?
Can operate independently, but hormones and extrinsic nerves exert a strong regulatory influence
46
how does it co-ordinate muscular , secretive and absorptive activities?
- 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
How does the parasympathetic innervate the GI tract?
Preganglionic fibres (releasing ACh) synapse with ganglion cells (in essence post-ganglionic neurones) within the ENS
48
what are the excitatory influences of the parasympathetic innervation?
Increased gastric, pancreatic and small intestinal secretion, blood flow and smooth muscle contraction
49
what are the inhibitory influences of the parasympathetic innervation?
Relaxation of some sphincters, receptive relaxation of stomach
50
How does the sympathetic innervate the GI tract?
Preganglionic fibres (releasing ACh) synapse in the prevertebral ganglia. Postganglionic fibres (releasing NA) innervate mainly enteric neurones, but also other structures
51
what are the excitatory influences of the sympathetic innervation?
Increased sphincter tone
52
what are the inhibitory influences of the sympathetic innervation?
Decreased motility, secretion and blood flow
53
what 3 neurones are involved in nerve reflexes?
- sensory neurone - interneurone - effector neurone - see slide for photo
54
what is an example of a local reflex?
peristalsis
55
what is an example of a short reflex?
intestino-intestinal inhibitory reflex (local distension activates sensory neurones exciting sympathetic pre-ganglionic fibres that cause inhibition of muscle activity in adjacent areas)
56
what is an example of a long reflex?
gastroileal reflex (increase in gastric activity causes increased propulsive activity in the terminal ileum)
57
what is peristalsis?
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
when GI tract walls detect distension from an abolus , what occurs on the oral side (propulsive segment)?
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
when GI tract walls detect distension from an abolus , what occurs on the aboral side (receiving segment)?
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
How is both NO/VIP and ACh released?
- release of VIP and NO from inhibitory motoneurone | - release of ACh and substance P from excitatory motoneurone
61
what is segmentation?
rhythmic contractions of the circular muscle layer that mix and divide luminal contents
62
where does segmentation occur?
- small intestine (in fed state) | - large intesine ( haustration)
63
what is colonic mass movement?
powerful sweeping contraction that forces faeces into the rectum – occurs a few times a day
64
what is migrating motor complex (MMC)?
powerful sweeping contraction from stomach to terminal ileum
65
what are the 2 tonic contractions?
Low pressure - organs with a major storage function (e.g. stomach) High pressure - sphincters
66
what are sphincters and how many are there?
- composed of specialised circular, generally smooth muscle | - 6 in total ( not sphincter of Oddi)
67
how to sphincters work?
Act as essentially as one way valves by maintaining a positive resting pressure relative to two adjacent structures (e.g. oesophagus and stomach)
68
what causes opening and closing of sphincters?
opening- proximal stimuli | closing - distal stimuli
69
what are the 6 sphincters?
- upper oesophageal sphincter - lower oesophageal sphncter - Pyloric sphincter - Ileocaecal valve - internal anal sphincters - external anal sphincters
70
what function does the upper oesophageal sphincter have?
i) relaxes to allow swallowing | (ii) closes during inspiration
71
what function does the lower oesophageal sphincter have?
(i) relaxes to permit entry of food to the stomach | (ii) closes to prevent reflux of gastric contents to the oesophagus
72
what function does the pyloric sphincter have?
(i) regulates gastric emptying | (ii) usually prevents duodenal gastric reflux
73
what function does the Ileocacecal valve have?
regulates flow from ileum to caecum (i) distension of ileum opens, distension of proximal colon closes
74
what regulates internal and external anal sphincters?
defaecation reflex

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