overview of digestionbjcampbl/lecture 1 - introduction to digesti… · digestion. schematic...
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School of Medicine @UoLmedicine
http://pcwww.liv.ac.uk/~bjcampbl/Digestion.htm
Cellular & Molecular Physiology, Institute of Translational Medicine
Prof. Barry Campbell
Overview of Digestion
Year 1 MBChB –
Gastrointestinal system
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Learning Outcomes:
➢ Describe the function of the regions of gut tube (i.e. from mouth to anus) and its associated secretory organs
➢ Differentiate between involuntary and voluntary muscle control and the context to gastrointestinal (GI) tract motility (e.g. swallowing, peristalsis, defecation)
➢ Define neural, endocrine and paracrine control of the gut➢ Define the motility patterns in fasted and fed states of the GI
tract
➢ Define motility in the small and large intestine (peristalsis, haustrations, mass movements).
➢ Define how we remove indigestible matter (anatomy of the ano-rectum, defecation and external anal sphincter control)
➢ Illustrate function versus dysfunction in the GI tract using clinical examples
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“The progressive breakdown of food into a form
suitable for absorption and the associated transport
processes”
Digestion therefore also includes: -
• The processes of secretion
• The processes of absorption
• Movement of the gut contents
• growth & differentiation
• The mechanisms protecting the gut from damage or attack, and
• the mechanisms controlling and integrating all of the above
DIGESTION
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SCHEMATIC REPRESENTATION OF
MAIN REGIONS OF THE GASTROINTESTINAL TRACT
Oesophagus
Stomach
duodenum
jejunum
ileum
Colon
rectum & anus
FUNCTION:
- fat, protein, carbohydrate
digestion & absorption, Ca2+/Fe2+
- water and electrolyte transport
- bile salt & vit B12 transport
REGION:
Small intestine
Large intestine
- Transit
- Storage
- water and electrolyte transport
- defaecation
- storage, H+/peptic digestion
& intrinsic factor
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Salivary glands -
synthesis/secretion:
amylase, mucus.
Water, electrolytes
Exocrine pancreas -
synthesis/secretion:
proteases,lipase,amylase.
HCO3- , water
Liver -
bile salt synthesis,
bile secretion.
Gall bladder -
storage and concentration of bile
SECRETIONS OF THE GUT
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• Endocrine - Gut hormones
• Paracrine - Local regulators
• Neural
Intrinsic - Myenteric & submucosal nerve plexuses
Extrinsic - Afferent & efferent nerves
vagal & splanchnic trunks (autonomic nervous system)
Physiological control systems in the gut
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Major hormones of the gut
STOMACH
Gastrin
Somatostatin
UPPER SMALL INTESTINE
Cholecystokinin (CCK)
Secretin
Gastric inhibitory peptide
Motilin
ILEUM AND COLON
Glucagon-like peptide 1 (GLP-1)
Peptide YY (PYY)
Neurotensin
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Overview of extrinsic innervation:
Voluntary (Conscious control)
Hypoglossal
nerve (XII)
Pudendal
nerve
Pelvic
splanchnic
nerve
Vagus
nerve (X)LOS
UOS
IASEAS
Autonomic(non-perception)
Superior
cervical
ganglion
Paraspinal cord
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The autonomic nervous systemSympathetic system:
Noradrenaline
Gut secretions (+) e.g acid, enzymes
& electrolytes
Sp
inal c
ord
Pons/medulla
Midbrain
Gut sphincters (-) reflex relaxation
Pancreas (+) exocrine &
endocrine secretion
ACTION
Rectum (+) defaecation
ACTION
Gut wall (+) increased
motility and tone
adrenaline
EFFECTSEFFECTS
Salivary glands (+) secretion of saliva
X IX
VII
Cranial
nerves
Parasympathetic system:
Acetylcholine (Ach)
(+) secretion of saliva salivary
glands
(+) vasoconstriction gut blood
2 (-) vasodilation vessels
1/2 (-) decrease motility gut wall,
(+) contraction sphincters
(+) secretion
Adrenal
medulla
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Organisation of the gut wall
epithelium
Lamina propria
Muscularis mucosa
SUBMUCOSA
Lumen
MUCOSA
Mesothelium (SEROSA)
Myenteric plexus
Longitudinal muscle
Circular muscle
MUSCULARIS
PROPRIA
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Intrinsic and extrinsic nerves of the digestive tract
Visceral
afferents
Parasympathetic
efferents
e.g. Stretch & chemosensitive
neurons
ABORAL ORALe.g. secretory cell
e.g. post-ganglionic non-
adrenergic or cholinergic fibres
INTRINSIC
NERVES
EXTRINSIC
NERVES
Submucosal plexus
Myenteric plexus
(Auerbach’s)
(Meissner’s)
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Action potentials
Mechanical recording1.5g
25 mV
BER
Control of smooth muscle contraction –
Interstitial cells of Cajal are pacemakers of the gut
Basal electrical rhythm (BER) originates in ICC, but itself does not cause
contraction; when spike potentials occur at maximum depolarisation of BER due to
Ca2+entry, the result is contraction
ICC cell network
L type Ca2+ current & action potential mechanism
Slow wave conducted to smooth muscle
Slow wave mechanism
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~130 min
antrum
duodenum
Distal small intestine
Colon
Motility in the interdigestive period -
The Migrating Myoelectic Complex (MMC)
III – motilin, ghrelin
& vagus nerve
III – somatostatin,
Serotonin (5-HT)
and xenin
Feeding
disrupts the
cycle
71%
29%
Phases of MMC
I
II
III
IV
I - quiescence; II - random contractions; III - burst of contractions (max.
amplitude & duration); IV - rapid decrease of contractions.
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Alternate contraction and relaxation of adjacent
segments causes thorough mixing of contents
Intestinal movement - segmentation
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Intestinal movement - peristaltic reflex
Descending relaxationAscending contraction
Orad Caudad
Circular smooth muscle
VIP/NO
Inhibitory motor
neurons
Ach = acetylcholineNO = nitric oxideTK = tachykininVIP = vasoactive intestinal
polypeptide5-HT = 5-hydroxytryptamine
(serotonin)
ACh/TK
ACh AChIntrinsic primaryafferent neurons(IPAN)
interneuronsinterneurons
Excitatory motor
neurons
TK/ACh>
>
> >
>>
Distension(5-HT)
Animation at www.westga.edu/~lkral/peristalsis/
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Removing indigestible material –
reflexes in the colon and rectum
Mass movements+
+
Food in stomach
Food in
duodenum
Faeces
Defaecation reflex
Colonrectum
anus
Ach
Sp
inal c
ord
IAS
EAS
FAECES
+Distention
-
VIP
ATP
+
-Pudendal
nerve
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Function & Dysfunction in the GI tract
Physiology Pathology
• Growth/development
• secretion
• absorption
• motility & signalling to
CNS
• surveillance (immuno/metabolic)
• co-ordination (neurons/hormones)
➢ cancer
➢ peptic ulcer, cystic fibrosis
➢ malabsorption
➢ oesophagitis, gastroparesis,
non-ulcer dyspepsia and
irritable bowel
➢ ulcerative colitis, Crohn’s
disease, Coeliac disease
➢ aganglionic colon
(Hirschsprung’s)