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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
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
“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
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
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
• 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
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
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
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
Organisation of the gut wall
epithelium
Lamina propria
Muscularis mucosa
SUBMUCOSA
Lumen
MUCOSA
Mesothelium (SEROSA)
Myenteric plexus
Longitudinal muscle
Circular muscle
MUSCULARIS
PROPRIA
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)
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
~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.
Alternate contraction and relaxation of adjacent
segments causes thorough mixing of contents
Intestinal movement - segmentation
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/
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
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)