Functions of the GI tract

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Functions of the GI tract
Motility - mechanical breakdown of food,
propulsion of food through gut
1. Ingestion
2. Digestion
Secretion - secretion of enzymes, water ions
3. Absorption
Control - of motility and secretion by nervous
system and hormones
4. Egestion
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Myenteric plexus (Auerbachs)
Submucosal plexus (Meissners)
Longitudinal

• circular

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Properties of GI smooth muscle
Form hollow tubes ? not contracting against
skeleton Form a syncitium - electrically coupled,
joined by gap junctions ? contractions
synchronous Actinmyosin ratio 151 (skeletal
muscle 21) Contractile elements not arranged in
sarcomeres?not striated Stimulated by
neurotransmitter released from varicosities Have
slow wave activity
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Membrane potential (mV)
Acetylcholine
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Slow waves in GI smooth muscle

• slow waves are changes in resting membrane
  potential (Em)
• 3-12 cycles per minute depending on area of GI
  tract - 3/min in stomach, 12/min small
  intestine
• always present but do not always cause
  contractions
• Frequency of contractions dictated by frequency
  of slow waves
• slow wave frequency and height modulated by -
• body temp metabolic activity
• intrinsic extrinsic nerves (increased by Ach,
  SP decreased by noradrenaline, NO, VIP),
• circulating hormones (esp CCK, motilin)

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Contraction of GI smooth muscle
Calcium activates contraction Depends on influx
of calcium from extracellular space through
calcium channels Calcium- calmodulin complex
activates myosin light chain kinase Contraction
explained by sliding filament theory
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Innervation of the GI tract
1. Intrinsic nerve plexuses
Located in the submucosa (submucosal or
Meissners plexus) and between circular and
longitudinal muscle layers (myenteric or
Auerbachs plexus)
Excitatory - Acetylcholine, Substance
P Inhibitory - VIP, nitric oxide
Control Motility - Myenteric plexus Secretion -
Submucosal plexus through release of
neurotransmitters
Excitatory - Acetylcholine
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Innervation of the GI tract
2. Extrinsic nerves
Parasympathetic innervation - via preganglionic
fibres in vagus and pelvic nerves - synapse on
ganglionic neurons in enteric nervous system -
excitatory through release of acetylcholine
Sympathetic nervous system - postganglionic
fibres from coeliac, superior and inferior
mesenteric ganglia - inhibitory through release
of noradrenaline
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Parasympathetic n.s
Sympathetic n.s.
CNS
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Effect of different stimuli on muscle contraction
Stimulus
Effect on muscle
more depolarised smooth muscle, ? more excitable.
Leads to action potential generation and smooth
muscle contraction
1. Stretch of GI tract wall 2. Acetylcholine
release 3. Parasympathetic stimulation
more hyperpolarised smooth muscle, ? Less
excitable and fewer contractions
4. Noradrenaline release 5. Sympathetic
stimulation
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The splanchnic circulation

• Large blood volume (30 cardiac output)
• Large reservoir - holds 30 blood volume
  under normal conditions

Blood flow through splanchnic circulation
controlled by many factors 1. Local nervous
system (enteric NS) causes post-prandial
vasodilation through VIP release. 2.
Parasympathetic stimulation causes
vasodilation 3. Sympathetic stimulation on
exercise adrenaline release during
haemorrhage 4. Local hormone release causes
vasodilation gastrin and secretin adenosine
released when local pO2 decreases
cause vasoconstriction