lecture 11 Flashcards

Neural control of digestion - a tale of multiple interacting systems and hierarchies - roles of different regions of digestive tract - components of digestive process regulated somatic NS and brain: where is control conscious - components of digestive process regulated by ENS: types of neurons in ENS and their connections

1
Q

What is the digestive process?

A
  • food taken into mouth
  • broken down into ever smaller components
  • separated chemically (and physically) into different nutrients that can be absorbed
  • e.g. fat will be on the surface - separates out due to different density in the stomach - has importance in different areas of digestion e.g. sense of satiation
  • biproducts transported along the intestinal tract for further processing and recovery of key materials
  • waste products excreted
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2
Q

How do the details of digestion vary between species?

A
  • digestion involves enzymatic breakdown of foods into components:
    • proteins to amino acids
    • sugars and starches into simple sugars e.g. glucose and fructose
    • lipids into fatty acids and glycerol
  • many species also use bacterial fermentation to break down cellulose – short chain fatty acids, sugars
    • in humans SFAs play important role in energy metabolism of mucosa in the ascending colon
  • animals with active lifestyles tend to depend on foods high in simple sugars and protein
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3
Q

What neurons are involved in digestion?

A
  • both somatic and visceral neurons are involved
  • somatic (voluntary) nervous system – chewing, swallowing, peristalsis in oesophagus and opening key sphincters
  • visceral neurons involved in salivation, primary peristalsis, all functions from stomach to anus
  • visceral neurons include sympathetic and parasympathetic nervous systems, many types of visceral afferents and the Enteric Nervous System
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4
Q

What happens when we see or smell food?

A
  • initiates the cephalic phase of digestion
    • salivation, gastric relaxation, gastric acid secretion: via parasympathetic NS (vagus) in particular
    • no food needs to be ingested
    • more palatable the food, the greater the response: i.e. requires memory of previous meals
    • depends on hunger and appetite: partly determined by nature of food previously eaten
  • food in mouth gives same behaviours
  • pavlov’s dog the first example of this
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5
Q

How is the mouth controlled?

A
  • chewing depends on motor pattern generator in brain stem
    • clearly under voluntary control, thus cortical involvement
    • sucking essential for early survival – seems to have hedonistic component
  • tongue again a voluntary set of muscles controlled via motor cortex
  • swallowing another voluntary process
    • motor pattern generator in the brain stem
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6
Q

How is the oesophagus controlled?

A
  • primary peristalsis controlled by CNS via vagus
    • in humans much of oesophagus is striated muscle
  • enteric nervous system also has a role
    • secondary peristalsis
  • upper and lower oesophageal sphincters important to regulate reflux
    • one major human disease – achalasia
    • important part of normal function in ruminants like cows, sheep, antelope, giraffes
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7
Q

How is the stomach controlled?

A
  • major peripheral control network under substantial central regulation via vagus
    • lesser role for sympathetic nervous system
  • also has important control system via pacemaker cells in antrum
    • interstitial cells of Cajal (ICC)
  • enteric nervous system less studied than vagal control systems
    • may act like a parasympathetic ganglion with some autonomy
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8
Q

How are the small and large intestines involved in digestion?

A
  • where the action is:
    • digestion (duodenum, jejenum)
    • absorption (nutrient: duodenum, jejenum; water: ileum, colon)
    • propulsion (whole length)
  • 6m small intestine and 1m large intestine in humans
    • guinea-pigs about 1m for small intestine
  • chemical refinery
    • mixing food (converted to slurry by stomach) with enzymes and water
    • neutralising acid
    • allowing for absorption
    • recovering reactants
    • disposing of waste products
  • control largely peripheral - enteric nervous system
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9
Q

What is the enteric nervous system?

A
  • contained entirely within the intestinal wall and running its full length
    • GI tract only organ with its own complete nervous system
  • contains more neuron cell bodies than the spinal cord (about 300,000,000 in humans)
  • can operate without CNS, but modulated by input from brain and via sympathetic nervous system
  • two rings that are two arrays of neurons from the back of the mouth through to the anus
  • pretty independent after the stomach
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10
Q

What does the ENS contain?

A

‘the little brain’

  • all neurons needed for complex behaviours like mixing and propulsion
    • intrinsic sensory neurons
    • excitatory and inhibitory motor neurons
    • interneurons
    • secretion and absorption of water and salt also requires secretomotor neurons
    • also directly regulates part of the nutrient absorptive process
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11
Q

What is a recent version of the neural circuit responsible for control of intestinal motility? What is an interesting detail?

A

very complex

  • recurrent excitatory circuit of intrinsic sensory neurons
  • requires intrinsic inhibition to prevent spasm
  • other recurrent excitatory circuits are also present
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