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What is the digestive process?

- 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


How do the details of digestion vary between species?

- 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


What neurons are involved in digestion?

- 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


What happens when we see or smell food?

- 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


How is the mouth controlled?

- 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


How is the oesophagus controlled?

- 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


How is the stomach controlled?

- 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


How are the small and large intestines involved in digestion?

- 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


What is the enteric nervous system?

- 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


What does the ENS contain?

'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


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

very complex

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