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
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
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
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
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
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
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
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
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
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
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