lecture 11

Question 1

What is the digestive process?

Answer 1

• 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

Question 2

How do the details of digestion vary between species?

Answer 2

• 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

Question 3

What neurons are involved in digestion?

Answer 3

• 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

Question 4

What happens when we see or smell food?

Answer 4

• 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

Question 5

How is the mouth controlled?

Answer 5

• 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

Question 6

How is the oesophagus controlled?

Answer 6

• 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

Question 7

How is the stomach controlled?

Answer 7

• 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

Question 8

How are the small and large intestines involved in digestion?

Answer 8

• 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

Question 9

What is the enteric nervous system?

Answer 9

• 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

Question 10

What does the ENS contain?

Answer 10

‘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

Question 11

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

Answer 11

very complex

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