Upper GIT

Question 1

Describe the enteric nervous system, its components and functions

Answer 1

• ~ 100,000,000 neurons
• Myenteric: Primarily motor control, throughout GIT
• Submucosal: Primarily secretion control
  
  • Only in small and large intestines
• ENS is a complete reflex circuit (No need for spinal cord or brain input)
• However, the brain can modify several GI functions

Question 2

Describe sympathetica and parasympathetic innervation fo the system, and discuss interactions with the ENS

Answer 2

• Sympathetic:
  
  • Reduces blood flow to GI tract
  • Inhibits non-sphincter muscles
  • Activates sphincter muscles
• Parasympathetic:
  
  • Vagus nerve - up to the proximal colon
  • Pelvic nerves - distal third of colon
  • increased Secretion, Motility
  • Vagovagal reflex
• myenteric and submucosal plexuses interact with each other
• sensors in mucosa send sensory information
• PSY and SY primarily interact with enteric nervous system

Question 3

Lis tand brieflydescribe the neurotransmitters of the ENS

Answer 3

• Acetylcholine
  
  • 1° neurotransmitter (pre- and post-ganglion)
  • Regulates secretory and smooth-muscle activity
• Vasoactive intestinal peptide (VIP)
  
  • Inhibits GI smooth muscle
  • Stimulates fluid & electrolyte secretion
• Others: somatostatin, serotonin, nitric oxide (NO), etc

Question 4

Describe the musculature, motor activity and types of contractions in the GIT

Answer 4

• Musculature: Two layers of smooth muscles, circular and longitudinal
• Motor Activity: Peristaltic contractions (propulsive), Segmental contraction (mixes food, no propulsion), Sphincter control (retains contents in stomach and large intestine)
  
  • all are achieved by smooth muscles
• Tonic and Rhythmic Contractions:
  
  • Tonic: Sustained contractions
  • Rhythmic: Alternating contraction and relaxation - slow wave activity; cycles of depolarisation and repolarisation
    
    • Function of Vm: Intrinsic slow-wave activity (-65 to -45 mV), Action Potential (increase in muscle tension)
    • key channels in slow wave: VGCC(K moves out – hyperpolarisation – closes VGCC – Ca decreases – K channels close, depolarising, VGCC opens, and K channel (iCa-dependent)

Question 5

Briefly describe mastication and swallowing

Answer 5

• Mastication (Chewing):
  
  • Breaks up large pieces of food
  • Mixes food particles with saliva to form a bolus, to facilitate swallowing and digestion
  • Optimal number of chews: 20 - 25
• Swallowing:
  • Note osophageal sphincter closes, epiglottis goes up, uvula goes down to ensure food goes the right way
• Primary peristalsis initiated by act of swallowing
  
  • Secondary peristalsis initiated by distension of oesophagus - flush out any potential reflux
  • On average, bolus takes 10 s to reach stomach

Question 6

Distinguish and describe the UOS and LOS

Answer 6

• Upper Esophageal Sphincter:
  
  • Striated muscle - potential to gain control
  • Highest resting pressure of all sphincters in GI tract
  • Under control of swallowing centre in medulla

Lower Esophageal Sphincter
- Smooth muscle
- Opens when swallowing begins - synchronous
- Tonically active (prevents reflux) but relaxes upon swallowing
- Under control of the vagus nerve (contract) and intrinsic properties of smooth muscle (release of NO and VIP causes relaxation).

Question 7

Describe the actions of the sphincters in swallowing

Answer 7

• Contraction is strongest and fastest during oropharyneal phase
• UES and LES relax simultaneously
• LES contracts after UES contraction i.e. stays relaxed until food has passed
• Vomiting - anti-peristalsis

Question 8

List the functions of the stomach

Answer 8

• Motor function:
  
  • Regulate food intake
  • Mixing with secretions
  • Reduction in particle size by grinding
  • Empties chyme duodenum
    
    • Liquids - proximal part
    • Solids - antral part

Question 9

List the secreted substances of the stomach

Answer 9

• Acid
  
  • Pepsinogen
  • Mucus
  • Bicarbonate
  • Intrinsic factor
  • H2O

Question 10

Describe gastric filling (in terms of relaxations)

Answer 10

• Three types of stomach relaxations:
  
  1. Receptive relaxation - triggered by swallowing
  2. Adaptive relaxation - distension (filling)
  3. Feedback relaxation - triggered by nutrients in small intestine - ensure all food making its way out
• ENS is the primary regulator of gastric accommodation
• Vagus nerve plays a modulatory role
• note: Increase in intragastric volume without increase in pressure - accommodates more food
  
  • as pressure builds: satiety
  • hence Gastric Banding

Question 11

Describe gastric emptying, which foods take least/most time, and the phases of emptying

Answer 11

• Saline and water empty first
  
  • Acidic and caloric fluids more slowly
  • Fatty material even slower
  • Solids must be reduced to less than ~ 2 mm - hence lag for solids
• Three distinct phases: Propulsion, Grinding, Retropulsion
  
  • propulsion: gastric contractions propel food towards antrum (initiated by gastric pacemaker). Pyloric valve is closed
  • grinding: chyme is churned in the antrum by propulsion. Pyloric valve is closed - helps to slam food, breaking down particles
  • retropulsion: most of chyme will not go through and is returned to stomach. Pyloric valve is slightly open
  • process cycles

Question 12

What factors can delay gastric emptying?

Answer 12

• Volume of liquid in stomach is of primary importance (emptying rate proportional to volume i.e. bigger volume quicker empty)
• Receptors in small intestine and duodenum monitor the fluid leaving the stomach.
  
  • Low pH
  • High caloric content
  • Lipid
  • Some amino acids
  • Osmolarity
• all of the above Trigger a delay in gastric emptying via neural & hormonal signals:
• This happens by: Fundic relaxation, Decrease in antral motor activity, Contraction of pyloric sphincter, Altered intestinal motor activity
• via:
• • Vagus nerve
• Secretin
• Cholecystokinin
• Gastric inhibitory peptide

Question 13

Describe teh hormonal control of motility

Answer 13

• Secretin: secreted by S cells in mucosa of duodenum in response to pH, Inhibitory effect on motility of most of GI tract, Stimulates secretion of bicarb from pancreas and liver.
• Cholecystokinin (CCK): Secreted by I cells in mucosa of duodenum and jejunum mainly in response to fat breakdown products, Moderately inhibits stomach motility, Potently increases contraction of gallbladder.
• Gastric inhibitory peptide (GIP): Secreted by K cells in mucosa of the proximal small intestine in response to macromolecules i.e.. fatty acids, AAs, and carbohydrates; Moderately inhibits stomach motility, Prevents overloading of small intestine.