L7 - Anatomy of GI tract Flashcards
Signals that regulate Hunger and Satiety
- Plasma glucose levels
- Hormones - leptin, ghrelin, insulin, glucagon
- Stretch receptors in GI tract
- Stress
- Body temperature
- Food palatability
What happens in absorptive state (well fed)?
- Ingested nutrients enter blood from GI tract
- Absorbed carbs converted to glucose by liver
- Glucose enter cells and catabolised to CO2 + H2O - energy released (ATP formation)
AFTER MEAL:
- ↑ blood glucose levels
- ↑ storage of fuels
- protein synthesis + glycogen synthesis ( storage)
- Insulin secretion, hence ↑ glucose uptake + metabolism in cells — causes ↓ blood glucose levels
What happens in post-absorptive state?
- Glycogen + triglycerides + proteins – net synthesis stops and net catabolism starts
- ↓ blood glucose as no more absorbed from GI tract
LEADS TO..
- Glycogenolysis (in liver) –> hydrolysis of glycogen into glucose (via glucagon enzyme)
- Gluconeogenesis –> creation of new glucose from amino acids
- Glucose-sparing:
–> lipolysis (fat utilisation) - when ↑glycerol + free fatty acids in blood
–> ↓ glucose catabolism - spared for use of nervous system
–> free fatty acids covered to ketones as source of energy during prolonged fasting.
How is Hunger signalled in the body? (Regulation of food intake)
- When:
- ↓ blood glucose, fat, protein
LEADS TO:
- ↑ ghrelin => hormone produced by enter-endocrine cells in GI tract
- Leads to stimulation of neuropeptide Y (NPY) and other neuropeptides in the Appetite Centre of lateral hypothalamus
- Causes ↑ hunger => leads to food intake
- NEUROPEPTIDE Y (NPY) ==> hypothalamic neurotransmitter that stimulates appetite + hunger
How is Satiety signalled in the body (stopping food intake)
When:
- ↑ blood glucose, fat, protein
- ↓ ghrelin
LEADS TO:
- ↑ leptin => hormone synthesised by adipocytes (adipose tissues)
- Leads to inhibition of NPY neurotransmitter in Satiety Centre of medial hypothalamus
- Causes ↓ hunger => ↓ food intake
- Satiety also stimulated by gastric emptying in GI tract
- NEUROPEPTIDE Y (NPY) ==> hypothalamic neurotransmitter that stimulates appetite + hunger
Effect of meal size and fat content on gastric emptying
- Different meal components = different rates of gastric emptying
- More food = longer digestive phase
- Nutrition density (calories) –> slows gastric emptying
- Fat content = delaying factor in gastric emptying:
-> fat in duodenum causes fungus to relax - lowering intragastric pressure
-> ↑ feeling of fullness for longer
-> prolongs elevation of pH in stomach
-> slows oral medications - delay of drug entering small intestine
What is the basic structure of the GI Tract? (know how to label a histology diagram)
IN TO OUT:
- Lumen
- Mucosa:
- Epithelium - enterocytes in villus + microvilli structures - larger surf area for absorption - Submucosa:
- Secretory glands
- Submucous plexus (Meissner’s plexus) - Muscularis externa:
- Circular muscles
- Longitudinal muscles
- Myenteric plexus (Auerbach’s plexus) - Serosa + Mesentery
- same basic structure along the length of the GI tract
What is the mesenteric plexus responsible for? (aka Auerbach’s plexus)
- Part of the Enteric Nervous System (ENS)
- Located in between outer longitudinal + inner circular smooth muscle layers of GI tract
- Controls motility of GI tract - influencing muscle activity
- Neurotransmitters involved:
Acetylcholine (ACh) - excitatory
Nitrogen oxide (NO) -inhibitory
What is the submucosal plexus responsible for? (Meissner’s plexus)
- Part of the Enteric Nervous System (ENS)
- Located within the gut wall in submucosa
- It is a local circuitry of neutrons + ganglia
- Largely sensory - receiving signals from epithelium + street receptors - influencing secretory activity
- Functions:
-> Regulates configuration of luminal surface
-> Controls glandular secretions
-> Alters electrolytes + water transport
-> Regulates local blood flow - Hormonal control via motilin hormone
Neuronal control of the GI tract
- CNS - brain + spinal chord
- PNS - connects CNS to limbs + organs
- Autonomic (ANS) - division of PNS influencing function of organs
- Enteric (ENS) - division of ANS which controls functions of GI tract
- ENS composed of 2 nerve plexuses:
- myenteric plexus
- submucosal plexus
- CNS and ENS connected via vagus nerve – largest parasympathetic nerve in the nervous system.
The components of the nervous system (branches)
Hierarchy (descending order):
Nervous system:
- CNS + PNS
CNS - brain + spinal chord
PNS - Efferent (motor)
- Afferent (sensory)
- Efferent:
- Autonomic NS (involuntary)
- Somatic NS (voluntary)
- ANS:
- Parasympathetic (rest & digest)
- Sympathetic (fight/flight response)
- Enteric (ENS) - GI tract
Afferent neurons
- aka ‘Sensory receptor neurons’
- Carry nerve impulses away from the receptors or sense organs towards the CNS
- Long neural reflexes from receptors in GI tract to CNS via afferent nerves
- Innervation of ENS by nerve plexuses linked to CNS via afferent fibres activated by:
- stretch receptors
- chemical stimulation
Efferent neurons
- aka ‘motor’ or ‘effector neurons’
- Carry nerve impulses away from CNS to effectors, such as muscle/glands
- Long neuronal reflexes from receptors in CNS to nerve plexuses + effector cells in GI tract via efferent fibres
- ENS receives efferent innervation from ANS:
- parasympathetic input - stimulates gut motility + secretory activity (rest&digest)
+ sympathetic input - causes presynaptic inhibition of parasympathetic-induced contractions (fight/flight)
