Neuronal communication I Flashcards
debate btw:
- reticulists
- neuronists
debate: reticulists
- Golgi
- said nervous system was diffuse network of continuous tissue (reticulum) from fused branches of dendrites/ axons
debate: neuronists
- Cajal
- neurons were discreet elements
- cytoplasm not continuous
- observed neurons had tiny gaps btw basket cell axons and Purkinje cell bodies
debate: who won?
- Cajal and used Golgi stain
4 major signalling mechanisms to allow cells to communicate:
- juxtacrine
- paracrine
- autocrine
- endocrine
signalling mechanisms: juxtacrine signalling
- membrane bound proteins (MBP) interact w other membrane proteins on adjacent cells
- MBP interact w ligands in extracellular matrix of adjacent cells
signalling mechanisms: paracrine signalling
- local communication btw cells that are in close proximity
signalling mechanisms: autocrine signalling
- chemical signalling released by cell bind to receptors on same cell
signalling mechanisms: endocrine signalling
- chemical signals released by endocrine cells into circulatory system to communicate w distant target organs
signalling mechanisms: juxtacrine travel
- travel via hydrophilic membrane channels/ pores (gap junctions) btw cells
signalling mechanisms: paracrine travel
- neurotransmitters released by one cell diffuse through the extracellular space to contact adjacent cells
cell-cell contracts may be:
- axodendritic
- axosomatic
- axoaxonic
neuron to neuron synapses:
- juxtacrine and/or paracrine signalling
- location of contact important functionally
- synaptic connections create neural circuits
- circuits trigger physiological or cognitive changes
synaptic delay:
- neural pathway has several synapses for signal to cross
- the more synapses= slower response time, more delay
name types of synapse:
- chemical
- electrical
define synapse
- specialised junction btw two neurons/ neuron + effector organ
electrical synapses: features
- cytosol of neighbouring cells linked via small pores/ channels = gap junctions
- electrical charge diffuses passively into adjacent cell
- no neurotransmitters required
electrical synapses: eg
- some neurons
- cardiac mm
- smooth muscle (single unit)
electrical synapses: properties
- juxtacrine signalling
- v rapid communicaion
- uni/ bidrectional communication
- can transmit excitatory/ inhibitory signals at same synapse
- signal transmission is always ‘sign conserving’ = preserves polarity passed from pre-synaptic to post-synaptic
electrical synapses: gap junctions large enough for
- small molecules not proteins/ DNA to diffuse
- allows secondary messengers to coordinate intracellular signalling processes across coupled cells
electrical synapses: conductance
- can be affected by secondary messengers (cAMP) in response various neuromodulators (eg. dopamine)
- complex/ dynamic mode of intracellular communication
electrical synapses: function
- sync electrical activity among neuronal populations
- info transfer/ signal processing
- rapid escape response circuitry
electrical synapses: sync electrical activity eg
- eg. electrical coupling of cardiac mm cells -> synchronised contraction of heart
electrical synapses: rapid escape eg
- tail flip escape response in crayfish
chemical synapses: significant parts
- presynaptic cell
- synaptic cleft
- postsynaptic cell
chemical synapses: presynaptic cell
- neuron
- axon terminal/ button/ synaptic knob
- synaptic vesicles contain neurotransmitter molecules
- voltage gated calcium ion channels
- neurotransmitter re-uptake molecule
chemical synapses: synaptic cleft
- gap btw presynaptic and postsynaptic neuron
chemical synapses: postsnynaptic cell
- neuron, muscle or gland
- receptor
- enzyme (neurotransmitter degradation)
chemical synapses: properties
- paracrine signalling
- gap is wide to prevent action potentials electrically passing
- mostly unidirectional
- synapses either excitatory (Type I) or inhibitory (Type II)
- slower than electrical synapses
chemical synapses: conversion
- electrical (as AP) - chemical - electrical
chemical synapses: pathway
- AP arrives at nerve terminal
- depolarisation causes voltage gated calcium channels open - calcium influx
- influx Ca triggers exocytosis of neurotransmitter
- neurotransmitter diffuses across cleft and binds to receptors on postsynaptic cell
- response triggered in postsynaptic cell
chemical synapses: pathway response terminated by removing neurotransmitter from synaptic cleft
- degradation/ inactivation of NT
- reuptake of NT
- diffusion of NT
neurotransmitters: features
- chemical messengers carry signals across chemical synapse from neuron -> postsynaptic neurons/ mm cells/ effector cells (glands)
neurotransmitters: criteria to be one
- present (and usually synthesised within) presynaptic neuron
- released in regulated fashion (usually exocytosis) following stimulation of presynaptic neuron
- receptors for substance must be present on postsynaptic target cell
- mechanisms must be present to remove/ inactivate substance
neuromodulator:
- messenger may/ not be released at synaptic sites
- tend to affect groups of neurons w appropriate receptors often w longer-lasting effects
neurohormone:
- messenger released into circulation that exerts an effect on distant target cells
neurotransmitters: name catagories
- classical
- neuropeptides
- purines
- others
neurotransmitters: classical NT
- single amino acids (glutamate)
- biogenic monamines (derived from aa. eg. GABA)
- acetylcholine
neurotransmitters: neuropeptides
- short chains of aa.
- eg. substance P -> central/ peripheral pain pathways
neurotransmitters: purines
- ATP
neurotransmitters: others
- endocannabinoids (appetite, mood, pain sensation)
neurotransmitters: most common excitatory NT in CNS
- glutamate
- excitatory over 90% synapses
neurotransmitters: most common inhibitory NT in CNS
- GABA (inhibitory at 90% synapses that do not use glutamate)
- glycine
neurotransmitters: synthesis
- in synaptic terminal from precursors transported from site of manufacture in cell body
drugs that affect synapses: name them
- drugs
- agonist
- antagonists
drugs that affect synapses: drugs features
- exogenous ligands
- similar chemical structure to endogenous neurotransmitter
drugs that affect synapses: agonist features
- similar enough to endogenous neurotransmitter that it binds to receptors on pre/post synaptic membrane and activates them (eg. morphine) mimic natural effect of natural endorphins
drugs that affect synapses: antagonists features
- similar enough to neurotransmitter binds to receptor site and blocks it (not enough to activate receptor)
- curare binds/ blocks nicotinic ACh receptors = muscle paralysis
inhibitory drugs: function
- mimic inhibitory NTs (hyperpolarise postsynaptic cell, decrease firing APs)
- eg. alcohol mimic GABA
- block receptors for excitatory NTs (prevent generation of AP in postsynaptic neuron)
- eg. heroin blocks synaptic transmission in pain pathways
- block release of excitatory NTs (prevent generation of AP in postsynaptic neuron)
- eg. botulinum toxin (botox) block release of ACh
excitatory drugs:
- mimic excitatory NTs (increase depolarisation, increase firing AP)
- eg. nicotine at autonomic ganglia
- block receptors for inhibitory NTs (prevent inhibition of AP in postsynaptic neuron)
- eg. caffeine inhibits inhibitory NT adenosine
- block NT reuptake pumps (NT remains in synaptic cleft longer)
- eg. cocaine blocks dopamine uptake
- block autoreceptors (no modulation of NT release)
- eg. clonidine blocks norepinephrine alpha2 autoreceptors
