Synapses + neurotransmitters Flashcards
What are the two kinds of synapse?
Electrical- gap junctions that allow electrical current to pass directly between neurons
Chemical- neurotransmitters released by exocytosis and then bind to specific receptors
How do you distinguish an electrical synapse from a chemical one?
Small molecules like dyes diffuse from one to the other
deleting connexin gene shakB2 will stop the passing of depolarization and hyperpolarization from one neuron to another in electrical synapses
What are the differences between chemical and electrical synapse transmission?
Electrical- signals pass in both directions, signals are passed directly so can only be attenuated, faster
chemical- signals pass in only one direction, signals can be radically transformed, slower
Steps in chemical synapse transmission
Package neurotransmitters and move them into presynaptic terminal > AP arrives so voltage gated Ca2+ channels open > Ca2+ influx means v SNAREs and t SNAREs zip together > vesicles fuse to membrane > neurotransmitters released by exocytosis > diffusion across synaptic cleft > activate receptors on postsynaptic cell triggering further signalling > neurotransmitters removed from cleft
What are the properties of synaptic vesicles?
Clear and small
small molecule neurotransmitters
filled by transporter proteins at the presynaptic terminal
recycled by endocytosis
What are the properties of dense-core secretory vesicles
Dense and large
Peptide neurotransmitters
Created and filled by ER/golgi secretory apparatus
One and done/ destroyed after use
Differences between ionotropic and metabotropic receptors
Ionotropic- directly depolarize or hyperpolarize the postsynaptic cell
Metabotropic- more complex effects such as enzyme cascades and secondary messengers
What three ways can a neurotransmitter be removed from the synaptic cleft?
Diffusing away
Actively being taken up by transporters for recycling
Destroyed in synaptic cleft by enzymes
Similarities between chemical and electrical synapses
They are both plastic (can be modified) but chemical more so than electrical
both allow summing up inputs by the postsynaptic neuron
Features of a neuromuscular junction
Fast and reliable transmission
Motor neuron APs always cause muscle cell APs
Uses ACh as neurotransmitter
What makes neuromuscular junctions efficient?
One of the largest synapses in the body with a large number of active zones in the presynaptic neuron
Postsynaptic cell contains junctional folds which are densely packed with receptor sites
Active zones and junctional folds are also precisely aligned for transmission
What are the criteria for determining if a molecule is acting as a neurotransmitter?
Neurotransmitters must: be present in presynaptic terminals be released in response to stimulation act on the postsynaptic neuron \+ blocking the neurotransmitter should prevent synaptic transmission
Features of amino acid and amine neurotransmitters
Small molecules
Stored in synaptic vesicles
Can bind to both ligand gated ion channels or G-protein coupled receptors
Features of peptide neurotransmitters
Large molecules
Stored in secretory granules
Only bind to G-protein coupled receptors
What is a co-transmitter?
A small molecule that neurons release along side a peptide neurotransmitter
Glutamate
Most common excitatory neurotransmitter in the CNS
Amino acid so found in all neurons
3 ionotropic receptors based on their agonists- AMPA, NMDA, Kainate
Action is terminated by selective uptake by presynaptic cells and glia
AMPA receptors
Mediate fast excitatory transmission
Triggers Na+ and K+ currents triggering EPSPs
NMDA receptors
Often co-exist with AMPA receptors
Have a voltage dependent Mg2+ blocks
so they only open when the membrane is already depolarized
Function as a coincidence detector- when a neuron is activated after just being activated this makes learning possible
Let in Ca2+ which causes downstream signalling
What are examples of metabotropic glutamate receptors? What do these do?
mGluR1, mGluR2
They allow glutamate to sometimes be inhibitory, like in the retina
GABA
Most common inhibitory neurotransmitter in the CNS
y-amino butyric acid
Synthesized from glutamate by the enzyme glutamic acid decarboxylase
Action is terminated by selective uptake into presynaptic neurons and glia
GABA A receptors
Causes IPSPs via GABA gated chloride channels
if the membrane potential is > chloride’s Nernst potential
Why is the right amount of inhibition by GABA important?
Too much- coma, loss of consciousness
Too little- seizures
What drugs can modulate GABA A receptors and what effects do they have?
They have no effect unless GABA is also bound so they are all allosteric drugs
Ethanol- alcohol
Benzodiazepines- treat anxiety
Barbituates- sedatives and anti-convulsants
Neurosteroids- metabolites of steroids
GABA B receptors
G-protein couples receptors that act in diverse ways
Can open K+ channels, close Ca2+ channels, trigger secondary messengers like cAMP
Often presynaptic or auto-inhibitory
