Synaptic Transmission Flashcards
What is a synapse?
A specialised junction where an axon terminal contacts another neuron or cell type
What are the two types of synapse?
Electrical
Chemical
What are the features of an electrical synapse?
Linked by gap junctions
These are formed by hemichannels (connexon)
Ionic current in presynaptic membrane flows passively into postsynaptic
Current can pass in both directions (non-rectifying synapse)
OR pass in one direction (rectifying synapse)
Rapid and allow ATP and 2nd messenger exchange
Synchronised activity among neuron populations
What is a hemichannel/ connexon?
It is composed of 6 connexin subunits
Connexins rotate to open a pore of the gap junction channel
What are the requirements for chemical synaptic transmission?
Mechanisms for:
Synthesising and packing neurotransmitters into vesicles
Causing vesicles to spill contents into cleft after an AP
Producing an electrical/ biochemical response to neurotransmitter in postsynaptic neuron
Removing transmitter from cleft
Must be very rapid!
What is the active zone of a chemical synapse?
Presynaptic site of neurotransmitter release
All vesicles wait ready to be activated
What is the postsynaptic density of chemical synapses?
The area of the postsynaptic neuron that contains the receptors to translate the extracellular signal into an intracellular signal
What ways can synapses be categorised by?
- Connectivity
Which part of the neuron is postsynaptic to the axon terminal - synapse Anatomy
There size and shape
What are the 4 types of synapses?
Axodendritic/ Axospinous
Axosomatic
Axoaxonic
Dendrodendritic
What are the connections of a axodendritic synapse?
Postsynaptic membrane is on the dendrite of another neuron
30% of neurons in CNS
What are the connections of an axosomatic synapse?
Postsynaptic membrane is on the cell body of another neuron
6% of neurons in CNS
What are the connections of an axoaxonic synapse?
Postsynaptic membrane is on the axon of another neuron
Rare
What are the connections of a dendrodentritic synapse?
Dendrites synapse with each other
Very rare
Describe Grays type l synapse
Postsynaptic membrane is thicker than the presynaptic membrane
Thought to be due to more receptors on the post
Usually excitatory
Describe Grays type ll synapse
Postsynaptic and presynaptic membranes similar thicknesses
Usually inhibitory
What are neurotransmitters?
Molecules released by the presynaptic neuron
Means of communication at a chemical synapse
What determines whether a signal is excitatory or inhibitory?
Neurotransmitters are specific to the receptors they bind to
One receptors will let in positive charge (Na+)
Another will let in negative charge (Cl-)
Name the different small molecule neurotransmitters that can be used
Glutamate
GABA
Acetylcholine
Dopamine
Noradrenalin
Adrenaline
Histamine
Serotonin
Name the purine/ neuro peptides that are used as neurotransmitters
ATP - cotransmitter (purine)
Substance P ]
Metenkephalin ] Neuropeptides
Opioids ]
What drugs use the GABAa receptor as their site of action?
Barbiturates
Benzodiazepines
Alcohol
What are the two methods of action for G protein coupled channels?
1) opening ion channels
2) activating enzymes that synthesise second messenger molecules
What are self regulating neurotransmitters?
At some synapses transmitters activate receptors on the terminal from which they are released as well as receptors on the postsynaptic membrane
What are the steps once the transmitter has left the presynaptic membrane?
Diffusion across the cleft
Enzyme degradation in the synapse
Presynaptic re-uptake followed by degradation or recycling
Uptake by glial cells
Uptake by postsynaptic neuron
Desensitisation
What are the two types of summation?
Spatial - occurs when stimuli are applied at the same time, but in different areas, with a cumulative effect upon membrane potential. Spatial summation uses multiple synapses acting simultaneously.
Temporal - occurs when multiple action potential signals fire down the axon hillock of a single neuron to be received by the synapse of another neuron. The impulses reach the postsynaptic neuron one at a time, accumulating enough voltage to produce an effective charge.
What is shunting inhibition?
It is the inward movement of Cl- anions that will negate the flow of positive ions
