8 - Synaptic Integration Flashcards
Dendrites integrate info….
Spatially and temporally
Is a single presynaptic AP that produces a small EPSP sufficient to produce an AP in post-synaptic neuron?
No - usually insufficient
EPSPs associated w/ simultaneous inputs…
From multiple neurons sum
EPSPs associated w/ sequential inputs from…
A single neuron can sum over time
Will neurotransmitter be released if there’s no extracellular Ca+2?
No — Ca+2 influx needed to initiate presynaptic neurotransmitter release
Neurotransmitter vesicle release is…
Quantal and PSPs reflect release of many vesicles
Does each vesicle contain different amount of neurotransmitter?
No — each contain similar amount and produce similar PSP
Miniature PSPs
Voltage change due to a single vesicle
Why is PSP smaller at soma than at synapse?
Passive ion diffusion dominant in dendrites (no APs). PSP reduced in amplitude at soma due to leakage of charge in dendrites from passive diffusion.
What’s most important consideration about whether an AP generated?
Depolarisation at trigger zone
Trigger zone
Contain highest density of Nav channels and easiest place to generate an AP
Passive conduction in dendrites
EPSP decays w/ distance from synapse due to leakage of ions, reducing size of depolarisation
Assuming there are no voltage-gated channels…
- effect of injected current decays exponentially w/ distance
- length constant -> distance at which depolarisation decays to 1/e of max size
What does length constant depend on?
- internal resistance (diameter, which increases towards soma)
- membrane resistance (density open channels, which varies)
Do signals propagate unidirectionally in dendrites?
No - bidirectional
For PSPs of same size synapses closer to soma have greater….
Influence on spiking probability
PSP needs to reach what in order to affect spiking probability?
Spike initiation zone
Active conduction always occurs in…
Some dendrites
What does active conduction in dendrites depend on?
Voltage-gated channels, which boost membrane depolarisations
Active conduction in dendrites improves probability that…
An EPSP reaches soma
Do dendritic APs function like axonal APs?
No — dendritic aren’t as reliable and don’t cause as large depolarisation
Synaptic current
Charge influx due to channels opening in response to neurotransmitter release
Temporal summation is affected by…
Membrane time constant (how quickly it leaks charge)
Spatial summation depends on…
Membrane length constant (how far charge can propagate)
Charge leakage limits…
Distance over which neuron can integrate input
Distal synapses are less effective at depolarising…
Soma
Distal synapses are associated w/
Larger EPSPs to help counteract decay along dendrite
Spiking threshold varies…
Across membrane
What is spiking threshold determined by?
Minimum mempot for which an AP will occur (sufficient Nav channels need to open to initiate positive feedback)
What’s another name for the axon hillock?
Trigger zone or spike initiation zone
T/F - if Vm already at -65mV, opening more Cl channels due to influx of GABA will change Vm
False — because Ecl = -65mv
Shunting inhibition
Inhibiting current flow from soma to axon hillock. Opening of Cl channels allows charge leakage, preventing EPSP reaching soma
Does arrival of an AP at axon terminal guarantee vesicles will be released?
No - release probability < 1
Modulating probability of vesicle release is another way to…
Control strength of synapse
Pr varies between areas
Spinal cord motor neurons Pr = 0-1
Motor neurons Pr = 1
Axo-axonal synapses regulate…
Ca+2 entry into axon terminal, modulating vesicle release
If a 2nd AP arrives before Ca+2 in axon terminal has cleared and not all vesicles have been released in response to 1st AP,
2nd AP associated w/ higher intracellular Ca+2 in pre-synaptic axon terminal, greater neurotransmitter release, larger post-synaptic change
What are autoreceptors?
Pre-synaptic metabotropic receptors that monitor own levels of neurotransmitter release and can inhibit neurotransmitter release and synthesis (-ve feedback)
Neuromodulation — metabotropic
Some metabotropic neurotransmitters can indirectly modify size of EPSPs associated w/ other synapses
Decreasing K+ leakage will…
Increase membrane resistance and length constant (makes neuron more excitable)
Astrocytes modulate…
Synaptic transmission by detecting and responding to many neurotransmitters
Back-propagation in dendrites
APs can travel retrogradely in dendrites
