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An electrophysiology quesiton

Xiaoshen Li xli6 at gmu.edu
Thu Feb 12 07:32:50 EST 2004

Matt Jones wrote:

> In real life, the voltage change during an EPSP should correspond
> approximately to what you would see in current clamp (because current
> clamp is essentially just observing the "natural" voltage change).
> In real life, the *current* flowing through the synapse follows this
> equation:
> Isyn = Gsyn * (V - Esyn),
> where Esyn can be taken as some constant (say, 0 mV). 
> Therefore, Isyn will also be changing during an EPSP in real life. In
> fact, if the cell fires a spike at the top of the EPSP, then V will
> actually change sign during the spike, and Isyn will also change sign.
> This means that the flow of ions actually reverses direction through
> the synaptic channels during a spike (cool, huh?).


I think this part needs some more discussion:

Soma fires action potential, so the membrane potential at soma will 
shoot up to a positive value, say +30mV. Only if the synapse is located 
at the soma and it is opened by presynaptic firing, then the flow of 
ions will reverse direction. Most synapses are at dendritic 
tree(proximal or apical location). The local membrane potential will 
rise to be closer to 0mV but may not exceed it to become positive value. 
So the direction of the ion flow on those synapses will not flip. (The 
value of the current flowing through the synapses will change due to 
smaller driving force).

If the dendrite also fires a spike locally, then the direction of ion 
flow will reverse. However, "dendrite fires spikes" is very new research 
information. (My impression is those spikes are calcium spikes, not 
classical Hodgkin-Huxley Na-K spike). Are those called "spikelet"?

I am also curious if this direction reverse has any physiology meaning.

Thank you very much.


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