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An electrophysiology quesiton

Xiaoshen Li xli6 at gmu.edu
Tue Feb 10 04:57:01 EST 2004

Dear Everybody:

Thank you so much for your help. I greatly appreciate it.

Please correct me if I am wrong:
(1)In voltage clamp mode, we usually clamp the neuron's membrane 
potential at its resting membrane potential (say -65mv) by using some 
electric engineering amplifier device. When presynapse fires, the 
postsynaptic channels open, in an excitatory case, the current will flow 
into the neuron (EPSC) which intends to depolorize the neuron membrane. 
So the V-clamp circuit will draw the current outward to mantain the 
neuron Vmem unchanged. The drawing out current in the amplifier is what 
is measured. It is equal to the current flowing at the synapse(EPSC). 
What is called "EPSC" is actually the amplifer reading.
(2)In current clamp mode, I am very unclear now. I guess, we clamp the 
neuron at a constant current through an amplifier too. (How much current 
should an electrophysiologist choose usually?) Say we choose to inject a 
small constant current(Io) into the neuron. So, neuron membrane 
potential will rise from its resting potential in the beginning with 
certain time constant. After a period, Vmem reaches a plateau, steady 
state (Vmem=Io*Rmem). Since Io is small, Vmem will not be high, so no 
action potential firing.
When presynapse fires, postsynaptic channels open. In an excitotary 
synapse case, the driving force will push current flow into the neuron 
through the opened channels. This current cancels part of Io. To keep 
the current constant, the amplifier device will increase its voltage 
potential to inject more current. The extra amount of current is equal 
to the current flowing in at the synapse. This amplifier volatage 
increase is measured and is called EPSP.

If I am correct most part above, what are the advantages/disvantages of 
EPSC and EPSP? How is an electrophysiologist going to choose to measure 
EPSC or EPSP in his today's experiment?

(3)In real life situation(no V-clamp, no I-clamp), a neuron sits there 
with Vrest=-65mv. When an excitotary synapse fires and the postsynaptic 
channels open, due to the driving force difference, the current will 
flow into the neuron. This current will charge the neuron membrane and 
depolarize it. Is this current equal to EPSC measured? Is this membrane 
potential change equal to EPSP measured?

Thank you very much for your help.


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