measuring neuronal excitability

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Labmind
Labmind's picture
measuring neuronal excitability

I was wondering if there is a consensus protocol to measure neuronal excitability?
I assume, the most straightforward way would be to measure membrane resistance and inject a series of depolarizing currents into the cell in current clamp and count the number of action potentials evoked. Are there particular criteria for consideration? I saw a whole range of different examples from 0-150 pA, 200ms-1s. is there consensus?
Or any alternative methods?
Thanks, 

Labmind
Labmind's picture
 No replies? Nobody knows

 No replies? Nobody knows what's the proper way of measuring excitability?

The FFM
The FFM's picture
You seem to have the method

You seem to have the method pretty much under control.

Use current clamp to hold the Vm close to the resting membrane potential.  Then record excitabilty as the number of action potentials evoked by a series of depolarizing current steps (The duration, magnitude of the depolaizations, and replicates are usually dictated by type of neuron from which you are recording). You then plot the intensity of the applied currents for each depolarization vs the number of action potentials evoked by each depolarizing current step.  The slope of the linear fit to the relationship between these two parameters is your quantified excitability.

Perhaps these refernces will also help?

http://brain.oxfordjournals.org/content/119/4/1099.abstract?ijkey=605e1a27a06b719336e2559da1af392d183f1974&keytype2=tf_ipsecsha

http://brain.oxfordjournals.org/content/123/12/2542.full

Labmind
Labmind's picture
 Dear Fraser,

 Dear Fraser,
Thank you very much for your reply. 
The papers might not be the best fit, as they are not really patch clamp, instead these are some measurements of the human motor axon properties. Although, it does look like there is some hyperpolarizing and depolarizing currents injection.

I am just trying to confirm my preliminary information. The final question is:
is there a minimum requirement for the longevity of depolarizing steps injection?

(in these papers at least for the axons there seems to be 100-200 ms - sufficient). While looking at some other papers of recordings in cortex, I saw a range from 400 ms to 800 ms and longer. there seems to be no consensus, almost random parameters.
thanks,

c.hill
c.hill's picture
I am also using current clamp

I am also using current clamp to measure neuronal excitability in CA1 pyramidal neurons.

My method involves holding Vm at -82 mV (in current clamp mode) and applying a hyperpolarising and a depolarising protocol. Each protocol produces eight 50 pA stepwise current pulses, which are 150 ms in duration. So, the hyperpolarising protocol goes 'down' in 50 pA steps (to -350 pA) from the holding current, and the depolarising protocol goes 'up' in 50 pA steps (to +350 pA) from the holding current.

The hyperpolarising protocol allows me to calculate the input resistance, and the depolarising protocol allows me to measure spike count, width, amplitude, firing frequency etc. The duration of the pulses are long enough to allow the hyperpolarised membrane potential to plateau a little, producing the 'steady' measurement that is required to calculate the input resistance. I guess it is also short enough for the depolarisation protocol not to run down the cell too much.

I hope that helps a little?

Labmind
Labmind's picture
Thanks!

Thanks!
It is helpful. I feel I've been thinking along the same lines. What range of firing rates do you usually measure in CA1? and is there a paper you usually reference for this protocol?

c.hill
c.hill's picture
On average I get 5 - 6 spikes

On average I get 5 - 6 spikes by the end of the depolarisation protocol (150 ms; +350 pA), but I have observed anything between 0 and 11!). I inherited this protocol from a previous student, so I'm afraid I can't give you a reference.

Philly
Philly's picture
 you could also consider

 you could also consider measuring rheobase