Assuming that during the depolarization phase of the action potential the membrane is mostly permeable to sodium through voltage-gated sodium channels. How to calculate the amplitude of the sodium current (per cm2) at the inflexion point of the depolarization phase of the action potential?
dV/dt = -I/C !!!
You could make the assumption that at the inflection point that any leak currents were negligible. You can estimate specific membrane capacitance as 1 uf/cm^2
I wanted to test if this was legitimate, so I set up a quick model in NEURON. I made a neuron with 1000 um^2 area (1x10-6 cm^2), a specific capacitance of 1uF/cm^2 and a default sodium and potassium conductance. It required 100 pA to make it fire, and at the inflection point, the voltage trajectory was about 6.5 V/s. If we use my idea, that means the sodium current should be 6.5 uA/cm^2. NEURON tells me that at the inflection, the actual sodium current was 25 uA/cm^2. Why the large disparity? Well even at such an early stage of the action potential, the potassium current was about 15 uA/cm^2... which together brings it pretty close to the ball park.
Seeing as your question involves the assumption that the potassium current is negligable, it should works out pretty well.
(In real life, you may also need to deal with the current supplied by any injected that makes the cell spike!)
1. Maintain cells in extracellular solution E 1 for in situ experiments or in extracellular solution E 2 for in vitro experiments.
2. Perform recordings in the perforated patch clamp or whole-cell mode. For whole-cell recordings, fi ll the patch pipette with intracellular solution I 1.
3. Clamp cells to a holding potential of −80 mV.
4. Apply 50 ms lasting voltage steps to potentials between −80 mV and +30 mV in 10 mV increments at an interval of 10 s.
5. Test whether 1 μM tetrodotoxin (TTX) induces Na + current inhibition.
wish to help you!
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