Questions regarding Spike Generation
Conduct a current clamp experiment
by inserting a short current pulse of 1 ms
duration and variable amplitude as follows:
Begin stimulation: 5 ms.
Duration: 1 ms
Amplitude: start first with a value of 0.2.
Repeat the experiment several times, lowering
the amplitude stepwise down to 0.02.
Set the end of the simulation to 30 ms.
- What is the minimal amplitude necessary to excite a spike?
- Does the form of the spike vary between different runs/different
stimulation amplitudes?
- Does the delay of spike initiation vary between different runs/different
stimulation amplitudes?
- Why is the sodium channel faster than the potassium channel?
What is the time difference between the maxima of sodium and potassium
current, respectively?
Questions regarding channel time constants
Conduct a voltage clamp experiment by applying a voltage step
as follows:
Begin stimulation: 5 ms.
Duration 25 ms.
Set the end of the simulation to 30 ms.
Repeat the exeriment several times with a target voltage after the step
between -50 mV and +10 mV.
This is pretty much the type of experiment that
Hodgkin and Huxley conducted to measure the (voltage-dependent) time
constant and equilibrium state of the K-channel.
For an overview, read the chapter in the book `Genesis',
edited by Jim Bower.
- Study the K-current (suppress all the other currents in the second graph).
Convince yourself that the `time constant' of the K-current
depends on the voltage.
Is the channel faster for high or for low voltage?
- Now look at the lower graph and study the dynamics of n,m, and h.
- Convince yourself from the graph that m faster than n and h.
- Convince yourself that h and n have roughly the same
dynamics except that (a) h is inverted with respect to n
and (b) h starts at a resting value of 0.6 whereas n has
a resting value close to zero.
The above two points are the basis of a reduction
of the Hodgkin-Huxley model to 2 dimensions
(Rinzel, Morris-LeCar, FitzHugh, Nagumo,
Abbott and Kepler).