The second tutorial, created by Mark Nelson, and described in Chapter 4 of The Book of GENESIS, lets one perform voltage or current clamp experiments on a compartment containing Hodgkin-Huxley channels. The tutorial also provides the ability to block channels, to plot the Hodgkin-Huxley activation variables, and to generate phase plots. The accompanying exercises help the student understand the Hodgkin-Huxley model and the procedure used to calculate rate parameters from voltage clamp experiments. Other exercises elucidate the basis of post-inhibitory rebound and refractory period.
UPPER LEFT shows the both the "command voltage" which is applied to the voltage clamp circuitry and the resulting membrane potential. A step from the resting potential (defined to be 0 mV) to 50 mV is applied after 10 msec.
LOWER LEFT plot shows the injection current used to maintain this voltage. The very sharp initial spike is due to the large current needed to quickly charge the membrane capacitance to the clamp voltage. It is followed by an inward Na current, shown as a negative current here, then an outward K current.
The channel conductance plots at the UPPER RIGHT reveal a rapid rise in the Na conductance, followed by a slower decay as it becomes inactivated. The K current has a much slower activation and no inactivation. It doesn't decay until the voltage is dropped back to zero. These conductance changes result in the channel currents shown in the LOWER RIGHT plots. As the Na equilibrium potential is greater than the resting potential, the Na current flow is inward (negative). The superposition of these two currents, plus the initial capacitive current give the injection current. (LOWER LEFT). From these, we can calculate the behavior of the channel activation and inactivation parameters.
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