Circuit diagram and related equation
The figure above diagrams the electrical circuit showing the four paths of current flow across the membrane. The diagram is oriented so that inside the cell is towards the top.
From left to right, the current paths are:
Beneath the circuit diagram is the Hodgkin-Huxley equation for the total current through the membrane. The terms of the equation for each of the four currents are in the same order as the corresponding elements in the diagram.
Variable resistors and batteries
The two paths for the voltage-sensitive conductances are diagrammed by resistors that are variable. That is, because of the voltage-sensitivity of the channels, the values of each resistor depend on the voltage across the membrane. Each resistor is in series with a battery that represents the driving force on the ion through that pathway. The polarity of the K battery is inside-negative because, at EK, the voltage across the membrane is negative inside with respect to outside. The polarity of the Na battery is the reverse because, at ENa, the voltage across the membrane is positive inside with respect to outside.
Conductance and current waveforms during a voltage clamp step
To the right of each element (except for the leak conductance) is a diagram. For the capacitance, Cm, the diagram shows the current flowing through the capacitance in response to a voltage step under voltage clamp. For conductances, gK and gNa, the diagrams show the change in each conductance in response to the clamp step as a function of time.
Conductance and current waveforms during an action potential
To see conductance and current waveforms during an action potential, click on a label or an element in the circuit where the cursor turns into a hand.