Gating Currents

Hodgkin and Huxley realized that their scheme for the gating of the sodium and potassium channel predicted that there should be an electrical current flow within the membrane associated with the movement ofthe charged gating particles. When a step change in membrane potential is made, the charged gating particles redistribute within the membrane; because the movement of charge through space is an electrical current (by definition), this redistribution of charges from one face of the membrane to the other should be measurable as a rapid component of membrane current in response to the voltage change. A current of this type flowing within a material is called a displacement current. The equipment available to Hodgkin and Huxley was inadequate to detect this small current, however. Almost 20 years later, Armstrong and Bezanilla managed to measure the displacement current associated with the movement of the gating particles.

The procedure for measuring the displacement currents, which have come to be called gating currents because of their presumed function in the membrane, is illustrated in Figure 7-17. The basic idea is to start by holding the membrane potential at a hyperpolarized level; this insures that all the gating particles are on the inner face of the membrane (assuming, once again, that the gating particles are positively charged). In addition, all the sodium and potassium currents through the channels are blocked by drugs, like tetrodotoxin and tetraethylammonium. A step is then made to a more hyperpolarized level, say 30 mV more negative. Because all the gating charges are already on the inner face of the membrane, no displacement current will flow as the result of this hyperpolarizing step. The only current flowing in this situation will be the rapid influx ofnegative charge necessary to step the voltage down. The voltage is then returned to the original hyperpolarized holding level, and a 30 mV depolarizing step is made. The influx of positive charge necessary to depolarize by 30 mV will be equal in magnitude, but opposite in sign, to the influx ofnegative charge necessary to make the previous 30 mV hyperpolarizing step. However, the depolarizing step will in addition cause some gating charges to move from the inner to the outer face of the membrane. Thus, there will be an extra

Figure 7-17 The procedure for isolating the gating current associated with the opening of voltage-sensitive sodium channels of an axon membrane. (a) Membrane voltage is stepped negative from a hyperpolarized level. With all ion channels blocked, the only current flowing is that required to move the membrane voltage more negative. (b) Membrane voltage is stepped positive from a hyperpolarized level. The current necessary to move the potential in the positive direction (dotted trace) will be the same amplitude, but opposite sign, as in (a). In addition, there will be an extra component of current in (b) caused by the movement of the charged gating particles in response to the depolarization. This component is seen in (c) on an expanded vertical scale.

component of current, due to the movement of gating charges, in response to the depolarizing step. By subtracting the current in response to the hyper-polarizing step from the depolarizing current, this extra gating current can be isolated. Experiments on this gating current suggest that it has the right voltage dependence and other properties to indeed represent the charge displacement underlying the gating scheme suggested by Hodgkin and Huxley. This is an important piece of evidence validating a basic feature of Hodgkin and Huxley's model of the membrane of excitable cells.

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