The total membrane current (as illustrated in Figure 4.1) satisfies Equation 3.48, which can be rewritten in the form: .Further, the positive direction of transmembrane current is chosen as outward (from the intracellular to the extracellular space).For historical reasons, however, membrane behavior and the voltage clamp method are discussed here first, before ionic channel behavior and the patch clamp method are explored.
(Hodgkin and Huxley further designed a compartment to eliminate any fringing effects at the ends.) Consequently, between the concentric electrodes, a membrane current will be measured that obeys the equation: (A) The physical structure of the device that ensures axial uniformity, hence current flow that is in the radial direction only. (B) The total current (im), through the membrane (per unit length), consisting of the components of ionic current im I and capacitive current im C.
In the space clamp procedure, the membrane current includes the capacitive component as a confounding source.
(Note that because we examine the ionic currents during the propagating nerve impulse, the membrane resistance r With appropriate instrumentation, it is possible to stimulate the axon simultaneously throughout the entire length of the preparation.
Then the membrane voltage at each instant of time is identical over the entire length of the axon.
This chapter describes the voltage clamp device, the experiments of Hodgkin and Huxley, the mathematical model into which their data were fitted, and the resulting simulation of a wide variety of recognized electrophysiological phenomena (activation, propagation, etc.). The Hodgkin and Huxley work is important not only for its ability to describe quantitatively both the active and the passive membrane, but for its contribution to a deeper understanding of the membrane mechanisms that underlie its electrophysiological behavior.