466 Chapter 12. Nerve impulses[[Student version, January 17, 2003]]
Figure 12.11: (Schematic.) A more detailed version of Figure 11.1. The long wire threaded through the axon
maintains its interior at a spatially uniform electric potential (space-clamping). A feedback circuit monitors the
transmembrane potentialVand sends whatever current is needed to keepVfixed at a “command” value chosen by
the experimenter (voltage clamping). The corresponding currentIis then recorded. Typically the axon is 30–40mm
long. [From L ̈auger, 1991]
and other reasons, Hodgkin and Huxley set up their apparatus in avoltage-clampmode. In this
arrangement the experimenter chooses a “command” value of the membrane potential; feedback
circuitry supplies whatever current is needed to maintainV at that command value, and reports
the value of that current.
Separation of ion currents Even with space- and voltage-clamping, electrical measurements
yield only the total current through a membrane, not the individual currents of each ion species.
Toovercome this problem, Hodgkin and Huxley extended Katz’s technique of ion substitution
(Section 12.2.3). Suppose we adjust the exterior concentration of ion speciesiso thatViNernstequals
the clamped value ofV.Then this ion’s contribution to the current equals zero, regardless of what
its conductancegi(V)maybe(see Equation 12.17 on page 460). Using an elaboration of this idea,
Hodgkin and Huxley managed to dissect the full current across the membrane into its components
at anyV.
Results Hodgkin and Huxley systematized a number of qualitative observations made by Cole and
Marmont. Figure 12.12 sketches some results from their voltage-clamp apparatus (Figure 12.11).
The command potential was suddenly stepped up from the membrane’s resting potential toV =
− 9 mV,then held there. One striking feature of these data is that the membrane conductance does
nottrack the applied potential instantaneously. Instead we have the following sequence of events:
1.Immediately after the imposed depolarization (Figure 12.12a), there is a very short spike of
outward current (panel b), lasting a few microseconds. This is not really current through the