12.3. The full Hodgkin–Huxley mechanism and its molecular underpinnings[[Student version, January 17, 2003]] 467
membranepotential,mVtotal membranecurrent,mA cm−^2-1 001240-65024imposed depolarizationtime, ms0024
time, ms00
24
time, mstime, mscapacitive
currentdelayed outward
currentdelayed outward
currenttransient inward
currenttransient inward
currenta
b
c
d
sodiumcurrentpotassium currentFigure 12.12:(Sketch graphs of experimental data.) Membrane currents produced by depolarizing stimuli. (a)Ap-
plied stimulus, a 56mVdepolarization of a squid axon membrane imposed by a voltage-clamp apparatus. (b)Currents
measured during the stimulus. The observed current consists of a brief positive pulse as the membrane’s capacitance
discharges, followed by a short phase of inward current, and then finally a delayed outward current. The inward and
delayed outward currents are shown separately in (c) and (d). (c)The transient inward current is due to sodium
entry. (d)Potassium movement out of the axon gives the longer outward current. Dividing the traces in (c,d) by
the imposedV−VNernsti yields the corresponding conductances,gi(V, t), which depend on time. [After Hodgkin &
Huxley, 1952a.]
membrane, but rather the discharge of the membrane’s capacitance (a “capacitive current”),
as discussed in Section 12.1.2.
2.Abrief, inward sodium current develops in the first half millisecond. Dividing byV−VNaNernst+
gives the sodium conductance, whose peak value depends on the selected command potential
V.
3.After peaking, however, the sodium conductance drops to zero, even thoughVis held constant
(Figure 12.12c).
4.Meanwhile, the potassium current rises slowly (in a few milliseconds, Figure 12.12d). Like
gNa+,the potassium conductance rises to a value that depends onV.UnlikegNa+,though,