The Electrochemical Gate

Alan Hodgkin, Andrew Huxley, and the Axonal Voltage Clamp in Plymouth

Measuring nearly one millimeter in diameter, the giant axon of the Atlantic squid (Loligo forbesii) is one hundred times thicker than a mammalian motor nerve fiber. In 1939, it was the only nerve fiber known to science large enough to let an experimenter thread a glass capillary electrode straight down its intracellular core without tearing the outer membrane. Zoologist John Zachary Young had identified the giant fiber three years earlier. Working at the Marine Biological Association laboratory in Plymouth, England, British physiologists Alan Hodgkin and Andrew Huxley used that anatomical structure to overturn the foundational theory of nerve conduction.

Since 1902, German physiologist Julius Bernstein's membrane theory had stated that a resting nerve is selectively permeable to potassium ions, creating a resting potential near $-65\text{ mV}$. Bernstein argued that an action potential is a brief, non-specific breakdown of membrane insulation that causes the voltage to collapse passively to zero. In August 1939, weeks before World War II forced them into radar and gunnery research, Hodgkin and Huxley inserted a fine electrode inside a squid axon. Recording on an oscilloscope, they photographed the true action potential waveform. The voltage did not stop at zero. It shot past zero by forty millivolts, reversing polarity to reach $+40\text{ mV}$ before plunging back down in two milliseconds.

Returning to Plymouth in 1947 after six years of wartime military electronics, Hodgkin and Huxley realized why the spike had defied analysis. Membrane ion permeability depends on voltage, creating an explosive positive-feedback loop where changing voltage alters ion flow and ion flow alters voltage. Borrowing the feedback amplifier principle designed by American biophysicist Kenneth Cole, they built an electronic voltage clamp. Using two internal silver wires—one measuring intracellular voltage $V_m$ and the other injecting compensating current—an operational amplifier held the membrane potential locked at any chosen step command ($dV_m / dt = 0$). Because total membrane current ($I_m$) equals capacitive current plus ionic current:

clamping $dV_m / dt$ to zero eliminated capacitive surge and allowed them to read pure ionic currents across the membrane. By replacing extracellular seawater sodium with impermeant choline ions, they split the current into an early inward sodium surge ($I_{\text{Na}}$) and a delayed outward potassium wave ($I_{\text{K}}$).

During the winter of 1951, denied access to Cambridge's EDSAC vacuum-tube computer while its memory was being upgraded, Andrew Huxley spent three weeks turning the crank of a hand-operated Brunsviga mechanical calculator. He numerically integrated their four-dimensional system of differential equations across tens of thousands of time steps:

Their equations predicted the exact waveform, firing threshold, refractory period, and conduction velocity ($18.8\text{ m/s}$ calculated versus $21.2\text{ m/s}$ measured) of the living nerve impulse. Depolarization past $-55\text{ mV}$ rotates the charged S4 voltage-sensing helices of the sodium channel outward. Three activation gates ($m^3$) snap open within 0.2 milliseconds, letting $\text{Na}^+$ rush inward toward its $+55\text{ mV}$ Nernst equilibrium. At the peak of the spike, a slower intracellular inactivation loop ($h$) plugs the sodium pore. Simultaneously, four potassium gates ($n^4$) open with a one-millisecond delay, venting $\text{K}^+$ outward to reset the line. In myelinated human nerves, insulating Schwann-cell myelin increases internodal membrane resistance ($r_m$) and slashes capacitance. That insulation forces the action potential to leap electrotonically between uninsulated nodes of Ranvier at velocities up to $120\text{ meters per second}$.