The Origins of Biological Electricity
For the 2012 Annual Question, my favorite deep, elegant, and beautiful explanation is the solution to the problem of the biological generation of electricity by animals and plants, provided by the British physiologists Alan Hodgkin and Andrew Huxley in 1952, and for which they received the Nobel Prize for Physiology or Medicine in 1963.
It had been known for over a century that nerves, muscles, and some other organs of animals and a few plants generate electricity. Most of that electricity is at low voltages of just a fraction of a volt. However, electric eels arrange 6,000 muscle membranes in series and thereby generate 600 volts, enough to kill their prey, shock horses wading rivers, and shock me when I was studying eel electricity generation as a graduate student and got so focused on thinking about physiological mechanisms that I forgot their consequences.
Electricity involves the movement of charged particles. In our light bulbs and electric grids, those charged particles are negatively charged electrons: what are they in biological systems? Already over a century ago, the German physiologist Bernstein speculated that the charged particles whose motion was responsible for biological electricity were not electrons but were positively charged ions.
Hodgkin and Huxley started the decisive experiments in the late 1930's. They expected to find that the voltage across a resting nerve membrane went transiently to zero during an electrical impulse, due to a loss of selective permeability to the positively charged potassium ion. To their surprise, they found that the nerve voltage didn't just go to zero, and nerve membrane didn't just become indiscriminately permeable: the voltage actually reversed in sign, requiring something special. But then Hitler invaded Poland, and Hodgkin and Huxley spent the next six years using their understanding of electricity to build radar sets for the British military.
In 1945 they resumed their experiments, using giant nerves that had been discovered in the backs of squid and that were big enough to insert an electrode for measuring the voltage across the nerve membrane. They confirmed their tantalizing pre-war discovery that the nerve voltage really did reverse in sign, and that that reversal got transmitted along a nerve to constitute an electrical impulse. In a series of experiments that define the word "elegance," they then artificially clamped the voltage across the nerve membrane at various levels, measured the electric currents going in and out of the membrane as a function of time at each level after the voltage clamp, translated those voltage measurements into changes of permeability to the positively charged potassium ion and then to the positively charged sodium ion as a function of voltage and time, and finally reconstructed the whole course of a nerve impulse from those time-dependent and voltage-dependent permeability changes. Today, physiology students do the necessary calculations to reconstruct an action potential in an afternoon on their desk computers. In 1952, before the era of modern computers, Andrew Huxley had to do the calculations much more laboriously with a desk calculator: it took him about a month to calculate one nerve impulse.
The four papers that Hodgkin and Huxley published in the British Journal of Physiology in 1952 were so overwhelming in their detailed unraveling of sodium and potassium movements, and in their reconstruction of nerve impulses, that the scientific world became convinced almost immediately. Those permeability changes to positive ions (not to negative electrons) make it possible for nerves to convey electrical impulses, for muscles to convey impulses that activate contraction, for nerve/muscle junctions to convey impulses by which nerves activate muscles, for nerve/nerve junctions (so-called synapses) to convey impulses by which one nerve activates another nerve, for sense organs to produce impulses that translate light and sound and touch into electricity, and for nerves and our brain to function. That is, the operation of animal electricity unraveled by Hodgkin and Huxley is what makes it possible for us to read this page, to think about this page, to pick up this page, to call out in surprise, to reflect about Edge questions, and to do everything else that involves motion and sensation and thought. The underlying principle (movement of positively charged particles) was simple, but God resided in the complex details and the elegant reconstruction.