The Song of the Cell: An Exploration of Medicine and the New Human

by Siddhartha Mukherjee

Imagine, now, that the army of invading ions, a charge (indeed, literally so) marches past the dendrites toward the neuron’s cell body—the soma—and reaches a pivotal point in the neuron, called the “axon hillock.” It is here that the critical biological cycle that enables nerve conduction is set into motion. If the pulse that reaches the axon hillock is greater than a set threshold, the ions begin a self-fulfilling loop. The ions stimulate the opening of more channels in the axon. In biology, when a chemical stimulates the release of the very same chemical, it sparks a positive feedback loop—more becomes more. The ion-sensitive ion channels are the linchpins in axonal conduction: they are self-propagating, as if the throng is now a perpetuating crowd—bludgeoning open more gates to the citadel, letting more of themselves in. More sodium rushes in through the channels, while potassium, another ion, rushes out. The process amplifies: the invading crowd of ions cracks open more gates, and even more sodium ions flow in. As more and more channels are unshackled open, a tidal wave of sodium ions enters and potassium ions exit, causing the large positive spike that Hodgkin and Huxley first saw in 1939. The net charge of the axon turns from negative to strongly positive. The cascade of conduction, once sparked, is now unstoppable: it moves farther and farther along the axon.I The process is self-propagating. One set of channels opens and closes, generating an electrical spike. That...