The Threshold of Everything

In the 1950s, British scientists Alan Hodgkin and Andrew Huxley inserted tiny electrodes into squid nerve fibers to study how electrical signals travel through the body. What they discovered earned them the Nobel Prize in 1963 — and reveals something profound about what it means to be brave. Every neuron operates on what scientists call the all-or-nothing principle. A nerve cell receives stimulation — heat, pain, a fingertip's pressure — and builds up electrical charge.…