resting membrane potential and action potential
Every nerve and muscle cell is like a tiny battery that is always kept charged and ready. At rest the inside of the cell sits at a slightly negative voltage compared with the outside, the way a charged battery holds a difference between its terminals. This standing charge is the resting membrane potential, and it is the reason a nerve can fire the instant it is asked to.
The charge exists because the cell membrane pumps and leaks charged particles (mostly sodium and potassium) unevenly, parking the cell at roughly minus seventy millivolts. When a signal arrives, tiny gates fly open and sodium rushes in, flipping the voltage briefly positive. That sharp flip, lasting about a millisecond, is the action potential, and it does not fade as it travels: it regenerates itself down the length of the nerve like a line of dominoes, each falling tile knocking over the next, so the signal arrives at the far end just as strong as it started. The cell then pumps the charge back out and resets, ready to fire again.
This is the bedrock of every electrodiagnostic test. The machine sends a small electrical pulse to trigger an action potential in a nerve, then records the wave that arrives downstream. How big that wave is, how fast it travelled, and how cleanly it propagated all depend on healthy membranes, healthy insulation, and enough surviving fibres, which is exactly what disease disturbs.
Think of the insulating sheath around a nerve, called myelin, as the rubber coating on a wire that lets the action potential jump quickly from gap to gap. Strip the rubber off and the signal still travels but limps along slowly. Cut enough copper wires and the signal still travels fast on the survivors, but the wave that arrives is smaller. These two pictures, slow versus small, are the heart of how electrodiagnosis tells one disease from another.
Damaged insulation slows the signal; lost fibres shrink it, the basis of demyelinating versus axonal patterns.
An action potential is all-or-nothing: a single fibre either fires fully or not at all. The graded, larger waves seen on the machine are the summed firing of many fibres at once, not one fibre firing harder.