demyelinating vs axonal patterns
Think again of the nerve as an insulated copper wire: a conducting core wrapped in a rubber sheath. There are really only two basic ways to wreck a wire, and they fail differently. You can damage the rubber insulation, leaving the copper intact, and the signal still gets through but slowly and unreliably. Or you can break the copper itself, and that fibre simply stops carrying anything. Nearly every nerve disease falls mostly into one of these two camps, and telling them apart is the central organizing question of electrodiagnosis.
Damaged insulation is the demyelinating pattern. Because the surviving fibres are intact but poorly insulated, the conduction velocity slows, the latency lengthens, and the signal can even be blocked in places, but the amplitudes from below the lesion can stay fairly full. Lost fibres make the axonal pattern. Here the surviving fibres conduct at normal or near-normal speed, but because fibres are gone, amplitudes shrink, and on the needle exam the orphaned muscle shows fibrillations and, later, large reinnervated motor units. In short: demyelinating means slow, axonal means small.
This distinction is not academic, it steers diagnosis and prognosis. Some demyelinating conditions, like Guillain-Barre syndrome or chronic inflammatory demyelinating polyneuropathy, are treatable in ways axonal ones are not, so the pattern can change what is done for the patient. Demyelination, if the axon survives, can also recover relatively quickly, whereas regrowing a severed axon is slow and often incomplete, which shapes what the rehabilitation team tells a patient about the road ahead. Many real diseases are mixed, and the examiner judges which process dominates.
Two patients both have numb, weak feet. In the first, conduction velocities are markedly slow and latencies long but amplitudes are preserved, a demyelinating picture suggesting an immune-mediated, potentially treatable neuropathy. In the second, velocities are near normal but amplitudes are tiny and the foot muscles fibrillate, an axonal picture more typical of a length-dependent neuropathy such as that of long-standing diabetes.
Slow with full amplitude versus small with normal speed: same symptom, opposite mechanisms.
The slow-versus-small rule is a useful first cut, not a law: severe axon loss can mildly slow conduction (the fastest fibres are gone), and the two processes often coexist, so the pattern is a judgement about which dominates, not a clean either-or.