compound muscle action potential (CMAP)
/ see-map / C-M-A-P /
Picture a choir where one cue makes everyone sing the same note at the same instant. From across the room you do not hear individual voices; you hear one big combined sound, and its loudness tells you roughly how many singers joined in. The compound muscle action potential is the electrical equivalent: when a motor nerve is shocked, all the muscle fibres it controls fire together, and the recording electrode over the muscle captures their summed response as a single wave.
In a motor nerve conduction study, the examiner places the recording electrode over the belly of a muscle, shocks the nerve that supplies it, and the muscle answers with a CMAP. Its size, the amplitude or area, reflects roughly how many working muscle fibres responded, which in turn reflects how many nerve fibres survived to drive them. A full-strength CMAP means a full complement of working motor wires; a shrunken CMAP means wires or muscle fibres have been lost. The time from shock to the start of the wave is the motor latency.
The CMAP is one of the workhorse measurements of electrodiagnosis. A small CMAP is the signature of axon loss, while a CMAP that is normal at the muscle but collapses when you stimulate above a certain point flags a block in the nerve, called conduction block, typical of focal demyelination. Tracking CMAP size over weeks can also gauge how much nerve has actually been lost after an injury, useful for setting realistic rehabilitation expectations.
After a wrist laceration cut part of a nerve, the CMAP from the affected hand muscle is half the size of the same muscle on the healthy side. That ratio gives the examiner a rough estimate that roughly half the motor fibres are out of action, helping the team judge how much hand function might realistically return.
Comparing the CMAP side to side estimates how much motor nerve survives.
A small CMAP early after an acute nerve cut can be misleading: the part of the nerve below the cut keeps conducting for a few days before it degenerates, so studies done too soon may underestimate the damage.