motor unit action potential
/ MUAP / M-U-A-P /
A single motor nerve fibre does not control one muscle fibre; it controls a whole little squad of them, and that nerve fibre plus all the muscle fibres it drives is called a motor unit. When you flex a muscle gently, you switch on these squads one at a time. The combined electrical blip from one squad firing, as heard by the needle, is the motor unit action potential, the basic unit of voluntary muscle activity that the EMG examiner reads during contraction.
Two things about these blips carry the diagnosis: their shape and their recruitment. Shape, the size and complexity of each blip, reflects how many muscle fibres are packed into the unit. After a nerve injury, surviving nerves sprout to adopt orphaned muscle fibres, so the units grow large and complex, big, long, jagged potentials, the signature of a healed or chronic nerve problem. In muscle disease the muscle fibres themselves are sick and dropping out, so units become small, brief, and broken-up. Recruitment is about teamwork: normally, as you push harder, more units join in smoothly. After nerve loss there are fewer units, so the survivors must fire faster to do the work (reduced recruitment); in muscle disease the units are intact but each is weak, so many fire even for a gentle effort (early or full recruitment).
Reading motor unit shape and recruitment together is how needle EMG tells a nerve problem from a muscle problem, the single most important distinction the study makes. Large units firing too fast say neuropathic; small units firing too readily say myopathic. Combined with the spontaneous activity and the nerve conduction numbers, this lets the examiner not just confirm a problem but name its nature and rough age, the heart of an electrodiagnostic answer.
A patient who recovered partially from a nerve injury a year ago shows large, long, jagged motor unit potentials that fire rapidly even on light effort, because surviving nerves took over the orphaned fibres and there are now too few units to share the load. The shape and recruitment together read as a chronic, partly reinnervated nerve injury, not an active new one.
Large units recruiting too fast mark a chronic, reinnervated nerve injury.
Recruitment can only be judged if the patient can actually contract the muscle; pain or unwillingness to push gives a falsely sparse pattern that can be mistaken for nerve loss, so the examiner must confirm the effort is genuine.