Foundations · Beginner

Start with the motor unit

Motor unit & electrophysiology

Identify whether you are recording a nerve response, a muscle response or one motor unit.

3 learning sectionsCompiled 2026-09-08English edition · Traditional Chinese available

01Three different signals

  • A SNAP sums synchronized sensory-fiber responses and is commonly measured in µV.
  • A CMAP sums muscle-fiber responses after motor-nerve stimulation and is commonly measured in mV.
  • A MUAP represents activity from one motor neuron and its muscle fibers within the needle's recording field.

02From signal to pathology

  • Amplitude, latency, velocity and duration describe different features; none captures the whole pathology alone.
  • NCS predominantly tests large myelinated fibers. Needle EMG assesses the motor system; routine testing does not directly measure small-fiber function.

03A useful sequence

  • Ask about location first, then axonal or myelin features, timing and severity.
  • Support each inference with the history, examination and multiple electrophysiological findings.

Pause and explain

Explain in your own words: “Identify the source of the signal before interpreting its numbers.” Then think of a situation in which it could be misinterpreted.

Show a review prompt

A low CMAP may reflect axonal loss, inadequate stimulation, NMJ dysfunction or muscle disease.

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References and further reading

Educational synthesis; independent neuromuscular specialist review is pending. Publication dates and compilation dates are distinct. This is an original teaching synthesis; consult the source for complete methods and criteria.