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Related Concept Videos

Generation of Action Potential in Skeletal Muscles01:24

Generation of Action Potential in Skeletal Muscles

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Every cell in the body maintains a membrane potential due to an uneven distribution of positive and negative charges across its plasma membrane. The membrane potential is measured in millivolts and quantifies the difference in charge across the membrane.
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Motor Unit Stimulation01:20

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Neurons communicate by firing action potentials—the electrochemical signal that is propagated along the axon. The signal results in the release of neurotransmitters at axon terminals, thereby transmitting information to the nervous system. An action potential is a specific "all-or-none" change in membrane potential that results in a rapid spike in voltage.
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In skeletal muscles, acetylcholine is released by nerve terminals at the motor endplate—the point of synaptic communication between motor neurons and muscle fibers. The binding of acetylcholine to its receptors on the sarcolemma allows entry of sodium ions into the cell and triggers an action potential in the muscle cell. Thus, electrical signals from the brain are transmitted to the muscle. Subsequently, the enzyme acetylcholinesterase breaks down acetylcholine to prevent excessive...
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Related Experiment Video

Updated: Jan 5, 2026

In Vivo Electrophysiological Measurement of Compound Muscle Action Potential from the Forelimbs in Mouse Models of Motor Neuron Degeneration
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Far-field potentials in the compound muscle action potential.

Masahiro Sonoo1

  • 1Department of Neurology, Teikyo University School of Medicine, Itabashi-ku, Tokyo, Japan.

Muscle & Nerve
|October 26, 2019
PubMed
Summary

The reference electrode significantly influences compound muscle action potentials (CMAPs) in motor conduction studies (MCSs), particularly for tibial and ulnar nerves. This far-field potential necessitates a re-evaluation of standard MCS interpretation.

Keywords:
compound muscle action potentialfar-field potentialmotor unit number estimationreference electrodetibial nerveulnar nerve

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Area of Science:

  • Neuroscience
  • Electrophysiology
  • Neuromuscular Studies

Background:

  • The compound muscle action potential (CMAP) in motor conduction studies (MCSs) is traditionally believed to reflect muscle activity directly beneath the active electrode.
  • Recent research indicates a significant, previously unrecognized contribution from the reference electrode to the CMAP waveform.

Purpose of the Study:

  • To investigate the contribution of the reference electrode to the CMAP in motor-nerve conduction studies.
  • To explore the implications of this reference electrode potential on the interpretation of MCS and related electrodiagnostic techniques.

Main Methods:

  • Analysis of CMAP recordings in response to tibial and ulnar nerve stimulation.
  • Investigation of the effects of altering reference electrode placement on CMAP characteristics.
  • Theoretical consideration of far-field potential (FFP) generation mechanisms.

Main Results:

  • The reference electrode contributes a notable potential to the CMAP, especially prominent in tibial and ulnar nerve studies.
  • This reference electrode potential is characterized as a far-field potential (FFP), with interosseous muscles being key contributors to ulnar CMAP FFPs.
  • The conventional understanding of action potential termination at the muscle-tendon junction is insufficient to explain these findings.

Conclusions:

  • The compound muscle action potential (CMAP) is not solely generated by muscle activity under the active electrode.
  • The "reference electrode potential" represents a significant far-field potential (FFP) that must be considered in MCS interpretation.
  • This new understanding may refine the interpretation of MCS and advanced techniques like motor unit number estimation.