Volume 34, Issue 1 pp. 34-43
Main Articles

The stimulus–response curve and motor unit variability in normal subjects and subjects with amyotrophic lateral sclerosis

R. D. Henderson MB, BS

Corresponding Author

R. D. Henderson MB, BS

Department of Neurology, Royal Brisbane and Women's Hospital, Brisbane, Queensland 4029, Australia

Department of Neurology, Royal Brisbane and Women's Hospital, Brisbane, Queensland 4029, AustraliaSearch for more papers by this author
G. R. Ridall PhD

G. R. Ridall PhD

School of Mathematical Sciences, Queensland University of Technology, Brisbane, Australia

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A. N. Pettitt PhD

A. N. Pettitt PhD

School of Mathematical Sciences, Queensland University of Technology, Brisbane, Australia

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P. A. McCombe PhD

P. A. McCombe PhD

Department of Neurology, Royal Brisbane and Women's Hospital, Brisbane, Queensland 4029, Australia

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J. R. Daube MD

J. R. Daube MD

Department of Neurology, Mayo Clinic, Rochester, Minnesota, USA

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First published: 21 April 2006
Citations: 49

Abstract

The behavior and stability of motor units (MUs) in response to electrical stimulation of different intensities can be assessed with the stimulus–response curve, which is a graphical representation of the size of the compound muscle action potential (CMAP) in relation to stimulus intensity. To examine MU characteristics across the whole stimulus range, the variability of CMAP responses to electrical stimulation, and the differences that occur between normal and disease states, the curve was studied in 11 normal subjects and 16 subjects with amyotrophic lateral sclerosis (ALS). In normal subjects, the curve showed a gradual increase in CMAP size with increasing stimulus intensity, although one or two discrete steps were sometimes observed in the upper half of the curve, indicating the activation of large MUs at higher intensities. In ALS subjects, large discrete steps, due to loss of MUs and collateral sprouting, were frequently present. Variability of the CMAP responses was greater than baseline variability, indicating variability of MU responses, and at certain levels this variability was up to 100 μVms. The stimulus–response curve shows differences between normal and ALS subjects and provides information on MU activation and variability throughout the curve. © 2006 Wiley Periodicals, Inc. Muscle Nerve, 2006

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