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NEUROLOGY 1996;47:1555-1561
© 1996 American Academy of Neurology

Reduced corticomotoneuronal excitatory postsynaptic potentials (EPSPs) with normal Ia afferent EPSPs in amyotrophic lateral sclerosis

M. Nakajima, MD, A. Eisen, MD, FRCP(C), R. McCarthy, MD, R. K. Olney, MD and M. J. Aminoff, MD

From the Neuromuscular Diseases Unit (Drs. Nakajima, and Eisen), Vancouver General Hospital and the University of British Columbia, Vancouver, British Columbia, Canada; and the Department of Neurology (Drs. McCarthy, Olney, and Aminoff), School of Medicine, University of California at San Francisco, San Francisco, CA.
Address correspondence and reprint requests to Dr. Andrew Eisen, The Neuromuscular Diseases Unit (EMG), Vancouver General Hospital, 855 West 12th Avenue, Vancouver, BC, V5Z 1M9, Canada.
Received February 29, 1996. Accepted in final form April 19, 1996.

We studied excitatory postsynaptic potentials (EPSPs) arising in single spinal motoneurons (composite EPSPs) induced by Ia afferent and magnetic cortical stimulation in 28 normal subjects ranging in age from 24 to 84 years and 28 patients with amyotrophic lateral sclerosis (ALS) aged 34 to 82 years.The subjects voluntarily recruited single motor units of the first dorsal interosseous muscle. Using peristimulus time histograms, we determined changes in the firing probability of the first dorsal interosseous motor units and measured the magnitude of the EPSP. An early period of increased firing probability (primary peak) occurred at approximately 30 msec after la afferent and 25 msec after cortical stimulation, reflecting underlying EPSPs arising in spinal motoneurons induced by either projection. The latency of the primary peaks for both Ia afferent and cortical stimulation was mildly prolonged in ALS, suggesting a loss of the fastest-conducting spinal motoneurons. Patients with ALS had la afferent-driven EPSPs whose amplitude and rise time were equivalent to those of normal subjects. However, the ratio of cortical to la afferent-driven composite EPSPs in ALS was significantly lower than that for normal subjects. Fourteen of 28 ALS motor units had cortically driven EPSPs that were small or large only because of a prolonged rise time. The findings suggest that in ALS, corticomotoneuronal attrition or dispersion of the descending volley occurs in the presence of normally functioning spinal motoneurons to which they project.

NEUROLOGY 1996;47: 1555-1561

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