Comparison of corticomotoneuronal EPSPs and macro-MUPs in amyotrophic lateral sclerosis

We correlated the size of the corticomotoneuronal excitatory postsynaptic potential (CM-EPSP) arising in a single spinal motor neuron with the function of the target motor unit as measured by conventional and macro EMG in early amyotrophic lateral sclerosis (ALS). Macro motor unit potentials (macro-MUPs) were recorded from a surface electrode after spike-triggered averaging in the extensor digitorum communis muscle. The size of the CM-EPSP projecting to the same motor unit was measured from changes in the firing probability of single motor units induced by transcranial magnetic stimulation using peristimulus time histograms. In controls, the amplitudes of CM-EPSPs and macro-MUPs correlated inversely, probably reflecting a lower input resistance of larger spinal motoneurons. In ALS the amplitude of macro-MUPs did not correlate with that of CM-EPSPs and one third of normal ALS motor units had a reduced or temporally dispersed CM-EPSP. The findings indicate primary dysfunction of the corticomotoneuronal projection system in ALS that is independent of functional changes of spinal motoneurons.