A single motor unit study of spinal cmd brainstem motor neuron in ALS

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The pathophysiology of corticospinal tract degeneration in amyotrophic lateral sclerosis (ALS) was investigated by studying the effect of transcranial magnetic stimulation on discharge characteristics of single motor units during voluntary activation. The occurrence of motor unit discharges from first dorsal interosseus muscle (FDI) was plotted in a peristimulus time histogram (PSTH) and evaluated the excitability of the motor neurons. For the analyses, we developed new computer programs. Next, we investigated the synaptic dysfunction of facial nucleus by trigeminal electrical stimulation using the same analysis method.PSTHs from FDI muscle revealed double primary peaks (PPs) in 6 out of 16 motor units (38%) in ALS with upper motor neuron sign as compared to only 2 out of 16 (13%) motor units in multiple sclerosis or cerebrovascular disease with upper motor neuron sign. None of the patients with lower motor neuron diseases or ALS without upper motor neuron sign had these abnormalities. These findings suggest typical ALS has abnormal hyperexcitable state of slow or polysynaptic corticospinal neurons.Trigeminal electrical stimulation produced trigemino-facial reflex. PSTH of the reflex from normal subjects showed an early short excitation (P1 : 10 msec) and a late dispersed one (P2 : 30 msec) with prolonged duration. That from 10 ALS patients revealed highly synchronized P1 peak with normal latency. These findings indicated facial nucleus of ALS had the abnormal hyperexcitability.Both of these findings suggest that the hyperexcitable state of motoneuron is a cardinal feature in the pathopysiology of ALS.