Changes in the recruitment pattern of single motor units in the blink reflex of patients with parkinsonism and hemiplegia.

The recruitment pattern of single motor units (SMUs) was studied in the early (R1) and late (R2) blink reflex components in normal subjects (15), patients with parkinsonism (10) and with hemiplegia due to hemispheral lesions (5). Reflexes were evoked by constant current stimuli applied to the supraorbital nerve. SMU discharges were recorded in the preseptal part of the lower eyelid using a bipolar needle electrode. Thresholds of R1 and R2, latencies at the thresholds and the number of discharges in R2 were determined. In parkinsonism, the recruitment of SMUs in R1 was impaired, suggesting that the malfunction of the basal ganglia in this disorder is associated with a reduced excitability of neurons in the pontine brain-stem. In hemiplegia, the recruitment in both R1 and R2 could be impaired. The orderly function of neurones in the pontine and medullary pathways of these components appears to require facilitatory hemispheral influences. Signs of disinhibition occasionally found in R1 may point to an imbalance between facilitatory and inhibitory hemispheral influences upon the pontine pathway.