Changes in motoneuron excitability of masseter muscle following exteroceptive stimuli in Parkinson's disease

Changes in motoneuron activity of masseter muscle to exteroceptive stimuli were evaluated in parkinsonian patients. Two different electrophysiological procedures were applied, consisting of exteroceptive suppression of the masseter or excitability curves of the masseteric reflex obtained by using exteroceptive conditioning stimuli. Seven patients not yet treated with dopaminergic or anticholinergic drugs were compared with 10 age-matched normal volunteers. Only the second phase of exteroceptive suppression was examined because correct measurement of the first phase was impossible due to the stimulus artefact. No significant differences were observed in exteroceptive suppression between parkinsonian patients and normals. Early and late inhibitory phases of the excitability curve of the masseteric reflex were obtained in both normals and patients. However, parkinsonians showed less inhibitory change than normals in both early and late phase of the curve. This study confirms that reduced inhibition of the masseteric reflex to exteroceptive stimuli is present in Parkinson's disease. The excitability curve of the masseteric reflex represents a more reliable method than exteroceptive suppression in detecting these abnormalities.