Characteristics of oscillatory pallidal neurons in patients with Parkinson's disease

Excessive neuronal activity in the globus pallidus internus (GPi) is believed to promote parkinsonian akinesia/bradykinesia, but not tremor. Parkinsonian tremor is thought to result from dysfunction in the basal ganglia and cerebello-thalamo-cortical circuits. Whether the GPi is involved in tremorgenesis has not been fully elucidated. This study was designed to quantify the characteristics of oscillatory GPi neurons in patients with Parkinson's disease.

Nine patients undergoing surgery were studied. Microelectrode recordings in the GPi and electromyographic (EMG) activity in the limbs were recorded and the mean spontaneous firing rates (MSFRs) were calculated. Spectral analysis was used to assess neuronal oscillatory patterns. Coherence analysis was applied to explore the relationship between oscillatory neurons and EMG.

Of 79 GPi neurons, 50.6% oscillated at the tremor frequency; 25.3% oscillated at β frequency, and 24.1% did not oscillate. The MSFR of all neurons was 81.5 ± 7.4 spikes/s. Among neurons oscillating at tremor frequency, 40% were coherent with the tremor. In four neurons, the pattern changed from tremor frequency to β frequency or vice versa. It appeared that the tremor began before the GPi fired bursts.

Some neuronal activity in the GPi correlates with tremor and this correlation might be due to either feedback, maintenance, or initiation. Since there were examples of EMG tremor prior to GPi activity, initiation seems least likely. The data further support the prediction of the classic pathophysiology model of Parkinson's disease.