Reflex origin of parkinsonian tremor

The 8-Hz wrist tremor seen in normal subjects results from an oscillation in the spinal stretch reflex arc but the resting 4-Hz tremor of Parkinson's disease is believed to result from synchronization of motor unit activity by periodic descending inputs driven by an oscillator which resides within the brain. Accelerometer and smoothed EMG (0.8 to 16.0-Hz pass) recordings of resting tremor were taken from the upper limbs of 10 volunteers with Parkinson's disease for several different limb positions and while the limb was fixed to prevent tremor movements. The smoothed EMG and accelerometer records produced a complex periodic waveform with prominent 4- and 8-Hz components. Spectral analysis of both records produced large peaks at those frequencies which were harmonically related. The appearance of the regular tremor waveform in accelerometer and smoothed EMG records was greatly altered by changes in limb posture in all patients. Fixing of the shoulder and elbow joints only, also altered the smoothed EMG waveform and reduced the tremor amplitude. Fixing of the entire limb removed all signs of synchronization of motor unit activity in raw and smoothed EMG records. Similarly, the prominent 4- and 8-Hz peaks, found in the smoothed EMG power spectra from trembling muscles, were eliminated if the limb was effectively prevented from trembling. These experiments showed that the synchronization of motor unit activity at Parkinson's tremor frequency is wholly dependent on the oscillation in limb position and thus proprioceptive reflex activity. It is suggested that the known properties of the 4-Hz resting tremor of Parkinson's disease can be attributed to a flip-flop oscillation involving the mutually inhibitory connections between the spinal stretch reflexes of antagonist muscles. The supraspinal contribution to the tremor may thus be confined to an "aperiodic" descending facilitation of spinal reflex pathways.