Pathophysiology of involuntary movements in Parkinson's disease

Tremor and drug-induced dyskinesia are major involuntary movements in Parkinson's disease. The rhythm of resting tremor is 4-6 cycles/s, driven by generators in the brain, and stabilized by reflex arcs involving the spinal cord, nerves and muscles. Its frequency is fixed: it is the same in proximal and distal muscles in each case and does not change in the course of amplitude reduction of tremor by levodopa injection. An approximate doubling of frequency occurs in action tremor which suggests a central mechanism liable to produce harmonics. In postural tremor as observed in the lower limbs while on standing, the frequency of grouped discharges falls into ranges of rhythm with either resting or action tremor. Levodopa-induced dyskinesia has a similar nature to chorea in both clinical observation and EMG. With EMG choreic discharges may appear concomitant with regular parkinsonian tremors in the same muscle, suggesting that the two are not opposites as expressions of dysfunction of the dopaminergic system.