Parkinson's disease tremor: pathophysiology.

There are a number of tremors that may affect patients with Parkinson's disease, but the classic is tremor-at-rest. The tremor is also seen during postural action after a short pause, and is often called re-emergent tremor although it appears that the physiology is the same. As a manifestation of Parkinson's disease, it is separate from bradykinesia and rigidity as the magnitude of tremor is not related to dopamine deficiency nor does it respond readily to dopamine treatment. Cellular activity in the different basal ganglia nuclei can be coherent with tremor, but cellular activity in the VIM nucleus of the thalamus, a cerebellar relay nucleus, is more coherent than cellular activity in basal ganglia. It is also notable that different body parts may have similar tremor frequencies, but are generally not exactly the same and are not phase locked. This suggests that each body part has a separate tremor generator. The ability to stay separate may be due to the somatotopic segregation of basal ganglia loops. Analysis of cellular behavior in the thalamus shows that the thalamus is not the generator of tremor. New data suggest that the basal ganglia trigger a cerebellar circuit to produce the tremor.