Clinical neurophysiology of akinesia

Akinesia refers to failure of willed movement to occur, and bradykinesia refers to slowness of movement that is ongoing. One mechanism of bradykinesia is failure to energize muscles up to the level necessary to complete a movement in a standard amount of time. Akinesia may occur for two possible reasons. One is that the movement is so slow (and so small) that it cannot be seen. A second is that the time needed to initiate the movement becomes excessively long; this can be studied by evaluation of reaction time. One simple factor in prolongation of reaction time is present in patients with rest tremor, who appear to have to wait for a beat of tremor in the agonist muscle of the willed movement in order to initiate the movement. Reaction time studies in patients with Parkinson's disease demonstrate that simple reaction time is delayed, while choice reaction time is normal. Additionally, there does not appear to be any slowness of thinking or difficulty with storage of a motor program. Hence, the difficulty with reaction time in these patients appears to be the time that it takes to execute a motor program. Studies with magnetic stimulation of the motor cortex during the reaction time period seem to support this hypothesis. Slowness of activation of the motor cortex to trigger a movement may well be analogous in mechanism to the slowness of bradykinesia.