Cortical myoclonus: an insight into the organization of normal movement in man.
The study of cortical myoclonus affords an insight into both the spatiotemporal organization of the human sensorimotor cortex and the nature of descending motor signals. Cortical activity tends to be rhythmic, whether in response to peripheral stimulation or intended movement. This activity leads to a series of descending motor discharges and a train of electromyographic bursts, with a frequency of around 50 Hz. Each descending motor discharge consists of a brief series of synchronous volleys, separated by 3 to 6 ms. Such a repetitive pattern of activity takes advantage of the additive facilitation seen with high-frequency inputs to the spinal motoneurone, so that movement is ensured. In health, cortical motor activity is focused and tempered by inhibitory inputs, both local and far afield. Studies in patients have highlighted two forms of inhibition, ipsilateral cortical and transcallosal, which are reduced in those cases with more extensive jerks. This lack of inhibition facilitates the transcallosal and cortical spread of myoclonic activity responsible for bilateral and generalized myoclonic jerks.