BRAIN-STEM MYOCLONUS

In brainstem myoclonus, muscle jerks arise from activity in neuronal c entres within the lower brainstem. The most common forms probably use the same circuitry as used by the normal startle reflex, which may be related to that described in animal experiments as the spino-bubo-spin al reflex. In man, the jerks are generalized with long-lasting electro myographic bursts (greater than 100 ms), and may be provoked by many d ifferent types of sensory stimuli, although cutaneous taps around the nose and face are particularly effective. The neural activity spreads down from the brainstem via a relatively slowly conducting spinal effe rent motor pathway. The cranial nerve nuclei are activated by rapid sp read up the brainstem. Other forms of brainstem myoclonus may also exi st, and these are characterized by more rapid conduction in spinal eff erent pathways and greater sensitivity to muscle stretch rather than c utaneous input. (C) 1996 Wiley-Liss, Inc.