Brainstem myoclonus.

In brainstem myoclonus, muscle jerks arise from activity in neuronal centres 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-bulbo-spinal reflex. In man, the jerks are generalized with long-lasting electromyographic bursts (greater than 100 ms), and may be provoked by many different 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 efferent motor pathway. The cranial nerve nuclei are activated by rapid spread up the brainstem. Other forms of brainstem myoclonus may also exist, and these are characterized by more rapid conduction in spinal efferent pathways and greater sensitivity to muscle stretch rather than cutaneous input.