The cholinergic system-dependent myoclonus of the baboon Papio papio is a reticular reflex myoclonus

Neurophysiological studies were performed on four Papio papio baboons presenting with nonepileptic myoclonus (a startle response resembling stimulus-sensitive jerk). Investigations of the EEG, back-averaged EEG, and somatosensory evoked potentials revealed the absence of cortical correlates preceding the jerks, and exclusion of cerebral cortex involvement. No long-latency reflexes could be recorded in these animals. The jerks were symmetric when evoked by unilateral stimulation in normal baboons as well as in a split-brain animal. Polymyographic records showed that the first muscle involved during the jerk was the trapezius; other muscles were involved with latencies increasing in both cranial and caudal directions. From these data, nonepileptic myoclonus of baboons can be classified as a reticular reflex myoclonus. The involvement of cranial nerves did not follow the layout of the nuclei in the brainstem, indicating that the jerk is most likely generated as a complete movement. The generating structure is probably under cholinergic control. Finally, the Papio papio baboon, which was already known as a model for cortical myoclonus elicited by intermittent photic stimulation in predisposed animals, can also be considered a model for the study of the reticular reflex myoclonus.