Additional Brain Stem Cortical Loops

A series of anatomical loops have been identified that are involved in the genesis of myoclonus, presumably through transmission of aberrant electrical discharges. A cortical loop has been proposed involving diffuse areas of the cortex, but predominantly the sensorimotor area, the pyramidal tract, medial lemniscus, and thalamus, with return fibers to the cortex. Complementing and interacting with the cortical loop is the spino-bulbar-spinal reflex, which primarily involves the brain stem reticular formation. Sensory impulses entering the spinal cord ascend bilaterally within the spinoreticular pathway, project to the nucleus reticularis gigantocellularis of the medial medullary reticular formation, descend within the reticulopsinal tract, and eventually terminate on interneurons at various spinal cord levels influencing alpha motor neurons. In several instances of myoclonus, involvement of nuclei and pathways involving the cerebellum has also been identified. Specifically, the network connecting the red nucleus, dentate nucleus, and the inferior olivary nucleus (triangle of Guillain and Mollaret) has been directly implicated as playing a role in rhythmical palatal myoclonus. In other clinical settings, myoclonus can be associated with damage to the dentate nucleus, superior cerebellar peduncles, and the spinocerebellar pathways, with preservation of the red nucleus and olives. Finally, a spino-spino loop with involvement of the flexor reflex afferents has been implicated in the generation of rhythmical myoclonus that occurs in a spinal segmental distribution. In contrast, repetitive, nonrhythmical jerks resulting in flexion at the trunk, hips, knees and neck may be produced by abnormal electrical activity generated within the cord and transmitted via the propriospinal system.

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