The human extrapyramidal system.

The term "extrapyramidal" originally described a collection of tegmentospinal pathways that conveyed the propagation of cortically-induced seizures to the spinal cord of dogs with bilateral transection of the pyramidal tracts. The extrapyramidal concept developed into unwarranted avenues to include forebrain structures involved in motor control, such as the basal ganglia, or as a broad synonym of clinical syndromes characterized by nonparalytic abnormal involuntary movements and postures. Clinicians and pathologists assumed that, similarly to mammals in general, the human extrapyramidal system was also organized in parallel with the pyramidal tracts. Several inconsistencies of this model, as applied to humans, were overlooked despite compelling clinicoanatomic evidence showing that (i) bilateral damage to the pyramidal tracts at any level from the pontomedullary transition to the motor cortex produces the locked-in syndrome and (ii) the extrapyramidal tracts are sparse in man. In the present essay I advance the hypothesis that the human extrapyramidal system is fundamentally different from its mammalian counterparts both anatomically and functionally. To test this hypothesis, a systematic analysis of the residual motor patterns observed on a natural model of a bilateral section of the human pyramidal tracts, the locked-in syndrome, is provided. This analysis reveals that the human extrapyramidal system underpins the organization of the following categories of motor synergies: (i) oculofacial and oculocephalic, (ii) faciorespiratory, (iii) axial-appendicular, and (iv) plurisegmental. Anatomically, these functional repertoires of motor integration are mediated by six collections of axons defined by their brainstem nuclei of origin and spinal cord destinations: reticulospinal (medial and lateral), vestibulospinal (medial and lateral), rubrospinal (lateral only), and tectospinal (medial only) fiber systems. I conclude that the extrapyramidal concept can reliably be extended to humans, albeit its physiological and anatomical scope is considerably narrower than traditionally assumed.