Who is who after spinal cord injury and repair? Can the brain stem descending motor pathways take control of skilled hand motor function?

Abstract

Over the last years, anatomical, electrophysiological and genetic studies have carefully dissected the pathways connecting the brain and the spinal cord. Lawrence and Kuypers (1968) described the organization of the descending motor pathways in the non-human primate spinal cord. Although there are some differences between species regarding the precise anatomical location of each spinal pathway and the selective connectivity onto spinal interneurons and motoneurons, the pattern of organization described is conserved among the mammalian spinal cord (Courtine et al., 2007). Based on their description, the major descending motor pathways are grouped depending on their anatomical origin and their terminal distribution pattern in the spinal grey matter. The motor cortex projects corticospinal axons to the spinal cord, which mostly run in the contralateral cord and innervate the mid and dorsal grey matter neurons. On the other hand, the spinal pathways originating in the brain stem are subdivided in the lateral and ventromedial systems. The ventromedial system fibers originate in the reticular formation and the vestibular complex, and terminate in the ventral and medial parts of the ventral horn grey matter. In contrast, the lateral system fibers originate in the red nucleus and preferentially terminate in the dorsal and lateral parts of the dorsal and medial grey matter (Figure 1). Together with the corticospinal fibers, the lateral brain-stem system contacts interneurons related to motorneurons of distal muscles, whereas on the other hand, the ventromedial system pathways contact interneurons related to motoneurons of proximal muscles. Functionally, the corticospinal and lateral brain stem pathways are involved in the control of distal arm and hand muscles involved in skilled forelimb movements. The ventral brainstem pathways are mostly involved in the control of the proximal musculature of the trunk and limbs involved in posture and locomotion. Behavioral and electrophysiological studies mostly performed in cats and non-human primates corroborate these anatomical findings (Lemon, 2012). Furthermore, rodents with injuries in discrete areas of the spinal cord selectively impair specific motor functions, evidencing the functional specificity of each spinal pathway and the spinal networks that they innervate (Schucht et al., 2002; Anderson et al., 2007)