Motor Cortex Bilateral Motor Representation Depends on Subcortical and Interhemispheric Interactions

Abstract

The corticospinal tract is a predominantly crossed pathway. Nevertheless, the primary motor cortex (M1) is activated bilaterally during unilateral movements and several animal studies showed that M1 has a bilateral motor representation. A better understanding of the uncrossed corticospinal system is especially important for elucidating its role in recovery of limb control after unilateral injury. We used intracortical microstimulation (ICMS) to determine the representation of contralateral and ipsilateral forelimb joints at single M1 sites in the rat. Most sites representing an ipsilateral joint corepresented the same joint contralaterally. We next determined whether ipsilateral responses evoked in one hemisphere depended on the function of M1 in the opposite hemisphere using reversible inactivation and pyramidal tract lesion. Ipsilateral responses were eliminated when the homotopic forelimb area of M1 in the opposite hemisphere was inactivated or when the pyramidal tract on the nonstimulated side was sectioned. To determine the role of transfer between M1 in each hemisphere we sectioned the corpus callosum, which produced a 33% increase in ipsilateral ICMS thresholds. Neither M1 inactivation nor callosal section changed contralateral response thresholds, indicating the absence of tonic excitatory or inhibitory drive to the opposite M1. Finally, ipsilateral responses following M1 inactivation and pyramidal tract lesion could be evoked after systemic administration of the K ϩ channel blocker 4-aminopyridine, suggesting the presence of latent connections. Our findings show important interactions between the corticospinal systems from each side, especially at the spinal level. This has important implications for recruiting the ipsilateral corticospinal system after injury.

Progressive Motor Cortex Functional Reorganization Following 6-Hydroxydopamine Lesioning in Rats

“…Since a parallel increase was observed in the extent of motor representations, it can be concluded that recovery of M1 output occurred through corepresentation of contralateral and ipsilateral movements at the same cortical site. Indeed, ipsilateral movement representation in M1 might be an important component of recovery in hemipakinsonian rats, as after unilateral damage of the pyramidal system (Brus-Ramer et al, 2009). Moreover, consistent with previous reports that behavioral changes after unilateral nigrostriatal degeneration are not restricted to the contralateral paw (Vergara-Aragon et al, 2003;Miklyaeva et al, 2007;Woodlee et al, 2008), we found a bilateral decrease in the distal-to-proximal ratio of forelimb representations.…”

Repair of complete nerve lacerations at the forearm: An outcome study using Rosén–Lundborg protocol

“…Several authors have showed that although the main organizational principle of primate motor systems is cortical control of contralateral limb movements, motor areas also appear to play a role in ipsilateral limb movements. [48][49][50] Numerous studies have also presented evidence that after unilateral damage, the ''contralesional'' intact hemisphere plays an increased role in ipsilateral movements and that the intact contralesional hemisphere may play a maladaptive role under certain conditions. 51 Approximately 15% in the normal population should be regarded as anastomosis, connections, or neurorrhaphies between median and ulnar nerve at the forearm named as Martin-Gruber connections.…”

Targeted mini-strokes produce changes in interhemispheric sensory signal processing that are indicative of disinhibition within minutes

“…In contrast, after a large stroke, tissue with similar function may only be found at more distant sites, such as the premotor cortex (for motor cortex stroke) (8,9) or regions within the unaffected contralateral hemisphere (10) where structural remodeling can be observed (11). Although the canonical view of sensory and motor processing is that body parts are controlled by neurons in the cerebral hemisphere on the opposite side of the body, ipsilateral pathways in which, for example, the right hemisphere processes information from the right side of the body are also present (12,13) and may provide a means to recovery. Evidence suggests that stroke recovering animals and patients can exhibit widespread changes in activity patterns that can even extend to the unaffected hemisphere (14)(15)(16).…”