How Can Corticospinal Tract Neurons Contribute to Ipsilateral Movements? A Question With Implications for Recovery of Motor Functions

Abstract

In this review, the authors discuss some recent findings that bear on the issue of recovery of function after corticospinal tract lesions. Conventionally the corticospinal tract is considered to be a crossed pathway, in keeping with the clinical findings that damage to one hemisphere, for example, in stroke, leads to a contralateral paresis and, if the lesion is large, a paralysis. However, there has been great interest in the possibility of compensatory recovery of function using the undamaged hemisphere. There are several substrates for this including ipsilaterally descending corticospinal fibers and bilaterally operating neuronal networks. Recent studies provide important evidence bearing on both of these issues. In particular, they reveal networks of neurons interconnecting two sides of the gray matter at both brainstem and spinal levels, as well as intrahemispheric transcallosal connections. These may form "detour circuits" for recovery of function, and here the authors will consider some possibilities for exploiting these networks for motor control, even though their analysis is still at an early stage.

Detection of the neuronal activity occurring caudal to the site of spinal cord injury that is elicited during lower limb movement tasks

“…The activity detected is in agreement with known physiology and together with previous findings of increased contralateral activation in response to stimulation of the injured spinal cord suggest that spinal fMRI is providing reliable results. 2,[16][17][18][19] This occurrence and the implications for recovery of function in injured persons have recently come under closer investigation. 19 Investigation of neuronal circuitry caudal to a spinal cord injury site is required.…”

“…2,[16][17][18][19] This occurrence and the implications for recovery of function in injured persons have recently come under closer investigation. 19 Investigation of neuronal circuitry caudal to a spinal cord injury site is required. There are specific difficulties with this clinical population that hinder spinal fMRI progress.…”

Bilateral Activity-Dependent Interactions in the Developing Corticospinal System

“…The ipsilateral CS tract projection, a notable feature after activity blockade and in hemiplegic CP, has been implicated in recovery of motor function after stroke in maturity (for a review of underlying circuitry, see Jankowska and Edgley, 2006). This role may be greater than that of the damaged contralateral projection after large strokes (Teasell et al, 2005).…”

“…Presumably, bilateral interactions normally persist during this late period of CS system development and into maturity because sparse ipsilateral terminations are present in the monkey and cat (Martin, 1996;Lacroix et al, 2004). In addition to a direct effect of ipsilateral CS axon terminations, indirect ipsilateral effects could be mediated at multiple levels of the CS system, including the corpus callosum, ipsilateral corticoreticular projections, and spinal commissural systems (Jankowska and Edgley, 2006).…”

Development of novel devices for upper-extremity rehabilitation

“…There, it damages the corticospinal pathways which originate in the motor cortices, travel through the internal capsule, and then project to the motoneurons in the spinal cord. It is thought that movement control in humans, as opposed to other animals, is highly dependent on these direct corticospinal projections [72,52,125,91,60,126,19,116]. Their loss is difficult to compensate for, although alternative, less efficient routes exist [90].…”