Review of descending motor pathways involved with transcranial stimulation

Abstract

The anatomical basis of the pyramidal tract is reviewed with respect to its proposed role in the conduction of the motor evoked potential. The fiber diameter profiles are discussed in relation to the measured conduction velocities of the corticospinal tract in humans. Stimulus parameters utilized to obtain the motor evoked potential are reviewed in relation to the laterality of response, response threshold, and properties of spatial and temporal summation. A discussion of the major descending tracts involved with walking as opposed to fine distal use of the digits is undertaken in the context of the possible prognostic capabilities of the motor evoked potential.

Different Characteristics of the Corticospinal Tract According to the Cerebral Origin: DTI Study

“…[1][2][3][4][5] The CST is known be involved mainly in movement execution of distal extremities, particularly fine-motor activities of the hand. [1][2][3][4][5] The CST originates from various cortical areas, such as the secondary motor area, the parietal cortex, and the primary motor cortex. The multiple cerebral origins of the CST appear to be important in terms of multiple functions of CST fibers and motor recovery mechanisms: perilesional reorganization following M1 injury.…”

Can stroke patients walk after complete lateral corticospinal tract injury of the affected hemisphere?

“…Previous animal studies [1][2][3]15] indicate that several possible motor pathways are responsible for the ability of these patients to walk, which include the crossing motor pathway from the lateral CST of the intact hemisphere. On the other hand, the ability in primates to take steps seems to require the sparing of at least one ventrolateral quadrant of the spinal cord, which allows the return of bilateral hindlimb locomotor activity [16].…”

“…It has been observed in the cat that bilateral sectioning of the ventrolateral quadrants of the cord prevents controlled evoked locomotion. Thus, the potential descending motor pathways that play a role in walking are the reticulospinal, vestibulospinal, and anterior CST pathways that lie in the ventrolateral quadrant of the cord [1]. Among these tracts, the role of the anterior CST pathway is not a feasible possibility, because it has been suggested that this tract in humans rarely descends below the upper thoracic portion of the spinal cord and that it primarily innervates the musculature of the neck, trunk, and proximal portions of the upper extremities [2].…”

“…Two separate CSTs exist. The largest is the lateral CST which is formed by 75-90% of CST fibers crossing the medulla [1][2][3], and the other is the anterior CST, which does not cross at the medullary decussation. The main function of the lateral CST is to control the distal musculature used for fine movements.…”

“…The main function of the lateral CST is to control the distal musculature used for fine movements. It is well known that stroke patients are unable to carry out fine motor activities of the hands after complete injury of the lateral CST [1][2][3]. The influence of such an injury on walking, however, is uncertain.…”