Systems Descending from the Brainstem: Functional Recovery Following Damage

Abstract

The descending brain stem-spinal systems could be classified largely into two major categories: ventromedial and lateral systems. The former system includes reticulospinal, vestibulospinal, tectospinal, and interstitiospinal tracts, which are mainly involved in the control of proximal muscles. The latter system includes the rubrospinal tract, which is involved in the control of distal part of the extremities. Injury to the corticospinal tract, either in the motor cortex or on the way to its target, causes severe deficit in the movements of distal extremities, especially the hand. Earlier studies showed that rubrospinal tract could compensate for the impaired function of the corticospinal tract. Recent studies focus on the capacity of the reticulospinal tract in the compensatory mechanism. The reticulospinal tract could compensate for the gross movements of the arm and hand; however, independent digit movements could not be fully compensated for by the tract. More recent studies have shown that the propriospinal neurons, which are located in the mid-cervical segments of the spinal cord and relay the corticospinal inputs to motoneurons, contributes to the compensation of individual digit movements. On the other hand, after damage to the motor cortex or corticospinal tract, the motor cortex of the intact side is activated and partly involved in the compensatory process. The compensation by the motor cortex ipsilateral to the affected extremity is mediated by the sprouting of recrossing corticospinal tract or reticulospinal tract either on the ipsilateral or contralateral side to the lesion. Furthermore, if the brain damage occurred at the early developmental period, such compensatory mechanism could accompany a large-scaled reorganization of the corticofugal pathways. Experimental studies on rats have shown that infants subjected to hemidecortication exhibit marked compensation of the impaired motor functions of the contralesional extremities by the remaining cortical hemisphere at their adult stage. In this case, descending pathways from the sensorimotor cortex of the intact side might maintain its projection to its targets in the spinal cord or in the brain stem contralateral to their normal target during development and control the ipsilateral limbs through such “aberrant” pathways.

All together, the brain stem-spinal cord pathways are involved in various aspects of compensatory process after damage to the corticospinal tract.