Figure 8. Hypothetical representation of propriospinal and direct bulbospinal systems involved in descending transmission of the locomotor command signal.

A, the present data suggest that propriospinal neurons alone may be sufficient to mediate descending activation of locomotor networks. The range of projection length of individual locomotor-related propriospinal neurons is unknown. In addition to the propriospinal system, some bulbospinal axons or their collaterals may activate contralateral locomotor-related neurons via commissural projections. B, there is no direct evidence that locomotor-related bulbospinal projections can cross the midline in the spinal cord and continue descending long distances, without an intervening synapse, on the contralateral side; however, this possibility is not absolutely excluded.