Determinants of locomotor recovery after spinal injury in the cat.

After a spinalization at the most caudal thoracic spinal segment, the cat can recover locomotion of the hindlimbs when they are placed on a moving treadmill. This chapter summarizes some of the determinants of such a dramatic recovery of motor function. Fundamental to this recovery is undoubtedly the genetically based spinal locomotor generator, which provides an essential rhythmicity to spinal motoneurons and hence the musculature. Other factors are also important, however. Sensory feedback is essential for the correct expression of spinal locomotion because spinal cats, devoid of cutaneous feedback from the hindfeet, are incapable of plantar foot placement. The neurochemical environment also adapts to spinalization, i.e., the loss of all modulation by descending monoaminergic pathways. Post-transection spinal rhythmicity then becomes more dependent on glutamatergic mechanisms. Finally, we argue that the mid-lumbar spinal segments evolve to play a crucial role in the elaboration of spinal locomotion as their inactivation abolishes spinal locomotion. In summary, the above findings suggest that the recovery of spinal locomotion is determined by a number of factors, each of which must now be more fully understood in the ever-continuing effort to improve the rehabilitation of spinal-cord-injured subjects.