Recovery of postural control following chronic bilateral hemisections at different spinal cord levels in adult cats.

Chronic cats with double hemisections of the spinal cord, first at a lower thoracic level followed by a contralateral midthoracic cord at intervals of 0 to 126 days (T-T preparations) or first at an upper cervical followed by a midthoracic at intervals of 15 to 74 days (C-T preparations), eventually recovered quadrupedal standing 7 to 53 days after the second hemisection. For about 7 days following the first hemisection at a lower thoracic level, floor reaction force (FRF) of the hind limb of the hemisected side decreased to 25 to 30% of the normal value, then recovered to the control value. A group of cats whose second hemisection was done within 7 days after the first hemisection needed 24 to 53 (mean 43) days to recover quadrupedal standing, whereas cats whose second hemisection occurred after 10 to 126 days needed 7 to 22 (mean 15) days. During the recovery period many unusual reflexes were elicited which eventually disappeared as the cats resumed standing and walking. Lateral stability of the double-hemisected cats deteriorated significantly, whereas segmental reflexes were augmented. These results indicate the importance of descending impulses over the segmental motoneuron pools to control standing posture and locomotion. It was assumed that the descending impulses were conveyed by polysynaptic pathways which had minimal functions before the hemisections.