Neurophysiological basis of functional recovery in the neonatal spinalized rat

It had been shown previously that, following spinalization of the neonatal rat on postnatal day 7 (PN7), at the middle thoracic level, there was spontaneous recovery of coordinated stepping in the hindlimbs, enabling the animal to execute quadrupedal locomotion, with an ataxic gait. No significant recovery occurred in rats that were similarly spinalized on PN14. Despite the functional recovery in the PN7 group, their hindlimbs were paralyzed when not in contact with a surface. In the present experiments, at 16-18 weeks after spinalization, muscle spindle GpIa and cutaneous afferents were tested for functional connectivity to the alpha motoneurons (a-MNs) that innervate the right triceps surae (TS) muscles. The Hoffmann reflex (H-reflex), the tonic stretch reflex (TSR), and cutaneous reflexes were recorded from the right TS muscles in the nonanesthetized, intercollicular decerebrate preparation. The H-reflex and the TSR were readily elicited from the PN7 animals, but not from the PN14 animals. The PN14 preparations were characterized by prolonged (> 18 h in 3/8 cases), spontaneous discharge of motor units, and prominent M responses. There was widespread, bilateral convergence of cutaneous afferents from the hindquarters to the a-MNs of the TS muscles in both the PN7 and the PN14 preparations. In the nonspinalized, control preparation, only ipsilateral, cutaneous afferents activated the right TS a-MNs. These results demonstrate that in the chronic (> 3 months after spinalization), spinalized PN7 rat, but not in the PN14 rat, there is a tight functional connectivity between the hindlimb GpIa afferents and their homonymous a-MNs.