Fictive motor activity in rat after 14 days of hindlimb unloading.

The goal of the present study was to examine the effects of chronic hindlimb unloading on fictive motor patterns which can be developed in hindlimb nerves of adult rats. The animals were divided into two groups. The first group was submitted to hindlimb unloading for 2 weeks by tail suspension. The second group served as controls. After this initial phase, the animals of both groups were acutely decorticated, paralysed and electroneurographic efferent activity was recorded from hindlimb muscle nerves under conditions of "fictive locomotion" in order to evaluate variations in central locomotor command. Fictive rhythmic motor episodes were either spontaneous or evoked by electrical stimulation of the mesencephalic locomotor region. Only the second ones were recognised as locomotor-like activities. The motor pattern was not fundamentally affected by unloading except that, after the unloading period, extensor muscle nerves were significantly more frequently activated and their burst durations were increased compared to activity in control animals, despite the fact that the phasic sensory afferent inputs were suppressed. This suggests that unloading induces plastic modifications of the central networks of neurons implicated in the locomotor command. The origin of this extensor hyperactivity is discussed. It is proposed that it could be the consequence of either changes in motoneuronal properties or of an increase in afferent input to motoneurones.