Spontaneous movements: Effect of denervation and relation to the adaptation of nociceptive withdrawal reflexes in the rat

Spontaneous movements are a ubiquitous phenomenon during development. Recently, we demonstrated that these movements play a key role in the functional adaptation of spinal reflex circuits. Here, we analyse the role of afferent input in the generation of spontaneous movements and characterize the occurrence of different types of spontaneous movements and their relation to the functional adaptation of the nociceptive withdrawal reflexes (NWR) up to postnatal day 22 (P22). Noxious thermal stimulation was used to evoke reflex responses in awake rats. Spontaneous tail movements occurring during active sleep were counted during the first three postnatal weeks and classified into two major classes: simple movements (unidirectional) and complex twitches (bi-directional and oscillating). All spinal nerves caudal to L2 were cut at P12 to study the effect of deafferentation on spontaneous movements. The number of simple movements and complex twitches in the deafferented animals did not differ as compared to control animals. The adaptation of tail NWR occurred during the period P7–P22. Spontaneous tail movements occurred relatively frequently and overlapped in time the adaptation of NWR. Notably, the relative number of simple movements increased in parallel with the functional adaptation of the NWR, suggesting a role of simple twitches in NWR adaptation. The present findings indicate that the spontaneous movements studied are driven by intrinsic mechanisms in the CNS and suggest that sensory feedback does not influence the spontaneous movement patterns. Moreover, the NWR adaptation appears to be related to a qualitative change in spontaneous movements caused by maturation in spinal reflex circuit connections.