Posture and locomotion in the rat: independent or interdependent development?

In this essay, recent research into the relation between postural control and the development of walking in the rat is reviewed. The adult-like walking pattern develops at the 15th to 16th day (P15-P16). Until this age, postural control, as indicated by EMG activity in the longissimus muscle in the trunk, is poorly phased in relation to the stepcycle. After P15-P16, accuracy increases but only after P21 is the adult pattern of EMG activity in the trunk muscles in relation to locomotor activity in the hindpaw muscles established. Neuroanatomical research revealed that those muscles in trunk and extremities which fulfil important postural tasks are innervated by motoneuronal pools containing conspicuous dendrite bundles. These bundles emerge at about the age when the adult type of postural control starts to develop. As spinal transsection at P10 abolishes their development, we hypothesize that the development of dendrite bundles is dependent on the ingrowth or the becoming functional of descending projections. On the basis of the neurophysiological and neuroanatomical results we conclude that the development of postural control, which reaches its final stage 5-6 days after the adult-like pattern of walking has developed, is dependent on the maturation of suprasegmental structures and their spinal projections.