Maturation of fastest afferent and efferent central and peripheral pathways: no evidence for a constancy of central conduction delays.

The maturation of the fastest afferent and efferent central and peripheral pathways was analysed using the recording of somatosensory evoked potentials of the median nerve stimulation for afferent conduction and magnetoelectrical stimulation of motor cortex to the thenar muscles for efferent conduction. Both afferent and efferent central pathways showed a prolonged maturational pattern with adult values being reached by the age of 5 to 7 years for the afferent and by the age of around 10 years for the efferent pathway. In contrast, the maturation for peripheral afferent and efferent pathways showed a similar trend with a fairly constant conduction velocity reached around the age of 3 years. There was no evidence for a constancy of central conduction delays in both afferent and efferent pathway which has been supposed as a possible mechanism for a temporal stability in feedback loops involved in motor learning processes during development. It is concluded that the prolonged maturational central conduction time precludes the usage of a fixed temporal timing pattern during development in the human sensory motor system.