Postnatal maturation of forelimb motor cortex and its impact on behavior in rats

The motor cortex expands postnatally in several species; however, the timeline of body representation development and its impact on hand movements remain unclear. We investigated the relationship between forelimb motor cortex maturation and forelimb behavior in 50 male Wistar rats aged 4 to 8 weeks. Intracortical microstimulation (ICMS) was used to examine motor representations in the motor cortex, and motor-evoked potentials (MEPs) were recorded from the wrist dorsiflexors. Grip strength and kinesiological features of pellet-reaching tasks were assessed weekly, and wrist dorsiflexor morphology was evaluated. The forelimb motor cortex expanded between weeks 4 and 5, followed by reorganization of its internal representations. During this period, MEP amplitude, the MEP/maximal evoked potential ratio, and muscle fiber diameter increased, while ICMS thresholds decreased. ICMS-evoked movements progressed from wrist dorsiflexion and elbow flexion at 4 weeks, to digit flexion at 5 weeks, and then to digit and shoulder extension at 6 weeks. Reaching performance and grip strength improved weekly and stabilized by 6 weeks. To determine the relationship between motor area expansion and forelimb motor development, an electric microlesion made in the expanded area at 8 weeks of age, 6 weeks after expansion, regressed forelimb motor function to the 6-week level. In contrast, damage to the original area led to impaired fundamental forelimb motor function. These findings indicate that postnatal expansion of the forelimb motor cortex, and strengthening of corticospinal synapses, are crucial for the maturation of skilled limb movements, highlighting the importance of cortical plasticity in the development of motor control.