The ability to correct hand movement trajectories after changes of target location is a key feature of primates' behavior. Recording neural activity during actions that require such online control allows the study of the evolution of motor intention and the coexistence of old and new motor plans in the brain. Single-unit activity has been recorded in the monkey's dorsal premotor (PMd), motor (M1) and parietal cortex (PPC) during direct and corrected reaches. In all the 3 areas and across conditions neural activity was highly correlated with hand kinematics suggesting that both corrected and unperturbed movements are subserved by the same neural mechanism. The specific role of each area mostly emerged from the temporal dynamics within the network. The signaling of the change of motor plan occurred before the hand started to move to the initial target's position. This signaling was earlier in PMd than in M1 and PPC. Thus, PMd encodes the higher-order command to update motor intention, while PPC seems responsible for a state estimate of the kinematics of the motor periphery, an essential step to allow motor cortex to modify hand trajectory. This indicates that the parieto-frontal system can update an original and not-yet-accomplished motor plan during its execution. The causal role of PPC in online control was established through muscimol inactivation that resulted in a defective online control similar to that observed in humans with Optic Ataxia.