Extensive motor skill training induces reorganization of movement representations and synaptogenesis within adult motor cortex. Motor skill does not, however, develop uniformly across training sessions. It is characterized by an initial fast phase, followed by a later slow phase of learning. How cortical plasticity emerges during these phases is unknown. Here, we examine motor map topography and synapse number within rat motor cortex during the early and late phases of motor learning. Adult rats were placed in either a skilled or unskilled reaching condition (SRC and URC, respectively) for 3, 7, or 10 d. Intracortical microstimulation of layer V was used to determine the topography of forelimb movement representations within caudal forelimb area of motor cortex contralateral to the trained paw. Quantitative electron microscopy was used to measure the number of synapses per neuron within layer V. SRC animals showed significant increases in reaching accuracy after 3, 7, and 10 d of training. In comparison with URC animals, SRC animals had significantly larger distal forelimb representations after 10 d of training only. Furthermore, SRC animals had significantly more synapses per neuron than URC animals after 7 and 10 d of training. These results show that both motor map reorganization and synapse formation occur during the late phase of skill learning. Furthermore, synaptogenesis precedes map reorganization. We propose that motor map reorganization and synapse formation do not contribute to the initial acquisition of motor skills but represent the consolidation of motor skill that occurs during late stages of training.

Recovery of motor function following stroke is believed to be supported, at least in part, by functional compensation involving residual neural tissue. The present study used a rodent model of focal ischemia and intracortical microstimulation (ICMS) to examine the behavioral and physiological effects of cortical stimulation in combination with motor rehabilitation. Adult rats were trained to criterion on a single pellet reaching task before ICMS was used to derive maps of movement representations within forelimb motor cortex contralateral to the trained paw. All animals then received a focal ischemic infarct within the motor map. A cortical surface electrode was implanted over the motor cortex. Low levels of electrical stimulation were applied during rehabilitative training on the same reaching task for 10 days and ICMS used to derive a second motor map. Results showed that both monopolar and bipolar cortical stimulation significantly enhanced motor recovery and increased the area of cortex from which microstimulation movements could be evoked. The results demonstrate the behavioral and neurophysiological benefits of cortical stimulation in combination with rehabilitation for recovery from stroke.

No abstract

No abstract

The functional organization of adult cerebral cortex is characterized by the presence of highly ordered sensory and motor maps. Despite their archetypical organization, the maps maintain the capacity to rapidly reorganize, suggesting that the neural circuitry underlying cortical representations is inherently plastic. Here we show that the circuitry supporting motor maps is dependent upon continued protein synthesis. Injections of two different protein synthesis inhibitors into adult rat forelimb motor cortex caused an immediate and enduring loss of movement representations. The disappearance of the motor map was accompanied by a significant reduction in synapse number, synapse size, and cortical field potentials and caused skilled forelimb movement impairments. Further, motor skill training led to a reappearance of movement representations. We propose that the circuitry of adult motor cortex is perpetually labile and requires continued protein synthesis in order to maintain its functional organization.