Plasticity of the Synaptic Modification Range

Activity-dependent synaptic plasticity is likely to provide a mechanism for learning and memory. Cortical synaptic responses that are strengthened within a fixed synaptic modification range after 5 days of motor skill learning are driven near the top of their range, leaving only limited room for additional synaptic strengthening. If synaptic strengthening is a requisite step for acquiring new skills, near saturation of long-term potentiation (LTP) should impede further learning or the LTP mechanism should recover after single-task learning. Here we show that the initial learning-induced synaptic enhancement is sustained even long after training has been discontinued and that the synaptic modification range shifts upward. This range shift places increased baseline synaptic efficacy back within the middle of its operating range, allowing prelearning levels of LTP and long-term depression. Persistent synaptic strengthening might be a substrate for long-term retention in motor cortex, whereas the shift in synaptic modification range ensures the availability for new synaptic strengthening.