Rapid development and plasticity of layer 2/3 maps in rat barrel cortex invivo

Cortical synaptic circuitry develops rapidly in the second postnatal week, simultaneous with experience-dependent turnover of dendritic spines. To rel ate the emergence of sensory maps to synaptogenesis, we recorded synaptic p otentials evoked by whisker deflection in layer 2/3 neurons from postnatal day (P) 12 to 20. At P12, synaptic responses were undetectable. Only 2 days later in life (P14), receptive fields had mature organization. Sensory dep rivation, if initiated before P14, disrupted receptive field structure. In layer 4, responses and maps were already mature by P12 and insensitive to d eprivation, implying that barrel cortex develops from layer 4 to layer 2/3. Thus, P12-14 is a critical period shared by layer 2/3 synapses and their s pines, suggesting that spine plasticity is involved in the refinement of ma ps.