Spread of dendritic excitation in layer 2/3 pyramidal neurons in rat barrel cortex in vivo

In layer 2/3 pyramidal neurons of barrel cortex in vivo, calcium ion concen tration ([Ca2+]) transients in apical dendrites evoked by sodium action pot entials are limited to regions close to the soma. To study the mechanisms u nderlying this restricted pattern of calcium influx, we combined two-photon imaging of dendritic [Ca2+] dynamics with dendritic membrane potential mea surements. We found that sodium action potentials attenuated and broadened rapidly with distance from the soma. However, dendrites of layer 2/3 cells were electrically excitable, and direct current injections could evoke larg e [Ca2+] transients. The restricted pattern of dendritic [Ca2+] transients is therefore due to a failure of sodium action-potential propagation into d endrites. Also, stimulating subcortical activating systems by tail pinch ca n enhance dendritic [Ca2+] influx induced by a sensory stimulus by increasi ng cellular excitability, consistent with the importance of these systems i n plasticity and learning.