CA2- AN APICAL BAND AND EVIDENCE FOR 2 FUNCTIONAL COMPARTMENTS( ACCUMULATIONS IN DENDRITES OF NEOCORTICAL PYRAMIDAL NEURONS )

Apical dendrites constitute a prominent feature of the microcircuitry in the neocortex, yet their function is poorly understood. Using fura- 2 imaging of layer 5 pyramidal neurons from slices of rat somatosensor y cortex, we have investigated the Ca2+ influx into dendrites under in tracellular, antidromic, synaptic, and receptor-agonist stimulation. W e find three spatial patterns of Ca2+ accumulations: an apical band in the apical dendrite approximately 500 mu m from the soma, an accumula tion restricted to the basal dendrites, soma, and proximal apical dend rite, and a combination of both of these. We show that the epical band can be activated antidromically and synaptically and that, under bloc ked Na+ and K+ conductances, it generates Ca2+ spikes. Thus, the apica l band may serve as a dendritic trigger zone for regenerative Ca2+ spi kes or as a current amplifier for distal synaptic events. Our results suggest that the distal apical dendrite should be considered a separat e functional compartment from the rest of the cell.