Ca²⁺ accumulations in dendrites of neocortical pyramidal neurons: An apical band and evidence for two functional compartments

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 somatosensory cortex, we have investigated the Ca²⁺ influx into dendrites under intracellular, antidromic, synaptic, and receptor-agonist stimulation. We find three spatial patterns of Ca²⁺ accumulations: an apical band in the apical dendrite approximately 500 μm from the soma, an accumulation restricted to the basal dendrites, soma, and proximal apical dendrite, and a combination of both of these. We show that the apical band can be activated antidromically and synaptically and that, under blocked Na⁺ and K⁺ conductances, it generates Ca²⁺ spikes. Thus, the apical band may serve as a dendritic trigger zone for regenerative Ca²⁺ spikes or as a current amplifier for distal synaptic events. Our results suggest that the distal apical dendrite should be considered a separate functional compartment from the rest of the cell.