The differences in dendritic length and diameter have implications for their electrical compactness and integrative properties ( Larkman et al., 1992 Zador et al., 1995). Their morphology is similar to L5 pyramidal neurons, but the apical dendrite is thinner and shorter ( Larkman et al., 1992). They are thought to be fundamental for cortical tasks such as feature selection and perceptual grouping ( Grossberg and Raizada, 2000 Binzegger et al., 2004). Pyramidal neurons in layer 2/3 (L2/3) constitute the majority of cells in the neocortex ( Zilles, 1990). We conclude that L2/3 pyramidal neurons can generate dendritic spikes, sharing with L5 pyramidal neurons fundamental properties of dendritic excitability and control by inhibition. As in L5 neurons, we found that distal dendritic calcium transients are sensitive to a long-lasting block by GABAergic inhibition. Initiation of dendritic spikes was facilitated by backpropagating somatic APs and could cause an additional AP at the soma. Regenerative dendritic potentials and large concomitant calcium transients were also elicited by trains of somatic action potentials (APs) above a critical frequency (130 Hz), which was slightly higher than in L5 neurons. Brief current injections into the apical dendrite evoked relatively short (half-width 2–4 ms) dendritic spikes that were isolated from the soma for near-threshold currents at sites beyond the middle of the apical dendrite. This was also consistent with a slight increase in input resistance with distance from the soma. Unlike L5 cells, L2/3 dendrites displayed little sag in response to long current pulses, which suggests a low density of I h in the dendrites and soma. Here we directly measured passive and active properties of the apical dendrites of L2/3 neurons in rat brain slices using dual dendritic–somatic patch-clamp recordings and calcium imaging. Despite their key position in the cortical microcircuit, synaptic integration in dendrites of L2/3 neurons is far less understood than in L5 pyramidal cell dendrites, mainly because of the difficulties in obtaining electrical recordings from thin dendrites. Layer 2/3 (L2/3) pyramidal neurons are the most abundant cells of the neocortex.
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