Widely integrative properties of layer 5 pyramidal cells support a role for processing of extralaminar synaptic inputs in rat neocortex

We have compared the length, strength, conduction velocity and divergence of horizontal connections onto layers 2/3 and 5 neurons in slices of rat primary somatosensory neocortex. Slices were cut along laminar borders to eliminate most vertical connections, and excitatory postsynaptic potentials (EPSPs) were recorded from pyramidal cells in adjacent uncut tissue. When electric stimuli were delivered within the same layer as the recorded cell, EPSPs could be evoked up to 2000 mum away for both layers 2/3 and 5 pyramids. Estimates of horizontal axonal conduction velocities (similar to0.4 m/s) and the thresholds for activation also did not differ between layers. However, layers 2/3 cells rarely responded to stimuli delivered to isolated deeper layers, while layer 5 neurons were often excited by horizontal inputs from isolated layers 2/3 and 4. (C) 2003 Elsevier Science Ireland Ltd. All rights reserved.