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 microm away for both layers 2/3 and 5 pyramids. Estimates of horizontal axonal conduction velocities ( approximately 0.4 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.