Responses of a layer 2/3 pyramidal neuron to proximal and distal photostimulation, and electrical stimulation of proximal or distal synaptic inputs.

<p><b>A:</b> Membrane potential response to fluctuating current injected through the intracellular recording electrode. <b>B, C:</b> Membrane potential response to a mixture of intracellular injection of the same fluctuating current and photoinduced optical EPSCs (oEPSCs) produced by 2 ms pulses of 488 nm light illuminating somatic region (B) or distal dendrites (C). Dashed vertical line shows the timing of an oEPSC which led to generation of an additional action potential. Note that other oEPSCs did not lead to additional spikes. <b>D:</b> Response of pyramidal neuron to intracellular injection of fluctuating noise current with immersed small artificial aEPSCs. <b>E:</b> A scheme of optical and electrical stimulation of a layer 2/3 pyramidal neuron. Black line on top shows cortical surface. Electrical stimuli were applied through patch stimulation electrodes located in layer 1 or in layer 2/3. For optical stimulation, either somatic region or distal part of apical dendrite was illuminated, as indicated by the circles. <b>F:</b> EPSCs evoked by electrical stimulation and photoinduced oEPSCs. All responses were recorded by intracellular pipette in the soma. Note that EPSCs induced by electrical or optical stimulation in layer 1 have slower onsets, indicative of their more distal origin, than EPSCs induced by electrical or optical stimulation in layer 2/3. <b>G:</b> PSTHs of spike responses to EPSCs immersed in fluctuating current. EPSCs were photoinduced in the somatic region or in distal dendrites, or produced by intracellular current injection. Note that in all three cases, the increase of the firing rate follows the dynamics of EPSC onset. Abscissa is population firing rate in Hz.