Fig. 2.

Reduction in input resistance during actual and simulated synaptic inputs in vitro. A: A layer 5 pyramidal cell (V_rest, −71 mV) received a train of afferent synaptic stimulation elicited by extracellular stimulations in layers 2/3. The shocks started a t₀, were Poisson distributed at a frequency of 120 Hz, their amplitude was random (gaussian around a mean that elicited a reliable epsp) and their width was 0.3 ms. The cell depolarized to about −62 mV and emitted occasional spikes. At t₁, 500 ms after the train onset, the cell was somatically injected with a current pulse (−50 pA) to evaluate its input resistance. B: Superposed individual traces (top) and average trace (middle) obtained with three different synaptic train patterns (two trains at 120 Hz and one train at 200 Hz, six trials each) in the same cell as in A. Stimulation artifacts have been removed with low-pass filtering (500 Hz) and action potentials are truncated. The I–V curve was constructed by repeated injection of five different hyperpolarizing current pulses amplitudes. Input resistance was obtained as the slope of the linear fit to the I–V curve. During these random synaptic inputs, the resistance of this cell was 185 MΩ, while it was 230 MΩ in the absence of synaptic stimulation (panel C, top). C: Point conductance clamp. The same cell as in A and B was injected with a 200 pA hyperpolarizing pulse. The two traces show the average response (six trials) of the cell in control condition (top) and when it was subjected to the point-conductance clamp (bottom: Ge₀=3 nS, Gi₀=15.5 nS, σ_e=5 nS, σ_i=12.5 nS). The input resistance was reduced from 23s0 MΩ to 52 MΩ.