Computational models and intracellular recordings were combined to
characterize synaptic bombardment in neocortical pyramidal neurons during
active states in vivo. Application of tetrodotoxin revealed that synaptic
activity accounts for up to 80% of the input conductance. These experiments
were replicated assuming that excitatory and inhibitory synapses release
randomly at high rates (1-5 Hz). A significant correlation between synaptic
events had to be introduced to account for the membrane potential fluctuations
recorded experimentally. We conclude that pyramidal neurons in vivo
experience high-frequency synaptic events with a significant correlation. The
consequences of this synaptic bombardment on cellular responsiveness are
investigated in the companion paper.
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