Dynamic spike threshold reveals a mechanism for synaptic coincidence detection in cortical neurons in vivo

Cortical neurons are sensitive to the timing of their synaptic inputs. They can synchronize their firing on a millisecond time scale and follow rapid stimulus fluctuations with high temporal precision. These findings suggest that cortical neurons have an enhanced sensitivity to synchronous synaptic inputs that lead to rapid rates of depolarization. The voltage-gated curren ts underlying action potential generation may provide one mechanism to ampl ify rapid depolarizations. We have tested this hypothesis by analyzing the relations between membrane potential fluctuations and spike threshold in ca t visual cortical neurons recorded intracellularly in vivo. We find that vi sual stimuli evoke broad variations in spike threshold that are caused in l arge part by an inverse relation between spike threshold and the rate of me mbrane depolarization preceding a spike. We also find that spike threshold is inversely related to the rate of rise of the action potential upstroke, suggesting that increases in spike threshold result from a decrease in the availability of Na+ channels. By using a simple neuronal model, we show tha t voltage-gated Na+ and K+ conductances endow cortical neurons with an enha nced sensitivity to rapid depolarizations that arise from synchronous excit atory synaptic inputs. Thus, the basic mechanism responsible for action pot ential generation also enhances the sensitivity of cortical neurons to coin cident synaptic inputs.