Clemens Boucsein (Institute of Biology III, Neurobiology and Biophysics, University of Freiburg)

Abstract:
Temporal precision of EPSP-spike coupling is an essential pre-requisite to sustain activity during sparse activity states, which have recently been reported to prevail in numerous cortical regions and layers. Most excitatory cells in mammalian brains, however, qualify as integrator or type-I neurons with notoriously imprecise coupling of spikes to transient depolarizing events. Such 'bumps' are typical for sparse activity states, and often push the membrane potential to just supra-threshold values. By combining experimental measures suitable for the classification of excitability and generic models of spike dynamics, we investigated the precision of EPSP-spike coupling in neocortical pyramidal cells. As expected for type-I cells, it can be highly imprecise in the bumpy regime. However, the majority of type-I cells display a striking temporal precision, which can be modulated by physiological variations in input resistance. Surprisingly, unlike previously described for other cells, transition between imprecise and precise spiking in neocortical pyramidal cells is not associated with a switch from integrator to resonator, or type-I to type-II, dynamics. We present a new theoretical model that can explain these surprising results within the framework of two-dimensional dynamical systems theory, and argue that the mechanism described here has a very general applicability, providing means to understand temporal precision of EPSP-spike coupling in the bumpy regime in a wide variety of neurons.