Subthreshold voltage noise due to channel fluctuations in active neuronal membranes

Voltage-gated ion channels in neuronal membranes fluctuate randomly between different conformational states due to thermal agitation. Fluctuations bet ween conducting and nonconducting states give rise to noisy membrane curren ts and subthreshold voltage fluctuations and may contribute to variability in spike timing. Here we study subthreshold voltage fluctuations due to act ive voltage-gated Na+ and K+ channels as predicted by two commonly used kin etic schemes: the Mainen et al. (1995) (MJHS) kinetic scheme, which has bee n used to model dendritic channels in cortical neurons, and the classical H odgkin-Huxley (1952) (HH) kinetic scheme for the squid giant axon. We compu te the magnitudes, amplitude distributions, and power spectral densities of the voltage noise in isopotential membrane patches predicted by these kine tic schemes. For both schemes, noise magnitudes increase rapidly with depol arization from rest. Noise is larger for smaller patch areas but is smaller for increased model temperatures. We contrast the results from Monte Carlo simulations of the stochastic nonlinear kinetic schemes with analytical, c losed-form expressions derived using passive and quasi-active linear approx imations to the kinetic schemes. For all subthreshold voltage ranges, the q uasi-active linearized approximation is accurate within 8% and may thus be used in large-scale simulations of realistic neuronal geometries.