  In terms of the long-distance communication of a single neuron, interpulse
intervals (IPIs) are a possible alternative to rate and binary codes. As a
proxy for IPI, the time-to-spike (TTS) for a neuron can be found in the
biophysical and experimental literature. Using the current, consensus layer V
pyramidal neuron, the present study examines the feasibility of IPI-coding and
examines the noise sources that limit the information rate of such an encoding.
In descending order of noise intensity, the noise sources are (i) synaptic
variability, (ii) sodium channel shot-noise, followed by (iii) thermal noise
with synaptic noise much greater than the sodium channel-noise. More
importantly, the biophysical model demonstrates a linear relationship between
input intensity and inverse TTS. This linear observation contradicts the
assumption that a neuron should be treated as a passive, electronic circuit (an
RC circuit, as in the Stein model). Finally, the biophysical simulations allow
the calculation of mutual information, which is about 3.0 bits/spike.
