The spike initiation in neurons with low input resistance

Abstract

Event Abstract Back to Event The spike initiation in neurons with low input resistance Simon Lehnert1* and Christian Leibold1, 2 1 Ludwig-Maximilians-Universität München, Department of Biology II, Germany 2 Bernstein Center for Computational Neuroscience Munich, Germany Spike initiation is generally assumed to occur in the axon's initial segment (AIS). In neurons with very low input resistance, however, this may no longer hold true, because the soma constitutes a huge current sink. As an example, we consider the principal cells of the medial superior olive (MSO). These neurons are very fast coincidence detectors in the auditory brainstem that encode low frequency azimuthal sound localization by interaural time differences. Their enormously low time constant of only a few hundred microseconds yields from a very low input resistance of about 5MΩ at rest [1], which arises from the expression of low-voltage-activated potassium channels and hyperpolarization-activated unspecific cation channels. Since these cells receive a huge amount of slowly-decaying inhibition [1], it is likely that their input resistance in vivo is even much smaller than at rest. Hence the question arises: how are these cells able to elicit action potentials (AP) at all? By using a multi-compartemental model of an MSO cell, we found that the spike initiation segment (SIS) in the model is still the AIS. The reason for this is the electrotonic independence of the AIS from the leaky soma. The input resistance of the axonal segments is considerably higher than that of the soma and hence an input current is able to evoke an AP in the AIS, while an AP in the soma is hardly distinguishable from a subthreshold response. We conclude, that the excitability of the axon enables the cell to convey information downstream, although the soma itself is not excitable. Thus, neurons with low input resistance cannot be realistically modeled using only a single compartment.