Impulse Coding of Electrical and Synaptic Input Actions by Nucl. Abducens Motoneurons with Active Dendrites: A Simulation Study

The study was carried out on models of nucl. abducens motoneurons with dendritic arborizations reconstructed with a high spatial resolution. The arborizations had membranes with nonlinear electrical properties due to the presence of glutamatergic NMDA-type synaptic conductivity with voltage dependent kinetics of ligand activation. We studied rules governing the transformation of electrical influences on the soma and of tonic excitatory synaptic actions on dendritic arborizations into output discharges of action potentials (APs), i.e., the processes of formation of “intrinsic” neuronal codes. The electrical action was a depolarizing current applied to the soma, and the synaptic action was a tonic synaptic excitation homogeneously distributed over the dendrites; this excitation was simulated by introducing a synaptic conductivity, which was homogeneous over the dendritic membrane surface and constant in time. We recorded impulse patterns generated at different intensities of the applied current or synaptic excitation. The only pattern generated in response to the current application was a continuous rhythmic discharge of APs with equal interspike intervals (ISIs); increases in the mean frequency of AP firing with increasing current intensity obeyed the logarithmic law. An increase in the synaptic activation intensity also led to an increase in the mean firing frequency, but in this case the “intensity-to-frequency” conversion obeyed the polynomial law. A feature of the patterns generated under these conditions was the existence of essential dissimilarities in the type and complexity observed at three ranges of the synaptic intensity, low, medium, and high. The medium range corresponded to complex multiburst output patterns, both periodical and non-periodical. At intensities corresponding to the low and high ranges, continuous AP discharges were generated with constant (or slightly varying) ISIs similar to those observed upon application of the depolarizing current. In the complex patterns, the interburst intervals demonstrated the greatest variability. Their duration was mainly determined by processes in the dendrites, whereas the variability of the intraburst ISIs was mostly due to fast processes in the trigger zone and was an order of magnitude smaller. Electrical states of the dendritic arborization were essentially heterogeneous during generation of AP bursts in the multiburst patterns, less heterogeneous during generation of simple patterns, and practically homogeneous within interburst time intervals.