Effect of a train of EPSPs in the repetitive discharge of motoneuron models with passive dendrite

Leonardo Abdala Elias
Escola de Engenharia El?trica e de Computa??o - UFG

Marcus Fraga Vieira
Laborat?rio de Biomec?nica - FEF/UFG

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     Last modified: February 28, 2007

Abstract
In motoneurons, an individual train of excitatory postsynaptic potentials (EPSPs) can be simulated by an injection of a suprathreshold step current in soma and a posterior addition of short duration current pulses. Thus, we can verify a modulation in the motoneuron firing rate owed to the current pulses superimposed, and a sublinear relationship between the increase in the mean firing rate and the pulses frequencies can be observed, with a saturation tendency at 400 Hz (Powers and Binder, 1996).
The aim of this work was to evaluate, by computational simulations, the repetitive response of motoneuron models with passive dendrite when an injection of currents simulating an individual train of EPSPs, in different frequencies, durations and amplitude, was performed.
The simulations were accomplished with compartmental motoneuron models of different types (S, FR and FF), whose geometry and electrotonic parameters were based on classic works of the experimental literature with lumbar-sacral motoneurons of cats, and the dendrite was passively modeled (Vieira, 2002).
Suprathreshold step currents with 1 s duration and enough amplitude to evoke discharges in basal frequencies of 12 and 22 imp/s were injected in the models? soma. Afterwards, those pulses of currents with variable amplitude, duration and frequency were superimposed, and the difference between the mean firing rate with or without the pulses was measured (Elias and Vieira, 2005). In order to compare the results obtained by the simulation and the experimental data, the function based on the effective synaptic current and the saturation function, reported by Powers and Binder were used(1996).
It could be verified that the current injection that simulates an individual train of EPSPs increases the mean firing rate of the motoneuron models, observing, in all cases, a sublinear characteristic between the increase in the firing rate and the pulses? frequencies. However, the saturation of these curves could be noticed in values that were different from those described in experimental literature. In the type S and type FR models, saturations were observed in the range of 50-300 Hz, and the type FF model was observed at pulse rate of 300-450 Hz.
When compared to the functions based on the effective synaptic current and the saturation function, all the results are similar to those reported on other papers, where the first function underestimates all the values and the second underestimates the lowest frequencies and overestimates in higher EPSPs frequencies, showing that the models well represent the phenomena studied.