A compartmental model of an external urethral sphincter motoneuron of Onuf's nucleus

This article discusses a model of the electrical behavior of an external urethral sphincter motoneuron, based on morphological parameters like soma size, dendritic diameters and spatial dendritic configuration, and several electrical parameters. Because experimental data about the exact ion conductance mix of external urethral sphincter neurons is scarce, the gaps in knowledge about external urethral sphincter motoneurons were filled in with known data of alpha-motoneurons. The constructed compartmental model of motoneurons of Onuf's nucleus contains six voltage-dependent ionic conductances: a fast sodium and potassium conductance and an anomalous rectifier in the soma; a fast delayed rectifier type potassium conductance and a fast sodium conductance in the initial axon segment; an L-type calcium channel in the dendritic compartments. This paper considers the simulation of external urethral sphincter motoneuron responses to current injections that evoke bistable behavior. Simulations show self-sustained discharge following a depolarizing pulse through the microelectrode; the firing was subsequently terminated by a short hyperpolarizing pulse. This behavior is highly functional for neurons that have to exhibit prolonged activation during sphincter closure. In addition to these 'on' and 'off ' responses, we also observed a particular firing behavior in response to long-lasting triangular current pulses. When the depolarizing current was slowly increased and then decreased (triangular pulse) the firing frequency was higher during the descending phase than during the initial ascending phase.