Electrotonic structure of motoneurons in the spinal cord of the turtle: inferences for the mechanisms of bistability

Understanding how voltage-regulated channels and synaptic membrane conductances contribute to response properties of neurons requires reliable knowledge of the electrotonic structure of dendritic trees. A novel method based on weak DC field stimulation and the classical method based on current injection were used to obtain two independent estimates of the electrotonic structure of motoneurons in an in vitro preparation of the turtle spinal cord. DC field stimulation was also used to ensure that the passive membrane properties near the resting membrane potential were homogeneous. In two cells, the difference in electrotonic lengths estimated with the two methods in the same cell was 6 and 9%. The majority of dendritic branches terminated at a distance of 1 electrotonic unit from the recording site. The longest branches reached 2 lambda. In the third cell, the difference was 36%, demonstrating the need to use both methods, field stimulation and current injection, for reliable measurements of the electrotonical structure. Models of the reconstructed cells endowed with voltage-dependent conductances were used to explore generation mechanisms for the experimentally observed hysteresis in input current-voltage relation of bistable motoneurons. The results of modeling suggest that only some dendrites need to possess L-type calcium current to explain the hysteresis observed experimentally and that dendritic branches with different electrotonical lengths can be bistable. Independent bistable behavior in individual dendritic branches can make motoneurons complex processing units.