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 conducta nces contribute to response properties of neurons requires reliable knowled ge of the electrotonic structure of dendritic trees. A novel method based o n weak DC field stimulation and the classical method based on current injec tion were used to obtain two independent estimates of the electrotonic stru cture of motoneurons in an in vitro preparation of the turtle spinal cord. DC field stimulation was also used to ensure that the passive membrane prop erties near the resting membrane potential were homogeneous. In two cells, the difference in electrotonic lengths estimated with the two methods in th e 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 bran ches reached 2 lambda. In the third cell, the difference was 36%, demonstra ting the need to use both methods, field stimulation and current injection, for reliable measurements of the electrotonical structure. Models of the r econstructed cells endowed with voltage-dependent conductances were used to explore generation mechanisms for the experimentally observed hysteresis i n input current-voltage relation of bistable motoneurons. The results of mo deling suggest that only some dendrites need to possess L-type calcium curr ent to explain the hysteresis observed experimentally and that dendritic br anches with different electrotonical lengths can be bistable. Independent b istable behavior in individual dendritic branches can make motoneurons comp lex processing units.