MODELING ACTION-POTENTIAL INITIATION AND BACKPROPAGATION IN DENDRITESOF CULTURED RAT MOTONEURONS

Regardless of the site of current injection, action potentials usually originate at or near the soma and propagate decrementally back into t he dendrites. This phenomenon has been observed in neocortical pyramid al cells as well as in cultured motoneurons. Here we show that action potentials in motoneurons can be initiated in the dendrite as well, re sulting in a biphasic dendritic action potential. We present a model o f spinal motoneurons that is consistent with observed physiological pr operties of spike initiation in the initial segment/ axon hillock regi on and action potential back-propagation into the dendritic tree. It a ccurately reproduces the results presented by Larkum et al. on motoneu rons in organotypic rat spinal cord slice cultures. A high Na+-channel density of (g) over bar(Na) = 700 mS/cm(2) at the axon hillock/initia l segment region was required to secure antidromic invasion of the som ato-dendritic membrane, whereas for the orthodromic direction, a Na+-c hannel density of (g) over bar(Na) = 1,200 mS/cm(2) was required. A '' weakly'' excitable ((g) over bar(Na) = 3 mS/ cm(2)) dendritic membrane most accurately describes the experimentally observed attenuation of the back-propagated action potential. Careful analysis of the threshol d conditions for action potential initiation at the initial segment or the dendrites revealed that, despite the lower voltage threshold for spike initiation in the initial segment, an action potential can be in itiated in the dendrite before the initial segment fires a spike. Spik e initiation in the dendrite depends on the passive cable properties o f the dendritic membrane, its Nai-channel density, and local structura l properties, mainly the diameter of the dendrites. Action potentials are initiated more easily in distal than in proximal dendrites. Whethe r or not such a dendritic action potential invades the soma with a sub sequent initiation of a second action potential in the initial segment depends on the actual current source-load relation between the action potential approaching the soma and the electrical load of the soma to gether with the attached dendrites.