EFFECTS OF TAPERING GEOMETRY AND INHOMOGENEOUS ION-CHANNEL DISTRIBUTION IN A NEURON MODEL

Recent experiments have produced direct evidence on the existence of v arious dendritic voltage-gated ion channels, indicating that these neu ronal components are not just a passive medium for the propagation of synaptic excitation but a putative source of neuronal excitability tha t is reflected in the activity patterns occurring on the soma. In orde r to study possible changes in neuronal excitability when the distribu tion of dendritic voltage-activated channels is non-uniform, and the d endritic geometry is not necessarily cylindric, we have developed a ne uron model that incorporates two voltage-activated currents (I-Na and I-K), and in which space-dependent distributions of the system paramet ers can be treated in a mathematically simple and efficient way. Simul ation results with the model showed that both linearly and exponential ly tapering geometries led to marked anisotropy of the propagation of excitation, favouring the soma-to-dendrite direction. Exponentially de caying densities of dendritic voltage-activated channels, with appropr iate choice of the parameters, induced bistable behaviour between the normal resting stare and an intrinsic, sustained oscillation with cyli ndric as well as linear and exponential tapering dendritic geometry. B istability could not be evoked when the model was reduced to a space-i ndependent one (point-like soma). These results suggest that both tape ring dendritic geometry and inhomogeneous distribution of ion channels may crucially affect the propagation and integration of synaptic pote ntials, and that changes in dendritic channel densities might underlie pathological electrophysiological activities. (C) 1998 IBRO. Publishe d by Elsevier Science Ltd.