On the invasion of distal dendrites of thalamocortical neurones by action potentials and sensory EPSPs

Abstract

The effects of different dendritic geometries and distal dendritic Na⁺ current distributions on the propagation of action potentials (APs) and sensory EPSPs were investigated using a multi-compartment model of thalamocortical (TC) neurones where the somatic and proximal dendritic distribution of voltage-gated channels matched the ones measured experimentally, i.e. a uniform distribution of K⁺ currents and a non-uniform distribution of Na⁺ and T-type Ca²⁺ currents.

Our simulations indicated that the distal dendritic Na⁺ channel density has not to be larger than 50% of the somatic density in order to reproduce the electrical activities recorded experimentally from the soma and proximal dendrites of TC neurones. Moreover, we could highlight the existence of a distinct threshold density of distal dendritic Na⁺ channels necessary to support the regeneration of APs in this part of the dendritic tree: this threshold density was smaller for non-branching than for heavily branching dendrites.

The amplitude of the somatic EPSP mainly depended on the number of simultaneously activated synapses on any dendritic branch, despite large differences in the size of the dendritic EPSPs. The amplitude of the EPSP on a proximal dendrite was also dependent on the number and relative location of simultaneously activated synapses on all other proximal dendritic branches. The dendritic geometry did not affect these features of the simulated sensory EPSPs. In addition, the duration of somatic and proximal dendritic EPSPs was markedly increased (100%) in the presence of somatic and proximal dendritic T-type Ca²⁺ current.

The backpropagation of EPSPs to distal dendrites was affected by the dendritic Na⁺ channel distributions, but even in the absence of distal dendritic Na⁺ channels the EPSP reached the dendritic ends with less than 40% decrease in amplitude. Overall, the amplitude of the backpropagating EPSP was not greatly affected by the dendritic geometry, though a smaller amplitude reduction in unbranched than in heavily branching dendrites was observed.