Transfer properties of neuronal dendrites with tonically activated conductances |
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Authors: | S M Korogod I B Kulagina S Tyč-Dumont |
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Institution: | (1) Dnepropetrovsk Division of International Center for Molecular Physiology, National Academy of Sciences of Ukraine, Ukraine;(2) Dnepropetrovsk State University, Ukraine;(3) Umté de Neurocybernétique Cellulaire, Centre National de la Recherche Scientifique, Marseille, France |
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Abstract: | The somatopetal current transfer was studied in the mathematical models of a reconstructed brainstem motoneuron with tonically
activated excitatory synaptic inputs uniformly distributed over dendritic arborization. The soma and axon provided a constant
passive leak. The extrasynaptic dendritic membrane was either passive or active (of a Hodgkin-Huxley type). The longitudinal
membrane current density (per unit path length) was used as an estimate of the current transfer effectiveness of different
dendritic paths. Introduction of a steady uniform voltage-independent conductance per unit membrane area simulated such a
synaptic activation. This actions always produced a spatially inhomogeneous membrane depolarization decaying from the distal
dendritic tips toward the soma. The reason for such an inhomogeneity was the preponderance of somatopetal over somatofugal
input conductance at every site in the dendrites with sealed distal ends and a leaky somatic end. In active dendrites, partial
voltage-dependent extrasynaptic conductances followed this depolarization according to their activation-inactivation kinetics.
The greater the local depolarization, the greater the contribution of the non-inactivating potassium conductance to the total
membrane conductance. The contribution of the inactivated sodium conductance was one order of magnitude smaller. Correspondingly,
the effective equilibrium potential of the total transmembrane current became spatially inhomogeneous and shifted to the potassium
equilibrium potential. In the passive dendrites, the equilibrium potential remained spatially homogeneous. Inhomogeneities
of the dendritic geometry (abrupt change in the diameter and, especially, asymmetrical branching) caused characteristic perturbations
in the voltage gradient, so that the path profiles of the voltage, conductances, and currents diverged. This indicated a geometry-induced
separation of the dendritic paths in their transfer effectiveness. Active dendrites of the same geometry were less effective
than passive ones due to the effect of the potassium conductance associated with the hyperpolarizing equilibrium potential. |
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