New biophysical methods for the characterization of signal transfer in neurons
Many neurons have extensive dendritic trees, and therefore somatic voltage clamp of dendritic synapses is often associated with substantial distortion and attenuation of the synaptic currents. A new method is presented which permits faithful extraction of the decay time constant of the synaptic conductance independent of dendritic geometry and the electrotonic location of the synapse. The decay time course of the synaptic conductance was recovered with high accuracy in all the tested geometries, even with high series resistances, low membrane resistances, and electrotonically remote, distributed synapses. The method also provides the time course of the voltage change at the synapse in response to a somatic voltage clamp step, and thus will be useful for constraining compartmental models and estimating the relative electrotonic distance of synapses. Action potential propagation in dendrites links information processing in different regions of the dendritic tree. In simulations using compartmental models with identical complements of voltage-gated channels, different dendritic branching patterns caused a range of backpropagation efficacies, similar to that observed experimentally. Dendritic geometry also determines the extent to which modulation of channel densities can affect propagation. Forward propagation of dendritically initiated action potentials is influenced by geometry in a similar manner. By determining the spatial pattern of action potential signalling, dendritic geometry thus helps to define the size and interdependence of functional compartments in the neuron.
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