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Determination of the Location and Magnitude of Synaptic Conductance Changes in Spinal Motoneurons by Impedance Measurements

Division of Neurobiology, Barrow Neurological Institute, St. Joseph's Hospital and Medical Center, Phoenix, Arizona 85013

Submitted 8 September 2003; accepted in final form 13 April 2004

The relation between impedance change and the location and magnitude of a tonic synaptic conductance was examined in compartmental motoneuron models based on previously published data. The dependency of motoneuron impedance on system time constant (), electrotonic length (L), and dendritic-to-somatic conductance ratio () was examined, showing that the relation between impedance phase and differed markedly between models with uniform and nonuniform membrane resistivity. Dendritic synaptic conductances decreased impedance magnitude at low frequencies; at higher frequencies, impedance magnitude increased. The frequency at which the change in impedance magnitude reversed from a decrease to an increase—the reversal frequency, F_r—was a good estimator of electrotonic synaptic location. A measure of the average normalized impedance change at frequencies less than F_r, cuZ, estimated relative synaptic conductance. F_r and cuZ provided useful estimates of synaptic location and conductance in models with nonuniform (step, sigmoidal) and uniform membrane resistivity. F_r also provided good estimates of spatial synaptic location on the equivalent cable in both step and sigmoidal models. Variability in relations between F_r, cuZ, and conductance location and magnitude between neurons was reduced by normalization with and . The effects on F_r and cuZ of noise in experimental recordings, different synaptic distributions, and voltage-dependent conductances were also assessed. This study indicates that location and conductance of tonic dendritic conductances can be estimated from F_r, cuZ, and basic electrotonic motoneuron parameters with the exercise of suitable precautions.

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