Voltage dependence of membrane properties of trigeminal root ganglion neurons.

1. Membrane potentials of trigeminal root ganglion neurons were varied systematically by intracellular injections of long-lasting step currents to determine the voltage dependence of their membrane electrical properties. The complex impedance and impedance magnitude functions were first determined using oscillatory input currents superimposed on these step currents. 2. Systematic step variations in the membrane potential led to qualitative changes in the impedance magnitude functions. Depolarization of neurons exhibiting resonance at their initial resting membrane potentials resulted in a reduction in the resonance behavior. Hyperpolarization of these neurons to membrane potentials of about -80 to -90 mV led to a disappearance of the resonant peak but increased the maximum of the impedance magnitude. 3. The complex impedance data were fitted with a neuronal model derived from linearized Hodgkin-Huxley-like equations, yielding estimates for the membrane properties. The four parameters of the model were 1) a time invariant, resting membrane conductance, Gr, 2) a voltage- and time-dependent conductance, GL, 3) a time constant, tau u, for the unknown ionic channels that are activated by the 2- to 5-mV oscillatory perturbation of the stepped membrane potential, and 4) Ci, the input capacitance. 4. The results of the curve-fitting procedures suggested that all parameters depended on membrane voltage. The most voltage-dependent parameters were GL and tau u throughout a 25- to 30-mV range that was subthreshold to the production of action potentials. Both Gr and GL increased with subthreshold depolarization. 5. These impedance data suggest the very important role of the membrane potential of the trigeminal root ganglion neurons on their abilities to synthesize and filter inputted electrical signals.