J Neurophysiol (May 1, 2003). 10.1152/jn.01000.2002
Submitted on Submitted 4 November 2002; accepted in final form 18 November 2002

Simulations to Derive Membrane Resistivity in Three Phenotypes of Guinea Pig Sympathetic Postganglionic Neuron

Prince of Wales Medical Research Institute, Randwick, New South Wales 2031; and the University of New South Wales, New South Wales 2052, Australia

Jamieson, John, Hugh D. Boyd, and Elspeth M. McLachlan. Simulations to Derive Membrane Resistivity in Three Phenotypes of Guinea Pig Sympathetic Postganglionic Neuron. J. Neurophysiol. 89: 2430-2440, 2003. The electrotonic behavior of three phenotypes of sympathetic postganglionic neuron has been analyzed to assess whether their distinct cell input capacitances simply reflect differences in morphology. Because the distribution of membrane properties over the soma and dendrites is unknown, compartmental models incorporating cell morphology were used to simulate hyperpolarizing responses to small current steps. Neurons were classified as phasic (Ph), tonic (T), or long-afterhyperpolarizing (LAH) by their discharge pattern to threshold depolarizing current steps and filled with biocytin to determine their morphology. Responses were simulated in models with the average morphology of each cell class using the program NEURON. Specific membrane resistivity, R_m, was derived in each model. Fits were acceptable when specific membrane capacitance, C_m, and specific resistivity of the axoplasm, R_i, were varied within realistic limits and when underestimation of membrane area due to surface irregularities was accounted for. In all models with uniform R_m, solutions for R_m that were the same for all classes could not be found unless C_m or R_i were different for each class, which seems unrealistic. Incorporation of a small somatic shunt conductance yielded values for R_m for each class close to those derived assuming isopotentiality (R_m approximately 40, 27, and 15 kcm² for T, Ph, and LAH neurons, respectively). It is concluded that R_m is distinct between neuron classes. Because Ph and LAH neurons relay selected preganglionic inputs directly, R_m generally affects function only in T neurons that integrate multiple subthreshold inputs and are modulated by peptidergic transmitters.