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

Abstract

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 k omega cm(2) 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.

Membrane properties and electrogenesis in the distal axons of small dorsal root ganglion neurons in vitro

“…Assuming a model with uniform axon diameter of 0.6 m (Fig. 1B), a resistivity of axoplasm of 200 ⍀·cm (Jamieson et al 2003;Rall et al 1992), and membrance capacitance of 1 F/cm 2 , we estimate that these axons have an average length constant of 207 Ϯ 13 m (n ϭ 20). While the axons under study displayed some variation in diameter along their length, diameter was often 0.5 m or less and rarely exceeded 0.8 m. The average diameter of the axon shown in Fig.…”

Regulation of the subthreshold chloride conductance in the rat sympathetic neuron

“…The resting conductance in an adult sympathetic neuron, when impaled with one high-resistance microelectrode in vitro (Jamieson et al, 2003) or in vivo, is lower than that measured in the same neuron under two-electrode voltage-clamp conditions; it is even lower when the neuron is brought within a culture and the patch-clamp technique is used. When potassium and chloride conductances are blocked, however, the values of cell input conductance of the naïve sympathetic neuron under two-electrode voltage-clamp become close to those observed by using a single high-resistance microelectrode or patch recording.…”

Effect of intracellular photosensitized singlet oxygen production on the electrophysiological properties of cultured rat hippocampal neurons

“…These changes in electrophysiological properties were accompanied by morphological changes characteristic of cell death, 49 including the formation of vacuoles on the cell. The latter could account, in part, for the observed increase in C M (recall that C M is related to the membrane surface area 48 ). In an independent assay, we ascertained that the cells irradiated under these conditions did not exclude propidium iodide, indicating that the membrane had indeed lost integrity and that the cells were necrotic.…”

“…The value of C M in a healthy cell is approximately 1 mF cm -2 , and is, with some restrictions, widely accepted as a biological constant. 48 Membrane resistance (R M ). This parameter reflects the ability of the membrane to control the passage of ions between intra-and extracellular domains.…”