title: What we talk about when we talk about capacitance measured with the voltage-clamp step method.
abstract: Capacitance is a fundamental neuronal property. One common way to measure capacitance is to deliver a small voltage-clamp step that is long enough for the clamp current to come to steady state, and then to divide the integrated transient charge by the voltage-clamp step size. In an isopotential neuron, this method is known to measure the total cell capacitance. However, in a cell that is not isopotential, this measures only a fraction of the total capacitance. This has generally been thought of as measuring the capacitance of the "well-clamped" part of the membrane, but the exact meaning of this has been unclear. Here, we show that the capacitance measured in this way is a weighted sum of the total capacitance, where the weight for a given small patch of membrane is determined by the voltage deflection at that patch, as a fraction of the voltage-clamp step size. This quantifies precisely what it means to measure the capacitance of the "well-clamped" part of the neuron. Furthermore, it reveals that the voltage-clamp step method measures a well-defined quantity, one that may be more useful than the total cell capacitance for normalizing conductances measured in voltage-clamp in nonisopotential cells.
authors.affiliation: Biology Department and Volen Center for Complex Systems, Brandeis University, Waltham, MA 02454, USA. altaylor@brandeis.edu
authors.forename: Adam L
authors.initials: AL
authors.lastname: Taylor
mesh_terms: D000818:Animals; D055592:Biophysical Phenomena / Q000502:physiology*; D001703:Biophysics; D004553:Electric Conductivity; D008564:Membrane Potentials / Q000502:physiology*; D008959:Models, Neurological; D009474:Neurons / Q000502:physiology*; D018408:Patch-Clamp Techniques / Q000379:methods*