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Cellular and Autonomic PhysiologyFAQ: What are the conventions for membrane potentials, currents, and the like?

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Here are a few basic definitions and conventions that you may have forgotten or maynever have known. Life will be vastly better once you get the terms down.1. Membrane potentials (Vm) are always expressed as intracellular relative to

extracellular potential. Thus, if we say a membrane potential is -70 mV, that meansinside negative. At rest, all excitable cells have negative Vm.

2. Equilibrium potentials, calculated with the Nernst equation for a given ion, arealways expressed as intracellular relative to extracellular. If we say an ion’sequilibrium potential is -90 mV, that means inside negative.

3. Current flow (or current) in physiology is carried by ions. So, when your professortalks about physiologic experiments in which current is measured, that means flow ofions (e.g., Na+, K+, Cl-).

4. Depolarization means the membrane potential becomes less negative, or positive.5. Hyperpolarization means the membrane potential becomes more negative.6. Repolarization means membrane potential is returning to its resting level, which for

excitable cells means it is becoming more inside negative.7. Conductance = g = 1/resistance, or 1/R. To use conductance instead of resistance

in Ohm’s law: V = I R make the substitution of g = 1/R V = I/g

FAQ: What is a reversal potential?Back to FAQs

We calculate the equilibrium potential for an ion at a given concentration difference withthe Nernst equation. The equilibrium potential says what potential difference would putthe ion at electrochemical equilibrium at that concentration difference.Reversal potential is measured in a voltage clamp experiment. In the experiment, thereis a pre-set concentration difference across the membrane for the ion. Then, voltage(potential difference) across the membrane is held, or “clamped,” at a series of values.At each clamped voltage, current is measured (both direction and quantity of currentflow). Let’s say, for illustration, that the membrane being clamped is highly permeable toK+ and no other ion. At each clamped voltage, therefore, the current measured is a K+

current. (Since no other ion is permeant, K+ is the only ion that can carry charge acrossthe membrane.) The direction and magnitude of the current flow (measured in theexperiment) depends on the electrochemical gradient for K+ across the membrane. Whenthe clamped voltage is exactly equal to the K+ equilibrium potential, the direction ofcurrent flow “reverses” (e.g., from out to in) and, at that potential, there is zero currentflow. (Perhaps a better name for reversal potential would be “zero current flowpotential.”) In the previous scenario, the reversal potential = the potential at which there