Resting Membrane Potential

The transmembrane ion concentrations at rest is dependent upon the passive diffusion of ions from the site of higher concentration to the site of lower concentration through ion channels, as well as the active adenosine triphosphatase (ATPase)-dependent transport of ions against a concentration gradient. At rest, the concentrations of sodium, chloride, and calcium ions are higher extracellularly, whereas the concentrations of potassium ions and impermeable protein anions are higher intracellularly. As a result, sodium and chloride are forced to move from the extracellular to intracellular space and potassium in the opposite direction. With the diffusion of ions across the cell membrane, a separation of charges develops because the nondiffusible negatively charged intracellular ions have a charge opposite that of the diffusible ions. As a result, an electrical potential difference develops between the intracellular and extracellular axon membrane. This electrical potential difference produces an electrical pressure that opposes the physical movement of the ion. The net ionic movement continues until the electrical pressure equals the diffusion pressure, and there is no net movement of ions. The resulting electrical potential across the membrane is called the equilibrium potential.

The equilibrium potential of each ion (E_ion) is the voltage difference across the membrane that exactly offsets the diffusion pressure of an ion to move down its concentration gradient. This potential is different for each ion and can be defined by the Nernst equation, which defines the equilibrium potential, E_m, inside the cell for any ion in terms of its concentration extracellularly [Xe] and intracellularly [Xi].

E_m = (61.5mV/Z_i ) log₁₀ ([X_e ]/[X_i ])

In the resting state, the approximate equilibrium potentials of the major ions are

K⁺ = −90 mV,Na⁺ = +50 mV, and Cl⁻ = −70 mV.

The contribution of a given ion to the actual resting transmembrane voltage depends not only on the ion’s concentration gradient but also the permeability of the membrane to that ion as a result of the opening or closing of the ion channel. Increased permeability (i.e., opening of the channel) to a particular ion brings the membrane potential toward the equilibrium potential of that ion. If a membrane is permeable to multiple ions that are present in differing concentrations on either side of the membrane, the resultant membrane potential is a function of the concentrations of each of the ions and of their relative permeabilities. The Goldman equation combines these factors for the major ions (Na⁺, K⁺, and Cl⁻) that influence the membrane potential and is used to calculate the resting membrane potential.

An electrical circuit model using Ohm’s law (E = IR) can be used to demonstrate the contribution of each ion to the resting membrane potential. The movement of ions across the membrane is expressed as an ion current. By Ohm’s law, this current depends on the driving force of the ion (the difference between the membrane potential and the equilibrium potential of that ion) and conductance of the ion (g). Using this model, the conductance (g) (or the reciprocal of the resistance) for a particular ion is dependent on the ion channel permeability of each ion. The concentration ratios of the different ions are represented by their respective equilibrium potentials (E_Na, E_K, E_Cl); their ionic permeabilities are represented by their respective conductances. Therefore, at rest, the conductance of the potassium ions is high, whereas sodium conductance is low and chloride conductance is moderate. As a result, the flow of potassium ions is the predominant contributor to the membrane potential at rest. The resting membrane potential (RMP) is the sum of the conductances of all the open channels permeable to each ion.

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