Membrane Potential

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Membrane Potential

Every neuron has a separation of electrical charge across its cell membrane, and the membrane potential results from a separation of positive and negative. The relative excess of positive charges outside and negative charges inside the membrane of a nerve cell at rest is maintained because the lipid bilayer acts as a barrier to the diffusion of ions. It gives rise to an electrical potential difference, which ranges from about 60 to 70 mV. The potential across the membrane when the cell is at rest (i.e., when there is no signalling activity) is known as the resting potential. Because, by convention, the potential outside the cell is arbitrarily defined as zero, and given the relative excess of negative charges inside the membrane; the potential difference across the membrane is expressed as a negative value: Vr = -60 to -70 mV, where Vr, is the resting potential voltage.

The charge separation across the membrane is disturbed whenever there is a net flux of ions into or out of the cell. A reduction of the charge separation is called depolarisation, and an increase in charge separation is called hyperpolarization. Transient current flow and, therefore, rapid changes in potential are made possible by ion channels, a class of integral proteins that traverse the cell membrane. Ionic species are not distributed equally on the two sides of a nerve membrane. Na⁺ and Cl^- are more concentrated outside the cell, while K⁺ and organic anions (organic acids and proteins) are more concentrated inside. The overall effect of this ionic distribution is the resting potential. There are essentially two forces acting on a given ionic species.

The driving force of the chemical concentration gradient tends to move ions down this gradient (chemical potential). On the other hand, the electrostatic force due to the charge separation across the membrane tends to move ions in a direction determined by its particular charge. Thus, for instance, Cl^- ions, which are concentrated outside the cell, tend to move inward down the concentration gradient through non-gated chloride channels. However, the relative excess of negative charge inside the membrane tends to push Cl^- ions back out of the cell. Eventually, an equilibrium can be reached so that the actual ratio of intracellular and extracellular concentration ultimately depends on the existing membrane potential.

The same argument applies to the K⁺ ions. However these two forces act together on each Na⁺ ion to drive it into the cell. First, Na⁺ is more concentrated outside than inside and, therefore, tends to flow into the cell, down its concentration gradient. Second, Na⁺ is driven into the cell by the electrical potential difference across the membrane. Therefore, if the cell is to have a steady resting membrane potential, the movement of Na⁺ ions into the cell must be balanced by the efflux of K⁺ ions. Although these steady ionic interchanges prevent irreversible depolarization, the process cannot be allowed to continue unopposed. Otherwise, the K⁺ pool would be depleted, intracellular Na⁺ would increase, and the ionic gradients would gradually run down, reducing the resting membrane potential. Dissipation of ionic gradients is ultimately prevented by ATP dependent Na⁺-K⁺ pumps, which extrude Na⁺ from the cell while taking in K⁺.