Resting potential Totally Explained

Everything about Cell Potential totally explained

The resting potential of a cell is the membrane potential that would be maintained if there were no action potentials (no voltage-gated channels), synaptic potentials, or other active changes in the membrane potential. In most cells the resting potential has a negative value of ~-70mV, which by convention means that there's excess negative charge inside compared to outside. The resting potential is mostly determined by the concentrations of the ions in the fluids on both sides of the cell membrane and the ion transport proteins that are in the cell membrane. How the concentrations of ions and the membrane transport proteins influence the value of the resting potential is outlined below.

Membrane transport proteins

For determination of membrane potentials, the two most important types of membrane ion transport proteins are ion channels and ion pumps. Ion channel proteins create paths across cell membranes through which ions can passively diffuse without expenditure of energy. They have selectivity for certain ions, thus, there are potassium-, chloride-, and sodium-selective ion channels. Different cells and even different parts of one cell (dendrites, cell bodies, nodes of Ranvier) will have different amounts of various ion transport proteins. Typically, the amount of certain potassium channels is most important for control of the resting potential (see below). Some ion pumps such as the Na+/K+-ATPase are electrogenic, that is, they produce charge imbalance across the cell membrane and can also contribute directly to the membrane potential. All pumps use energy to function.

Equilibrium potentials

For most animal cells potassium ions (K⁺) are the most important for the resting potential. Due to the active transport of potassium ions, the concentration of potassium is higher inside cells than outside. Most cells have potassium-selective ion channel proteins that remain open all the time. There will be net movement of positively-charged potassium ions through these potassium channels with a resulting accumulation of excess positive charge outside of the cell. The outward movement of positively-charged potassium ions is due to random molecular motion (diffusion) and continues until enough excess positive charge accumulates outside the cell to form a membrane potential which can balance the difference in concentration of potassium between inside and outside the cell. "Balance" means that the electrical force (potential) that results from the build-up of ionic charge, and which impedes outward diffusion, increases until it's equal in magnitude but opposite in direction to the tendency for outward diffusive movement of potassium. This balance point is an equilibrium potential as the net transmembrane flux (or current) of K⁺ is zero. The equilibrium potential for a given ion depends only upon the concentrations on either side of the membrane and the temperature. It can be calculated using the Nernst equation: ť

E_

, where P_tot is the combined permeability of all species, again in arbitrary units. The latter equation portrays the resting membrane potential as a weighted average of the reversal potentials of the system, where the weights are the relative permeabilites across the membranes (P_X/P_tot). During the action potential, these weights change. If the permeabilities of Na⁺ and Cl^- are zero, the membrane potential reduces to the Nernst potential for K⁺ (as P_K⁺ = P_tot). Normally, under resting conditions P_Na+ and P_Cl- are not zero, but they're much smaller than P_K+, which renders E_m close to E_eq,K+. Medical conditions such as hyperkalemia in which blood serum potassium (which governs [K⁺]_o) is changed are very dangerous since they offset E_eq,K+, thus affecting E_m. This may cause arrhythmias and cardiac arrest. The use of a bolus injection of potassium chloride in executions by lethal injection stops the heart by shifting the resting potential to a more positive value, which depolarizes and contracts the cardiac cells permanently, not allowing the heart to repolarize and thus enter diastole to be refilled with blood.

Measuring resting potentials

In some cells, the membrane potential is always changing (such as cardiac pacemaker cells). For such cells there's never any “rest” and the “resting potential” is a theoretical concept. Other cells with little in the way of membrane transport functions that change with time have a resting membrane potential that can be measured by inserting an electrode into the cell. Transmembrane potentials can also be measured optically with dyes that change their optical properties according to the membrane potential.

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