Refers to the concentration gradient of an ion or molecule. The concentration gradient may exist across a biological membrane, where the concentration is higher on one side of the membrane compared to the other side. Concentration gradient may also exist in a solution without an apparent barrier separating the area of higher concentration from the area of lower concentration. In both cases, the free energy that results from the concentration difference drives the movement of the ion/molcule from the area of higher concentration to the area of lower concentration. In free solution, the ion/molecule simply diffuses down its gradient. Movement across a biological membrane is more complicated and is a function of lipid solubility of the ion/molecule as well as the presence of channels or transport proteins that can allow the ion/molecule to cross the membrane (see Lipid Bilayer Permeability and Summary of Membrane Transport Processes).

Electrical gradient
Electrochemical gradient

The flow of charge. In electrical wires and electronic circuits, current is carried by electrons. In physiological solutions, current is carried by ions in solutions.

Voltage

Refers to a change in the value of the membrane potential, where the membrane potential becomes less negative (or more positive) than the resting membrane potential.

Repolarization
Hyperpolarization

Resting Membrane Potential - Introduction
Figure showing depolarization, repolarization, and hyperpolarization

Movement of a substance out of the cell.

Efflux is reported as a rate. It is the amount of substance that moves through a given area of the plasma membrane per unit time.

Flux
Influx
Unidirectional flux
Net flux

In biological solutions, electrical gradient refers to the electrical potential that acts on an ion to drive the movement of the ion in one or another direction (see Resting Membrane Potential - Establishment of the Membrane Potential).

Chemical gradient
Electrochemical gradient

V_DF

When an ion is not at its electrochemical equilibrium, an electrochemical driving force (V_DF) acts on the ion, causing the net movement of the ion across the membrane down its own electrochemical gradient.

The electrochemical driving force is generally expressed in millivolts and is calculated according the following equation:

V_DF = V_m − V_eq

where V_DF is the electrochemical driving force, V_m is the membrane potential, and V_eq is the equilibrium potential.

Membrane potential
Equilibrium potential
Electrochemical gradient

Resting Membrane Potential - Electrochemical Driving Force Acting on Ions
Electrochemical Driving Force Calculator

Refers to the balance of chemical and electrical gradients that act on an ion, particularly as it relates to the movement of an ion across a biological membrane (see Resting Membrane Potential - Establishment of the Membrane Potential and Resting Membrane Potential - Nernst Equilibrium Potential).

Chemical gradient
Electrical gradient

Electrochemical gradient

Electrochemical gradient

Electrochemical gradient

Electrochemical gradient

An electrogenic transport process is one that leads to the translocation of net charge across the membrane. For example, ion channels such as Na⁺, K⁺, Ca²⁺, and Cl⁻ channels are electrogenic.

The Na⁺/K⁺ ATPase is electrogenic because for every ATP molecule hydrolyzed, 3 Na⁺ ions are transported out of the cell and 2 K⁺ ions are transported into the cell (leading to the translocation of one net positive charge out of the cell).

Many secondary active transporters are also electrogenic. For example, the Na⁺/glucose cotransporter (found in the small intestine and kidney proximal tubules), transports 2 Na⁺ ions and 1 glucose molecule into the cell across the plasma membrane (leading to the translocation of two net positive charges into the cell per transport cycle).

Electrophysiology is the study of the electrical properties of biological macromolecules, cells, tissues, and organs. Electrical signals such as voltage and/or current are generally measured. Examples include measuring changes in the membrane voltage of excitable cells (e.g., neurons, muscle cells, and some endocrine cells) during an action potential. The current carrried by ions as they permeate the pore of ion channels can also be measured - both at the single-channel level (single-channel current), as well as the macroscopic current resulting from the activity of a population of channels. As another example, electrical measurements may involve recording voltage changes at the surface of the skin that result from the activity of skeletal muscles (electromyogram, EMG), cardiac myocytes (electrocardiogram, ECG), or neurons in the brain (electroencephalogram, EEG).

Endocrine cells are responsible for producing and releasing hormone molecules into the bloodstream. Endocrine cells are typically grouped together in organs referred to as endocrine glands.

Endocrine gland
Hormone

V_eq. or E_eq.

Refers to the membrane potential at which there is no net movement of an ion across the plasma membrane into or out of the cell.

Resting Membrane Potential - Establishment of the Membrane Potential
Resting Membrane Potential - Nernst Equilibrium Potential
Nernst Potential Calculator

Refers to the ability of some cells to be electrically excited resulting in the generation of action potentials. Neurons, muscle cells (skeletal, cardiac, and smooth), and some endocrine cells (e.g., insulin-releasing pancreatic β cells) are excitable cells.

Resting Membrane Potential - Introduction

Refers to a change in the value of the membrane potential, where the membrane potential becomes more negative than the resting membrane potential.

Depolarization
Repolarization

Resting Membrane Potential - Introduction
Figure showing depolarization, repolarization, and hyperpolarization

Movement of a substance into the cell.

Influx is reported as a rate. It is the amount of substance that moves through a given area of the plasma membrane per unit time.

Flux
Efflux
Unidirectional flux
Net flux

The voltage difference across a cell plasma membrane.

The membrane potential is generally inside negative with respect to the outside, where the outside potential is generally set as the reference value. In electrically excitable cells, the value of the membrane potential can be positive (inside with respect to the outside) during electrical activity (i.e., during action potentials).

Resting membrane potential

Resting membrane potential

An equation used to calculate the equilibrium potential (V_eq.) of an ion. The equilibrium potential for an ion is also referred to as the Nernst potential for that ion. It is the membrane potential at which no net movement of the ion in question occurs across the membrane.



where V_eq. is the equilibrium potential, R is the universal gas constant, T is the temperature in Kelvin, z is the valence of the ionic species, F is the Faraday's constant, and [X]_o and [X]_i are the extracellular and intracellular, respectively, concentrations of the ion in question.

Resting Membrane Potential - Nernst Equilibrium Potential
Derivation of the Nernst Equation

Net flux represents the amount of substance moved in or out of the cell. It is the mathematical difference between influx and efflux.

Net flux = Influx − Efflux

Similar to influx and efflux, net flux is reported as a rate. It is the net amount of substance that moves through a given area of the plasma membrane per unit time.

Flux
Influx
Efflux
Unidirectional flux

Refers to cells that do not generate action potentials. With the exception of neurons, muscle cells, and some endocrine cells, all cells in the body are non-excitable.

Resting Membrane Potential - Introduction

Ouabain binds to and inhibits the transport activity of the Na⁺/K⁺/ATPase (i.e., sodium pump).

Ouabain is plant derived and belongs to the class of drugs referred to as cardiac glycosides. Similar to other cardiac glycosides, ouabain increase heart muscle contractility. However, ouabain is used only experimentally and not in humans (as for example digoxin is for the treatment of congestive heart failure).

There is some evidence that ouabain may be produced endogenously in humans.

Vanadate

Ouabain (Wikipedia)

Permeability refers to the ease with which molecules cross biological membranes. It may also refer to the ease with which ions or molecules pass through the pore of channel proteins.

Permeable
Impermeable
Permeant
Impermeant

Lipid Bilayer Permeability

Refers to the return of the membrane potential toward the normal resting value after a membrane depolarization.

Depolarization
Hyperpolarization

Resting Membrane Potential - Introduction
Figure showing depolarization, repolarization, and hyperpolarization

The voltage difference across a cell plasma membrane in the resting or quiescent state. It is also simply referred to as the resting potential (V_rest). The value of the resting membrane potential varies from cell to cell. Depending on the cell type, it can range from −90 mV to −20 mV.

For example, V_rest is −90 mV in skeletal and cardiac muscle cells as well as in astrocytes. In a typical neuron, V_rest is approximately −70 mV. In many non-excitable cells, V_rest ranges from −60 to −50 mV. In photoreceptors, V_rest is about −20 mV.

Resting membrane potential

An inhibitor of the Na⁺/K⁺/ATPase (i.e., sodium pump). The form commonly used for this purpose is sodium orthovanadate.

Ouabain

The difference in electrical potential between two points.