All cells, including neurons, are inherently electrical, because the fluid inside and outside a cell contains many ions and charged molecules. The different concentrations of ions inside and outside a cell results in a concentration gradient across the plasma membrane. Sodium ions (Na+) and chloride ions (Cl-) are more concentrated outside a cell than inside. Potassium ions (K+) and negatively charged proteins are more concentrated inside a cell than outside. A voltmeter can be used to measure the voltage, or electrical potential, across a plasma membrane. A recording electrode is placed in the cellâs cytoplasm, while a reference electrode is placed in the extracellular fluid. If there is an equal distribution of positive and negative charges on both sides of the membrane, no membrane potential exists, and the voltmeter reads zero millivolts (0 mV). The plasma membrane contains ion channels. A type of potassium channel, sometimes called a leak channel, is always open, allowing K+ to cross the membrane. Because the inside of the cell has a greater concentration of K+ than the outside, K+ tends to flow down its concentration gradient to the outside of the cell, leaving the inside with a net negative charge. This charge forms an electrochemical gradient across the membrane. At a voltage called the equilibrium potential, the negative charge is great enough that it opposes the outward net flow of the positively charged K+ ions. Most cells have a membrane potential of around -70 millivolts when they are at rest. This is called the resting potential. The leakage of K+ is largely responsible for this potential. Na+ and Cl- also leak across the membrane, but to a lesser extent. All animal cells have potassium leak channels, but neurons and other electrically excitable cells also have voltage-gated channels that allow them to produce a neural signal by making rapid changes in the membrane potential. The voltage-gated sodium and potassium channels are closed at the cellâs resting potential but open in response to specific changes in voltage, which may occur when the cell is stimulated by another cell. Researchers can artificially change the membrane potential to study the behavior of voltage-gated channels. Recall that at -70 millivolts, the channels are closed. As the membrane potential becomes less negative, a point is reached where the channels will open. The most negative voltage at which point these channels will open is -50. The concentration gradients of Na+ and K+ across the membrane provide a cell with a form of potential energy. These gradients are maintained by a pump in the membrane called the sodium-potassium pump. With each cycle, the pump takes up three Na+ ions from inside the cell. ATP phosphorylates the pump, giving it the energy to change shape and expel the Na+ ions to the outside. The pump then picks up two K+ ions from outside the cell, releases its phosphate group, and returns to its original conformation, releasing the K+ into the cell. If this pump did not operate, the ion concentration gradients across the membrane would soon disappear. These concentration gradients are essential for neurons to generate electrical signals, called action potentials.
