Membrane potential is the voltage across a cell membrane, important in nerve cells. Ion channels allow potassium to move across the membrane, while ion pumps move sodium out and potassium in, creating a negative charge inside. An action potential occurs when a nerve stimulus opens sodium channels, making the membrane less negative. Potassium channels then open, repolarizing the membrane and returning it to its resting state.
A membrane potential is the voltage that exists across the membrane of a cell. It is also known as the transmembrane potential and is particularly important in nerve cells or neurons. Membrane potential is caused by an electrical potential difference between the inside and outside of the cell. When a neuron is at rest, i.e. not emitting a nerve impulse, the inside of its cell membrane has a negative charge relative to the outside of the cell. This results from different concentrations of charged ions immediately inside and outside the membrane.
Membrane potentials arise because cell membranes do not allow sodium and potassium ions to pass freely in and out of cells and reach an equilibrium. Instead, special steps known as ion channels allow potassium ions to move across the cell membrane, reducing the positive charge within the cell. Ion pumps in the membrane use energy to pump sodium ions out of the cell, while they pump potassium in. For every pair of potassium ions that are moved into the cell by these ion transporters, three sodium ions are moved out, causing an overall loss of positive charge from the cell. Even the negatively charged protein molecules inside the cell cannot get out.
Together, these factors create a negative charge inside the cell relative to the outside, which forms the membrane potential. The potential is constant at rest, but changes in nerve cells as impulses are transmitted from one neuron to another. During nerve impulse transmission, what is known as an action potential occurs, in which the cell membrane goes through a process called depolarization. After the action potential, the membrane potential returns to its normal resting state, which is normally measured as a -70 millivolt difference between the inside and outside of the membrane.
The action potential begins when a nerve stimulus arrives at the cell, opening special sodium channels in the cell membrane. Positively charged sodium ions pass into the cell and the membrane potential changes, becoming less negative. When a point known as the action threshold is reached, many more sodium channels open and the inside of the cell membrane becomes positively charged, the opposite of normal.
Around the peak of the action potential, potassium channels open and potassium flows out of the cell. This makes the inside of the cell more negative so that the membrane is repolarized. The sodium channels also close during this time. Usually, repolarization overcomes and gradually returns to the normal resting membrane potential. This process of inverting the membrane potential to create an action potential is what allows for the transmission of impulses along the nerves.
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