Nerve cells have a resting potential due to a negative charge inside the cell compared to the outside. When stimulated, depolarization occurs, leading to an action potential. Repolarization occurs after a refractory period, restoring the resting potential. This is due to the unequal concentration of sodium and potassium ions, with more potassium inside the cell. Sodium channels open during depolarization, allowing sodium to enter the cell and increasing the number of positively charged ions inside. Repolarization involves potassium ions leaving the cell first, followed by active pumping of sodium out.
Nerve cells have a negative electrical charge across their plasma membranes, known as the resting potential. The plasma membrane is a thin boundary layer that encloses the nerve cell and the resting potential exists because the inside of the cell is negatively charged compared to the outside. When a neurotransmitter, a chemical that carries signals between nerve cells, arrives at the membrane, or the membrane is mechanically disturbed, the charge across the membrane changes, becoming more positive. This change is known as depolarization, and if it reaches a certain level, what is called an action potential occurs, in which an electrical impulse is sent along the nerve. Following an action potential, the membrane is repolarized, becoming negatively charged again and restoring the resting potential.
The resting potential of nerve cell membranes is created by unequal concentrations of positively charged sodium ions and potassium ions on each side of the membrane. There is more potassium inside the cell and more sodium outside the cell. The reason for this is a sodium-potassium pump located in the cell membrane, which actively moves sodium out of the cell and potassium into the cell.
There are channels in the membrane through which sodium and potassium ions can travel, but when the membrane is at rest the sodium channels are closed and only some of the potassium channels are open. Sodium ions are forced out of the cell, while some potassium ions leave the cell to join them through the open channels. The end result is that more positively charged ions end up outside the cell than inside, and this creates the negative charge across the membrane, known as the resting potential, which is necessary if depolarization of neurons is to occur.
For an action potential to take place, the nerve cell must first be stimulated by the stretch or arrival of a neurotransmitter. A depolarizing effect then occurs because sodium channels open and allow sodium to enter the cell, increasing the number of positively charged ions inside and making the electrical potential across the membrane more positive. Once depolarization reaches a threshold level, many sodium channels open simultaneously and an action potential occurs, in which complete depolarization of the membrane suddenly occurs, with the depolarization also passing along the nerve cell in a wave.
After depolarization, repolarization occurs after a short interval known as the refractory period. During that time any further stimuli applied to the cell have no effect. The refractory period lasts only a fraction of a second, allowing a nerve to fire many times in the space of a second. Repolarization involves potassium ions leaving the cell first, before sodium is actively pumped out. Once the membrane potential has reached the necessary negative charge, the resting potential is reached and the nerve is ready to fire again.
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