Definition of Action Potential and Membrane

The action potential involves a rapid change in the membrane potential of an excitable cell that spreads rapidly across it. Action potentials are the basic mechanism for transmitting information in the nervous system and in all types of muscles.

All the functions carried out by our nervous system, the contraction of the muscles that allows us to move and the heartbeat that allow blood to be carried to all the cells of our body are orchestrated by electrical signals that propagate through the tissues involved.

Membrane potential

From a purely physical point of view we can think of cells as if they were small batteries. There are electrical charges in the extracellular medium and the intracellular medium whose different concentrations result in an electrical potential difference across the membrane of the cell. The generated electrochemical gradient gives rise to several of the electrical phenomena that occur in the cell.

The ions that are especially relevant when determining the membrane potential of cells are the sodium ion (Na

⁺) and the potassium ion (K⁺). The average concentration of Na⁺ in the extracellular medium it is 142 mEq/l while in the intracellular medium it has a concentration of only 14 mEq/l. On the other hand, the concentration of K⁺ outside the cell it is 4 mEq/l and inside the cell it is approximately 140 mEq/l.

The difference in concentrations of these two ions between the extracellular and intracellular medium generates a difference in electrical potential across the cell membrane. However, there are other factors that also influence this. The cell membrane is more permeable to the K ion⁺, that is, this ion can pass more easily through it. There are ion channels known as potassium leak channels that allow the passage of K ions⁺ from the inside of the cell to the outside of it. They also allow some Na^+ ions to escape, although these channels are approximately 100 times more permeable to potassium.

Another element that plays a fundamental role in the formation of the membrane potential is the Na pump.^+- K.⁺. It is a protein that uses ATP to produce a continuous pump of 3 Na ions⁺ out of the cell and 2 K ions⁺ inwards, thus causing a greater accumulation of positive charges in the extracellular medium. When all these mechanisms act together, a net membrane potential of approximately -90 mV is generated in the nerve fibers. The value of being potential is with respect to the interior of the cell, that is, the potential is more negative in the intracellular medium.

action potential

An action potential begins with a sudden change from the normal negative membrane potential. up to a positive potential, and ends with an almost equally rapid change back toward the potential negative. The actors required for the generation of an action potential and the recovery thereafter are voltage-gated sodium channels and voltage-gated potassium channels. These are ionic channels that open and close depending on the value of the membrane potential at a certain moment.

An action potential begins with the cell at rest and with its membrane potential at the typical value of -90 mV. During this phase the membrane is said to be “polarized”. Under certain conditions the membrane suddenly becomes very permeable to Na ions.⁺, in such a way that these begin to move towards the interior of the cell and the potential begins to become more positive.

If the potential reaches a value that can be between -70 and -50 mV, the voltage-gated sodium channels open and there is a rapid movement of more Na ions.⁺ towards the intracellular medium. During this phase the permeability of the membrane to sodium can increase up to 5,000 times and the potential membrane reaches values ​​that oscillate between + 35 and + 40 mV, it is then said that the membrane is “depolarized”. The voltage-gated sodium channel remains open for a few ten thousandths of a second and then closes.

When the membrane potential reaches 0 mV, voltage-gated potassium channels begin to open, allowing the flow of K ions.⁺ towards the outside of the cell. However, due to the delayed opening of the potassium channels, they open fully when the sodium channels begin to close. The combination of both events causes a rapid recovery of the membrane potential to the resting value of -90 mV, in this phase it is said that there is a "repolarization".