Neurons are the basic unit of our nervous system and, thanks to their work, it is possible to transmit the nerve impulse so that it reaches encephalic structures that allow us to think, remember, feel and much more.
But these neurons are not transmitting impulses all the time. There are times when they rest. It is during these moments that the resting membrane potential occurs, a phenomenon which we explain in more detail below.
What is membrane potential?
Before further understanding of how the resting membrane potential is produced and also how it is altered, it is necessary to understand the concept of membrane potential.
In order for two nerve cells to exchange information it is necessary for them to modify the voltage of their membranes , which will result in an action potential. In other words, the action potential is understood to be a series of changes in the membrane of the neuron axon, which is the elongated structure of the neurons that serves as a cable.
Changes in membrane voltage also imply changes in the physicochemical properties of this structure. This allows for changes in the permeability of the neuron, making it easier and more difficult for certain ions to enter and exit.
Membrane potential is defined as the electrical charge on the membrane of the nerve cells. It is the difference between the potential inside and outside the neuron .
What is resting membrane potential?
Resting membrane potential is a phenomenon that occurs when the nerve cell membrane is not altered by either excitatory or inhibitory action potentials. The neuron does not signal, i.e. it is not sending any signals to other nerve cells to which it is connected and is therefore in a resting state.
The resting potential is determined by the concentration gradients of the ions , both inside and outside the neuron, and by the permeability of the membrane when these same chemical elements are allowed to pass through or not.
When the neuron membrane is at rest, the inside of the cell has a more negative charge in relation to the outside. Normally, in this state, the membrane has a voltage close to -70 microvolts (mV). That is, the inside of the neuron is 70 mV less than the outside, although it should be mentioned that this voltage can vary between -30 mV and -90 mV. In addition, at this time there are more sodium (Na) ions outside the neuron and more potassium (K) ions inside it .
How does it occur in the neurons?
The nerve impulse is nothing more than the exchange of messages between neurons by electrochemical means. That is, when different chemicals enter and leave the neurons, altering the gradient of these ions in the environment inside and outside the nerve cells, electrical signals are produced . As ions are charged elements, changes in their concentration in these media also involve changes in the voltage of the neuronal membrane.
In the nervous system the main ions that can be found are Na and K, although calcium (Ca) and chlorine (Cl) are also highlighted. Na, K and Ca ions are positive, while Cl is negative. The nerve membrane is semi-permeable, allowing some ions to enter and exit selectively.
Both outside and inside the neuron, the ion concentrations try to balance out ; however, as mentioned above, the membrane makes it difficult, since it does not allow all the ions to go out or come in in the same way.
In the resting state, K ions pass through the neuronal membrane with relative ease, while Na and Cl ions have more trouble passing through. During this time, the neuronal membrane prevents the exit of negatively charged proteins to the neuronal exterior. The resting membrane potential is determined by the non-equivalent distribution of ions between the inside and outside of the cell.
One element of fundamental importance during this state is the sodium-potassium pump. This structure of the neural membrane serves as a regulatory mechanism for the concentration of ions inside the nerve cell. It works in such a way that for every three Na ions that come out of the neuron, two K ions enter . This makes the concentration of Na ions higher on the outside and the concentration of K ions higher on the inside.
Resting membrane changes
Although the main topic of this article is the concept of resting membrane potential, it is necessary to explain, very briefly, how changes in membrane potential occur while the neuron is resting. In order for the nerve impulse to be given, the resting potential must be altered. Two phenomena occur for the electrical signal to be transmitted: depolarization and hyperpolarization.
In the resting state, the inside of the neuron has an electrical charge with respect to the outside.
However, if electrical stimulation is applied to this nerve cell, i.e. receiving the nerve impulse, a positive charge is applied to the neuron. On receiving a positive charge, the cell becomes less negative with respect to the outside of the neuron , almost at zero charge, and therefore the membrane potential is decreased.
If in the resting state the cell is more negative than the outside and, when depolarized, it does not have a significant difference in load, in the case of hyperpolarization it happens that the cell has a more positive load than its outside.
When the neuron receives several stimuli that depolarize it, each of them causes the membrane potential to be changed progressively .
After several of them, the point is reached where the membrane potential changes a lot, making the electrical charge inside the cell very positive, while the outside becomes negative. The resting membrane potential is exceeded, making the membrane more polarized than normal or hyperpolarized.
This phenomenon occurs for about two milliseconds . After this very short period of time, the membrane returns to its normal values. The rapid inversion in the membrane potential is, in itself, what is called action potential and is what causes the transmission of the nerve impulse, in the direction of the axon to the terminal button of the dendrites.
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