Neurons are the basic unit of our nervous system and, thanks to their work, it is possible to transmit nerve impulses so that it reaches the brain structures that allow us to think, remember, feel and do well. Moreover.
But these neurons don’t transmit impulses all the time. There are times when they rest. It’s during these times that it happens the membrane potential at rest, A phenomenon that we explain in more detail below.
What is the potential of the membrane?
Before better understanding how resting membrane potential occurs and how it is changed, it becomes necessary to understand the concept of membrane potential.
For two nerve cells to exchange information they must modify the tension of their membranes, Which will result in a potential for action. In other words, the action potential is understood as a series of changes in the membrane of the neuronal axon, which is the elongated structure of neurons that serves as a cable.
Changes in membrane tension also involve changes in the physicochemical properties of this structure. This allows the permeability of the neuron to be changed, making it easier and more difficult for certain ions to enter and exit.
Membrane potential is defined as the electrical charge in the membrane of nerve cells. This is the difference between the potential between the inside and the outside of the neuron..
What is the potential of the membrane at rest?
Resting membrane potential is a phenomenon that occurs when the membrane of nerve cells is not altered by action potentials, neither exciters nor inhibitors. The neuron does not signal, that is, it does not send any signals to other nerve cells to which it is connected and is therefore in a state of rest.
The rest potential is determined by ionic concentration gradients, Both inside and outside the neuron, and the permeability of the membrane to let through, or not, these same chemical elements.
When the neuron’s membrane is at rest, the inside of the cell has a more negative charge compared to the outside. Normally, in this state, the membrane has a voltage close to -70 microvolts (mV). That is, the inside of the neuron has 70 mV less than the outside, although it should be mentioned that this voltage can vary, between -30 mV and -90 mV. Also, now there are more sodium (Na) ions outside the neuron and more potassium (K) ions inside.
How does this happen in neurons?
Nerve impulse is nothing more than the exchange of messages between neurons by electrochemical means. That is, when different chemicals enter and exit neurons, changing the gradient of these ions in the internal and external environment of nerve cells, electrical signals are produced. Since ions are charged elements, changes in their concentration in these media also involve changes in the tension 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) also stand out. Na, K and Ca ions are positive, while Cl is negative. The nerve membrane is semi-permeable, allowing certain ions to enter and exit selectively.
Both outside and inside the neuron, ion concentrations try to balance out; but, as has already been said, the membrane makes it difficult, since it does not allow all the ions to exit or enter in the same way.
At rest, K ions cross the neuronal membrane with relative ease, while Na and Cl ions have a harder time passing. During this time, the neuronal membrane prevents the release of negatively charged proteins from the neuronal exterior. The resting membrane potential is determined by the unequal distribution of ions between the interior and exterior of the cell.
One element of fundamental importance during this state is the sodium-potassium pump. This structure of the neuronal membrane serves as a mechanism for regulating the concentration of ions inside the nerve cell. It works so that for every three Na ions that exit the neuron, two K ions enter. This results in a higher Na ion concentration on the outside and a higher K ion concentration on the inside.
Changes in the resting membrane
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 when the neuron is at rest. In order for the nerve impulse to be given, the resting potential must be changed. Two phenomena occur so that the electrical signal can be transmitted: depolarization and hyperpolarization.
At rest, the inside of the neuron has an electrical charge relative to the outside.
However, if electrical stimulation is applied to this nerve cell, i.e. receiving the nerve impulse, it is applied to the positively charged neuron. By receiving a positive charge, the cell becomes less negative compared to the outside of the neuron, With almost zero charge, and therefore the membrane potential decreases.
If at rest the cell is more negative than the outside and, when it is depolarized, does not have a significant difference in charge, in the event of hyperpolarization it happens that the cell has a more positive charge than its outside.
When the neuron receives various stimuli that depolarize it, each of which gradually changes the potential of the membrane.
After several of them, one reaches the point where the potential of the membrane changes a lot, which makes the electric 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 of the membrane potential is, in itself, what is called the action potential and it is what causes the transmission of the nerve impulse, in the direction of the axon towards the terminal button of the dendrites.
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