The resting membrane potential or resting potential occurs when the membrane of a neuron is not altered by excitatory or inhibitory action potentials. It occurs when the neuron is not sending any signal, being in a moment of rest. When the membrane is at rest, the inside of the cell has a negative electrical charge relative to the outside..
The resting membrane potential is approximately -70 microvolts. This means that the inside of the neuron is 70 mV less than the outside. In addition, at this time there are more sodium ions outside the neuron and more potassium ions inside..
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For two neurons to exchange information, action potentials need to be given. An action potential consists of a series of changes in the membrane of the axon (extension or "wire" of the neuron).
These changes cause various chemicals to move from inside the axon to the fluid around it, called extracellular fluid. The exchange of these substances produces electrical currents.
The membrane potential is defined as the electrical charge existing on the membrane of nerve cells. Specifically, it refers to the difference in electrical potential between the inside and outside of the neuron.
The resting membrane potential implies that the membrane is relatively inactive, resting. There are no action potentials that affect you at that time.
To study this, neuroscientists have used squid axons because of their large size. To give you an idea, the axon of this creature is one hundred times larger than the largest axon of a mammal..
Researchers place the giant axon in a container of seawater, so it can survive for a couple of days.
In order to measure the electrical charges produced by the axon and its characteristics, two electrodes are used. One of them can provide electrical currents, while another serves to record the message from the axon. A very fine type of electrode is used to avoid any damage to the axon, called a microelectrode..
If an electrode is placed in the seawater and another one inserted inside the axon, it is observed that the latter has a negative charge with respect to the external liquid. In this case, the difference in electrical charge is 70 mV.
This difference is called the membrane potential. That is why it is said that the resting membrane potential of a squid axon is -70 mV.
Neurons exchange messages electrochemically. This means that there are various chemicals inside and outside the neurons that, when they increase or decrease their entry into nerve cells, give rise to different electrical signals.
This occurs because these chemicals have an electrical charge, which is why they are known as "ions".
The main ions in our nervous system are sodium, potassium, calcium, and chlorine. The first two contain a positive charge, calcium has two positive charges and chlorine has a negative charge. However, there are also some negatively charged proteins in our nervous system..
On the other hand, it is important to know that neurons are limited by a membrane. This allows certain ions to reach the interior of the cell and blocks the passage of others. That is why it is said to be a semi-permeable membrane..
Although the concentrations of the different ions are tried to balance on both sides of the membrane, it only allows some of them to pass through its ion channels.
When there is a resting membrane potential, potassium ions can easily pass through the membrane. However, sodium and chlorine ions have a more difficult time passing through at this time. At the same time, the membrane prevents negatively charged protein molecules from leaving the interior of the neuron..
In addition, the sodium-potassium pump also starts. It is a structure that moves three sodium ions out of the neuron for every two potassium ions that it introduces into it. Thus, in the resting membrane potential, more sodium ions are observed outside and more potassium inside the cell..
However, for messages to be sent between neurons, changes in membrane potential must occur. That is, the resting potential must be altered.
This can occur in two ways: depolarization or hyperpolarization. Next, we will see what each of them means:
Suppose that in the previous case the researchers place an electrical stimulator on the axon that alters the membrane potential in a specific place.
As the interior of the axon has a negative electrical charge, if a positive charge is applied in this place, a depolarization would occur. Thus, the difference between the electrical charge on the outside and inside the axon would be reduced, which means that the membrane potential would decrease..
In depolarization, the membrane potential becomes at rest, to decrease towards zero.
Whereas, in hyperpolarization there is an increase in the cell's membrane potential.
When several depolarizing stimuli are given, each one of them changes the membrane potential a little more. When it reaches a certain point, it can be abruptly reversed. That is, the inside of the axon reaches a positive electrical charge and the outside becomes negative..
In this case, the resting membrane potential is exceeded, which means that the membrane is hyperpolarized (more polarized than usual).
The whole process can take about 2 milliseconds, and then the membrane potential returns to its normal value..
This phenomenon of rapid inversion of the membrane potential is known as the action potential, and it involves the transmission of messages through the axon to the terminal button. The value of the voltage that produces an action potential is called the "excitation threshold".
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