Resting membrane potential: what it is and how it affects neurons

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 brain structures that allow us to think, remember, feel and much further.

But these neurons are not transmitting impulses all the time. There are times when they rest. It is during those moments is when it occurs resting membrane potential, a phenomenon which we explain in more detail below.

• Related article: "Types of neurons: characteristics and functions"

What is membrane potential?

Before further understanding how the resting membrane potential is produced and how it is altered, it is necessary to understand the concept of membrane potential.

For two nerve cells to exchange information it is necessary that they modify the voltage of their membranes, which will result in an action potential. In other words, 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 the membrane voltage also imply changes in the physicochemical properties of this structure. This allows there to be 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 nerve cell membrane. It is the difference between the potential between the inside and the outside of the neuron..

What is the resting membrane potential?

The resting membrane potential is a phenomenon that occurs when the nerve cell membrane is not altered by action potentials, neither excitatory nor inhibitory. The neuron does not signal, that is, it is not sending any type of signal to other nerve cells to which it is connected and, therefore, it is in a state of rest.

resting potential is determined by the concentration gradients of the ions, both inside and outside the neuron, and the permeability of the membrane by allowing these same chemical elements to pass through, or not.

When the neuron membrane is in a resting state, the inside of the cell has a more negative charge relative 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 is worth mentioning that this voltage can vary between -30 mV and -90 mV. Furthermore, at this time there are more sodium (Na) ions outside the neuron and more potassium (K) ions inside the neuron.

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How is it produced in neurons?

The nerve impulse is nothing more than the exchange of messages between neurons via electrochemical means. That is, when different chemical substances enter and leave the neurons, altering the gradient of these ions in the internal and external environment of the nerve cells, electrical signals are produced. Since ions are charged elements, changes in their concentration in these media also imply changes in the neuronal membrane voltage.

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, selectively letting some ions in and out.

Both outside and inside the neuron, ion concentrations try to balance; however, as already mentioned, the membrane makes this difficult, since it does not allow all the ions to leave or enter in the same way.

In the resting state, K ions cross the neuronal membrane with relative ease, whereas Na and Cl ions have more trouble passing through. During this time, the neuronal membrane prevents negatively charged proteins from leaving the neuronal exterior. The resting membrane potential is determined by the non-equivalent distribution of ions between the interior and exterior of the cell.

An element of fundamental importance during this state is the sodium-potassium pump. This structure of the neuronal membrane serves as a regulatory mechanism for the concentration of ions inside the nerve cell. It works so that for every three Na ions that leave the neuron, two K ions enter. This causes the concentration of Na ions to be higher on the outside and the concentration of K ions to be higher on the inside.

Membrane changes at rest

Although the main theme 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 in resting. In order for the nerve impulse to be given, it is necessary that the resting potential be altered. There are two phenomena that occur so that the electrical signal can be transmitted: depolarization and hyperpolarization.

1. Depolarization

At rest, the interior of the neuron has an electrical charge with respect to the exterior.

However, if electrical stimulation is applied to this nerve cell, that is, receiving the nerve impulse, a positive charge is applied to the neuron. When receiving a positive charge, the cell becomes less negative with respect to the outside of the neuron, with almost zero charge, and therefore the membrane potential is lowered.

2. hyperpolarization

If in the resting state the cell is more negative than the outside and, when it depolarizes, it does not have a difference of significant charge, in the case of hyperpolarization it happens that the cell has a more positive charge than its abroad.

When the neuron receives various stimuli that depolarize it, each of them causes the membrane potential to change progressively.

After several of them, the point is reached that 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, causing the membrane to be more polarized than normal, or hyperpolarized.

This phenomenon occurs for about two milliseconds.. After that very brief period of time, the membrane returns to its normal values. The rapid reversal in the membrane potential is itself what is called the action potential and is the that causes the transmission of the nervous impulse, in the direction of the axon to the terminal button of the dendrites.

Bibliographic references:

• Cardinali, D.P. (2007). Applied neuroscience. Its fundamentals. Panamerican Medical Editorial. Buenos Aires.
• Carlson, N. R. (2006). Physiology of behavior 8th Ed. Madrid: Pearson.
• Guyton, C.A. & Hall, J.E. (2012) Treatise on Medical Physiology. 12th edition. McGraw Hill.
• Kandel, E.R.; Schwartz, J.H. & Jessell, T.M. (2001). Principles of neuroscience. Fourth edition. McGraw-Hill Interamericana. Madrid.