NERVE IMPULSE: CHARACTERISTICS, STAGES, FUNCTIONS - BIOLOGY - 2025

The nerve impulse is a series of action potentials (AP) that occur along the axon and other electrically excitable cells (muscle and glandular). In principle, it occurs when a message is transmitted from one neuron to another, or from a neuron to an effector organ due to an external or internal stimulus being received.

The message is essentially an electrical signal that is generated in the dendrites or the body of the neuron and travels to the end of the axon where the signal is transmitted. This action potential is the primary electrical signal generated by nerve cells, neurons, and is caused by changes in the permeability of the membrane to specific ions.

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The kinetics and voltage dependence of the permeabilities to certain ions provide a complete explanation of the generation of the action potential.

characteristics

The action potential is then an explosive phenomenon that will propagate without decrease along the nerve fibers. The axon conducts the AP from its point of origin, which is the spike initiation zone (near the axonal cone of the neuron), to the axonal terminals.

Neurons are therefore cells specialized in receiving stimuli and transmitting impulses. The active electrical responses of neurons and other excitable cells depend on the presence of specialized proteins, known as voltage-gated ion channels, in the cell membrane.

For the nerve impulse to be generated, a change must necessarily occur in the membrane of the neuron, which extends throughout the entire axon. The electrochemical difference between the cell cytoplasm and the extracellular environment allows a potential difference to occur on both sides of the membrane.

If we measure this difference in electrochemical potential inside and outside the membrane we would observe a difference of approximately -70mV. In this sense, the inner side of the neuron membrane is negative with respect to the outer side when there is no stimulus.

Ion channels and their importance

Voltage-gated ion channels allow ions to move across the membrane in response to changes in the membrane's electric field. There are several types of ion channels existing in the neuron, each of which will allow the passage of a specific ionic species.

These channels are not evenly distributed on the membrane. However, in the axonal membrane we can find fast-acting channels for Na + and K +, while in the axonal terminal we find Ca + channels.

K + channels are responsible for maintaining the resting state of electrically excitable cells when there are no stimuli to trigger AP, a phenomenon called passive changes in membrane potential.

While the Na + channels react quickly, intervening in the depolarization of the membrane when a PA or active change in the membrane potential is generated.

On the other hand, the Ca + channels, although they open more slowly during depolarization, play the fundamental role of propagating electrical signals and triggering the release of neurotransmitter signals at synapses.

Bioelements that participate in the excitability of the neuron

The impulse occurs due to the asymmetry in the concentration of bioelements and biomolecules between the cytoplasm and the extracellular medium. The most important ions that participate in the excitability of the neuron are Na +, K +, Ca2 + and Cl-.

There are also some organic anions and proteins that are located only in the intracellular fluid and cannot leave it because the plasma membrane is impermeable to these components.

Outside the cell there is a higher concentration of ions such as Na + (10 times more) and Cl- and inside up to 30 times more K + and a large amount of organic anions (proteins) that generate a negative charge in the cytoplasm.

As soon as the voltage-sensitive Na + and K + channels are open, the voltage changes will be transmitted to areas adjacent to the membrane and induce the opening of voltage-sensitive components in those areas and the transmission of the voltage change to others. farthest sectors.

After the closing of the Na + and K + channels, the gates are inactivated for a short period, which means that the momentum cannot go back.

Action potential dependencies

The production of the action potential then depends on three essential elements:

First, the active transport of ions by specific membrane proteins. This generates unequal concentrations of an ionic species or several on both sides of it.

Second, the uneven distribution of the ions generates an electrochemical gradient across the membrane that generates a source of potential energy.

Finally, ionic species selective gated ion channels allow ionic currents to flow driven by electrochemical gradients through these membrane-spanning channels.

Stages

Resting potential

When an action potential is not being transmitted, the neuron's membrane is at rest. In this instance, the intracellular fluid (cytoplasm) and the extracellular fluid contain different concentrations of inorganic ions.

This results in the outer layer of the membrane having a positive charge while the inner layer having a negative charge, meaning that the membrane at rest is "polarized". This resting potential has a value of -70mv, that is, the potential inside the cell is 70 mV more negative than the extracellular potential.

Na + entry and K + exit normally exist in the cell due to the effect of the concentration gradient (active transport). As there is more Na + outside the cell, it tends to enter and as there is more K + inside the cell, it tends to exit to equalize its concentration on both sides of the membrane.

The different ionic concentration is maintained by the action of a membrane protein called the "sodium and potassium pump". To preserve the potential difference, the Na + and K + pump removes 3 Na + ions from the cell for every two K + it introduces.

Nerve impulse formation

When a stimulus is presented in the receptor area of ​​the neuronal membrane, a generating potential is produced which increases the permeability to Na + in the membrane.

If this potential exceeds the excitability threshold that is -65 to -55 mV, a nerve impulse is generated and the Na + is introduced so fast that even the Na + and K + pump is inactivated.

The massive influx of positively charged Na + causes the aforementioned electrical charges to reverse. This phenomenon is known as membrane depolarization. The latter, stops at around + 40mv.

Upon reaching the threshold, a standard BP is always generated, as there are no large or small nerve impulses, consequently all action potentials are equal. If the threshold is not reached, nothing happens, which is known as the “all or nothing” principle.

The PA is very short lasting 2 to 5 milliseconds. The increase in the permeability of the membrane to Na + ceases quickly because the Na + channels are inactivated and the permeability to the K ions that flow from the cytoplasm increases, re-establishing the resting potential.

Impulse shift

The impulse does not remain in the neuronal membrane where it is generated as a consequence of a generator potential, but instead travels through the membrane along the neuron until it reaches the end of the axon.

The transmission of the impulse consists of its movement in the form of electrical waves along the nerve fiber. Once it reaches the terminal feet of the axon, it must cross a synapse, which is done by means of chemical neurotransmitters.

The AP travels along the nerve fiber continuously, if it does not have myelin, however, if it does, the myelin layers isolate the nerve fiber membrane over its entire surface except in the nodules of Ranvier. The PA in this situation advances in jumps from one node to the next, which is known as saltatory conduction.

This type of transmission saves a lot of energy and increases the speed of the impulse and transmission of the information since depolarization occurs only in the nodes of Ranvier. Speeds of up to 120 m / sec have been recorded, while for fibers not covered by myelin the approximate speed is 0.5 m / sec.

Synaptic transmission

The flow of the nerve impulse goes from the afferent end of the neuron that comprises the body and the dendrites to the efferent end formed by the axon and its collateral branches. The axonal endings at whose ends are the terminal feet or synaptic buttons are included here.

The area of ​​contact between one neuron and another or between a neuron and a muscle or glandular cell is called a synapse. For the occurrence of the synapse, neurotransmitters play a fundamental role so that the transmitted message has continuity on the nerve fibers.

Cyclical behavior of impulse

In essence, an action potential is a change in the polarity of the membrane from negative to positive and back to negative in a cycle that lasts from 2 to 5 milliseconds.

Each cycle comprises an ascending phase of depolarization, a descending phase of repolarization and a sub-descending phase called hyperpolarization at figures below -70 mv.

Features

The nerve impulse is an electrochemical message. It is a message because there is a recipient and a sender and it is electrochemical since there is an electrical component and a chemical component.

Through the nerve impulse (action potential), neurons transport information quickly and accurately to coordinate the actions of the entire body of an organism.

PAs are responsible for every memory, sensation, thought, and motor response. This in most cases occurs at great distances to control effector responses that include ion channel opening, muscle contraction, and exocytosis.

References

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