What is an action potential train?

The action potential train, or "spike train" is a concept widely used in neuroscience.

A train or chain of action potentials (spike train is a sequence of time recordings in which a neuron fires electrical signals or nerve impulses. This particular form of communication between neurons is the subject of interest and study by the neuroscientific community, although there are still many unanswered questions.

In this article we will see what these trains of action potentials are, what their duration and structure are, what the concept of neuronal coding consists of, and the current state of research in this area.

What is a train of action potentials?

To understand what action potential trains are, let's first look at what an action potential consists of.

Our brains contain about hundred billion neurons that fire signals to communicate with each other constantly.. These signals are electrochemical in nature and travel from the cell body of one neuron, through its axon or neurite, to the next neuron.

Each of these signals or electrical impulses is known as an action potential. Action potentials are produced in response to stimuli or spontaneously, and each firing usually lasts 1 millisecond. each firing usually lasts 1 millisecond.

A train of action potentials is simply a combined sequence of firings and non-firing. To make it easier to understand: imagine a digital sequence of zeros and ones, as in a binary system; we would assign a 1 for firing and a 0 for not firing. In that case, a train of action potentials could be coded as a numerical sequence, like this: 00111100. The first two zeros would represent the latency time between stimulus presentation and the first firing or action potential.

Trains of action potentials can be generated by direct sensory stimuli from vision, touch, sound, or smell; and can also be induced through abstract stimuli triggered by the use of cognitive processes such as memory (by evocation of memories (by evocation of memories, for example).

Duration and structure

The duration and structure of a train of action potentials generally depend on the intensity and duration of the stimulus. Such action potentials usually last and remain "active" as long as the stimulus is present.

However, some neurons possess special electrical properties that cause them to produce a sustained response to a very brief stimulus. In such neurons, stimuli of greater intensity tend to elicit longer trains of action potentials..

When action potentials are repeatedly recorded from a neuron in response to changing stimuli (or when an organism generates different behaviors), they usually maintain a relatively stable shape. However, the firing pattern of each train of action potentials varies as the stimulus changes; generally, the rate at which firing occurs (the firing rate) changes with different conditions.

Neuronal coding

Action potential trains have been and continue to be of interest to the neuroscientific community, given their particularities.given their particularities. Many researchers try to find out in their studies what kind of information is encoded in these action potentials and how neurons are able to decode it.

Neural coding is a field of neuroscience that studies how sensory information is represented in our brain by means of neural networks. Researchers often encounter great difficulties when trying to decipher trains of action potentials.

It is difficult to think of a train of action potentials as if it were a purely binary output device.. Neurons possess a minimum threshold of activation and fire only if the stimulus intensity is above that threshold. If a constant stimulus is presented, a train of action potentials will be generated. However, the activation threshold will increase over time.

The latter, which is what is referred to as sensory adaptation, is the result of processes such as synaptic desensitization. the result of processes such as synaptic desensitization, a decrease in the response to the constant stimulus produced at the synapse (the chemical connection between two neurons).a decrease in the response to the constant stimulus produced at the synapse (the chemical connection between two neurons).

This result will lead to a reduction in the firing associated with the stimulus, which will eventually decrease to zero. This process helps the brain not to be overloaded with information from the environment that remains unchanged.. For example, when after a while we stop smelling the perfume we have applied or when we adapt to a background noise that initially disturbs us.

Recent research

As we already know, neurons communicate through the generation of action potentials, which can propagate from one neuron (transmitter or presynaptic) to another (receptor or postsynaptic) through the synapse. Thus, when the presynaptic neuron generates the action potential, the postsynaptic neuron is able to receive it and generate a response that can eventually produce a new action potential, in this case postsynaptic.

Different sequences or trains of presynaptic action potentials generally produce different chains of postsynaptic action potentials. This is why the neuroscientific community believes that there is a "neuronal code" associated with the temporality of action potentials, i.e. that the same neuron produces different action potentials.i.e., the same neuron could be using several different action potential sequences to encode different types of information.

On the other hand, the electrical activity of a neuron is usually quite variable and is rarely determined in its entirety.and is rarely determined entirely by the stimulus. In the face of successive repetitions of the same stimulus, the neuron will respond each time with a different chain of action potentials. So far, researchers have not been able to characterize the response of neurons to stimuli nor have they been able to determine clearly how the information is encoded.

What had been thought so far was that all the information stored in a train of action potentials was encoded in its frequency; that is, in the number of action potentials produced per unit of time. But in recent years, research is investigating the possibility that the precise instants at which each action potential is produced may contain critical information and even a "neuronal signature. a "neuronal signaturethat is, a kind of temporal pattern that would make it possible to identify the transmitting neuron.

The most recent investigations point to the design of a new method that would make it possible to characterize a chain of action potentials based on the timing of each of the action potentials in the chain. With the application of this procedure, it would be possible to align the different sequences and determine which action potentials are equivalent in each of the chains. And with that information, the statistical distribution that each action potential follows in a hypothetical "ideal train" could be calculated..

This ideal train of action potentials would represent the common pattern, of which each of the real trains is only a concrete realization. Once characterized, it would be possible to know whether or not a new chain of action potentials might fit the distribution, and thus to know whether it is encoding the same information. This concept of the ideal train could have interesting implications for the study and interpretation of the neural code, as well as for strengthening the theory of neural signatures.

Bibliographical references:

• Strong, S.P., Koberle, R., de Ruyter van Steveninck. R.R., Bialek, W. (1998). Entropy and information in neural spike trains. Phys Rev Lett; 80: pp. 197 - 200.