What is a train of action potentials?

A train or chain of action potentials (spike train in English) is a sequence of temporal records in which a neuron fires electrical signals or nerve impulses. This particular form of communication between neurons is the object of interest and study by the neuroscientific community, although many answers remain to be answered.

In this article we will see what these trains of action potentials are, what is their duration and structure, in what consists of the concept of neural coding and what state of research is currently in this subject.

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

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 firing signals to communicate with each other constantly. These signals are electrochemical in nature and travel from the cell body of a neuron, through its axon or neurite, to the next neuron.

Each of these electrical signals or impulses is known as an action potential. Action potentials occur in response to stimuli or spontaneously, and

each shot typically lasts 1 millisecond.

A train of action potentials is simply a combined sequence of firing and non-firing. To make it easier to understand: let's imagine a digital sequence of zeros and ones, as in a binary system; we would assign a 1 for the trip and a 0 for the no trip. In that case, a train of action potentials could be encoded as a numerical sequence, such as: 00111100. The first two zeros would represent the latency time between the presentation of the stimulus and the first firing or action potential.

Action potential trains can be generated through direct sensory input from sight, touch, sound, or smell; and they can also be induced by abstract stimuli triggered by the use of cognitive processes such as memory (by evoking memories, for example).

• You may be interested in: "Action potential: what is it and what are its phases?"

duration and structure

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

However, some neurons have special electrical properties that cause them to produce a sustained response to a very brief stimulus. In this type of neurons, the stimuli of greater intensity tend to provoke longer trains of action potentials..

When action potentials are repeatedly recorded from a neuron in response to stimuli changing (or when an organism generates different behaviors), they tend to maintain a relatively stable. However, the firing pattern of each train of action potentials varies as the stimulus changes; Generally, the speed at which shots are fired (the rate of fire) changes depending on different conditions.

neural coding

action potential trains have been and continue to be of interest to the neuroscientific community, given its peculiarities. 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 run into great difficulty trying to decipher action potential trains.

It is difficult to think of a train of action potentials as being a purely binary output device.. Neurons have a minimum activation threshold and fire only if the intensity of the stimulus 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 called sensory adaptation, is 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).

This result will lead to a reduction in the firing associated with the stimulus, which will eventually decrease to zero. said 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 that 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 are can spread from one neuron (sending or presynaptic) to another (receiving 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, eventually, can 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. It is because of that the neuroscientific community believes that there is a "neural code" associated with the timing of action potentials; that is, that the same neuron could be using several different sequences of action potentials to encode, for its part, different types of information.

On the other hand, the electrical activity of a neuron is usually certainly variable, and is seldom entirely determined by the stimulus. Before 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 clearly determine how information is encoded.

What had been thought up to now is that all the information stored in a train of action potentials was encoded in its frequency; that is, in the number of action potentials that occur per unit of time. But in recent years, the possibility is being investigated that the precise instants at which each action potential occurs may contain critical information and even a “neural signature”; that is, a kind of temporal pattern that would allow the emitting neuron to be identified.

The most recent investigations point to the design of a new method that would allow characterizing a chain of action potentials based on the times of each of the action potentials of the same. By applying 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 follows each action potential in a hypothetical “ideal train” could be calculated.

That ideal train of action potentials would represent the common pattern, of which each of the actual trains is only a concrete realization. Once characterized, it would be possible to know if a new chain of action potentials could fit the distribution or not, and therefore know if 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 reinforcing the theory of neural signatures.

Bibliographic 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.