An action potential train ( spike train is a sequence of time records 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 neuroscience 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 is, what the concept of neuronal coding consists of and what state the research in this area is currently in.

What is an action potential train?

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

Our brains contain about one 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 shot usually lasts 1 millisecond .

An action potential train is simply a combined sequence of shots and no shots. 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 triggering and a 0 for not triggering. In that case, a train of action potentials could be coded 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 trigger or action potential.

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

Duration and structure

The duration and structure of an action potential train generally depends on the intensity and duration of the stimulus. This type of action potential usually lasts and remains “active” as long as the stimulus is present.

However, some neurons have special electrical properties that cause them to respond in a sustained manner to a very brief stimulus. In this type of neurons, higher intensity stimuli usually cause trains with longer action potentials .

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

Neural coding

The action potential trains have been and continue to be of interest to the neuroscientific community , given their particularities. Many researchers are trying to find out in their studies what kind of information these action potentials carry in code and how the 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 the trains of action potentials.

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

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

This result will lead to a reduction in the triggers 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 to ourselves 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 spread from one neuron (emitting or presynaptic) to another (receiving or post-synaptic) through the synapse. Thus, when the presynaptic neuron generates the action potential, the postsynaptic neuron is capable of receiving it and generating a response that, eventually, can produce a new action potential, in this case postsynaptic.

Different sequences or trains of pre-synaptic action potentials generally produce different chains of post-synaptic action potentials. This is why the neuroscientific community believes that there is a “neuronal code” associated with the temporality 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 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 has it been possible to clearly determine how information is encoded.

What had been thought until now is that all the information stored in an action potential train was coded in its frequency; that is, in the number of action potentials that occur per unit of time. But in recent years, research is being carried out into the possibility that the precise moments in which each action potential is produced may contain critical information and even a “neuronal signature” ; that is, a kind of temporal pattern that would allow the identification of the emitting neuron.

The most recent research points to the design of a new method that would make it possible to characterize a chain of action potentials based on the times of each of its action potentials. 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 follows each action potential in a hypothetical “ideal train” could be calculated .

This 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 whether a new action potential train could fit the distribution or not, and therefore, to know if it is coding the same information. This concept of the ideal train could have interesting implications for the study and interpretation of the neuronal code, as well as for reinforcing the theory of neuronal signatures.

Bibliographic references:

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