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 there are still many answers to be answered.
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 what state the research in this area is currently in.
To understand what trains of action potentials 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 electrical signals or impulses is known as an action potential. Action potentials occur in response to stimuli or spontaneously, and each shot usually lasts 1 millisecond
A train of action potentials is, simply, a combined sequence of firing and non-firing. To make it better understood: let’s imagine a digital sequence of zeros and ones, as in a binary system; we would assign a 1 for the shot and a 0 for the non-shot. 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.
Trains of action potentials can be generated through direct sensory stimuli that come from vision, touch, sound, or smell; and They can also be induced by abstract stimuli triggered by the use of cognitive processes such as memory (by evocation of memories, for example).
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 “active” while 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, higher intensity stimuli usually provoke 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 form. However, the firing pattern of each train of action potentials varies as the stimulus changes; Generally, the speed at which shots occur (the firing rate) changes depending on different conditions.
Neural coding
Trains of action potentials have been and continue to be an object of interest for the neuroscientific community, given its particularities. Many researchers try to find out in their studies what type 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 through 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 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 constant stimulation produced at the synapse (the chemical connection between two neurons).
This result will lead to a reduction in 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 spread from one neuron (sender or presynaptic) to another (receiver or postsynaptic) 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 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 temporality of action potentials ; That is, the same neuron could be using several different sequences of action potentials to encode, in turn, different types of information.
On the other hand, The electrical activity of a neuron is usually variable, and is rarely entirely determined by the stimulus. When faced with 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 until 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 produced per unit of time. But in recent years, research has been conducted into the possibility that the precise moments in 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 sending neuron to be identified.
The most recent research points to the design of a new method that would allow the characterization of a chain of action potentials based on the times of each of the action potentials in it. 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 each action potential follows 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 whether a new chain of action potentials could fit the distribution or not, and therefore, 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 reinforcing the theory of neural signatures.
