The action potential train, or “spike train” is a widely used concept in neuroscience.
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 answer.
In this article we will see what these action potential trains are, what their duration and structure are, what the concept of neuronal coding consists of, and in what state the research in this matter is currently.
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 a hundred billion neurons that constantly fire signals to communicate with each other. 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 are produced in response to stimuli or spontaneously, with each trigger typically lasting 1 millisecond.
A train of action potentials is simply a combined sequence of shots and no shots. To be better understood: imagine a digital sequence of zeros and ones, as in a binary system; we would assign a 1 for the trigger and a 0 for the non-trigger. 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.
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).
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 “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 these types of neurons, stimuli of greater intensity tend to 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 shape. However, the firing pattern of each train of action potentials varies as the stimulus changes; Generally, the speed at which shots are produced (the rate of fire) changes depending on different conditions.
The action potential trains have been and continue to be of interest to the neuroscientific community, given their particularities. Many researchers try to find out in their studies what kind of information these action potentials are encoded 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 find it difficult to decipher action potential trains.
It’s hard to think of a train of action potentials as 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 trigger 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 in 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 that we have applied or when we adapt to a background noise that initially disturbs us.
As we already know, neurons communicate through the generation of action potentials, which can spread from one neuron (emitter 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, 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. That is why 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 action potential sequences to encode 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. 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 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 are produced per unit of time. But in recent years, the possibility is being investigated 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 emitting neuron to be identified.
The most recent research points to the design of a new method that would allow characterizing a chain of action potentials based on the times of each of its action potentials. With the application of this procedure, it could 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 if a new chain of action potentials could fit the distribution or not, and therefore, to 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 to reinforce the theory of neural signatures.
- Strong, SP, Koberle, R., by Ruyter van Steveninck. RR, Bialek, W. (1998). Entropy and information in neural spike trains. Phys Rev Lett; 80: pp. 197-200.