Nociceptors (pain receptors): definition and types

Your body is full of neurons that specialize in alerting you when they detect possible damage and dangers.

We call "nociceptor" the cell endings that detect Pain sensations and transmit them to other areas of the central nervous system. The different types of nociceptors respond to mechanical, thermal or chemical stimuli, both external and those provoked by the body itself.

In this article we will describe what nociceptors are and how the 5 main types differ.. We will also briefly explain how the experience of pain works at the level of the medulla and the brain, and the ways in which it can be inhibited.

What are nociceptors? A definition

The nociceptors are sensory receptors that respond to stimuli that injure tissues or could injure tissues, and are located at the end of the axon of a sensory neuron. This response, known as nociception, consists of the emission of pain signals to the central nervous system, i.e., the brain and spinal cord.

Nociceptors are located in different parts of the body, both in external and internal tissues. Thus, the sensation of pain is not only produced in the brain, but also in the spinal cord, Thus, the sensation of pain is not only produced in the skin or mucous membranes, but also in the muscles, the intestine or the bladder.

Activation of nociceptors can occur either by direct stimulation of the tissues or indirectly, by the release of chemicals in the damaged tissue. These compounds include histamine, bradykininpotassium, serotonin, acetylcholine, substance P and ATP.

Nociceptor axons can be of two types: A delta (Aδ) fibers and C fibers. The former are myelinated, so action potentials are transmitted with great speed through these fibers. C-fibers, on the other hand, are much slower because these axons contain a lower amount of myelin.

Nociceptor types

The transmission of nociceptive signals is triggered when tissues detect injurious stimulation of different types, such as compression or intense heat.

We can divide nociceptors according to the types of stimuli to which they respond.Some of them, however, react to several sensory modalities.

1. Mechanical (mechanoreceptors)

Mechanical nociceptors are activated by intense tactile sensations, such as punctures, pressure or deformation; they therefore respond to cuts and shocks. Their frequency of response is greater the more damaging the stimulus.

This type of nociception involves very fast responses because mechanical receptors transmit afferents via A delta fibers, fast-conducting myelinated nerves..

2. Thermal (thermoreceptors)

Conduction from thermal nociceptors also takes place through A delta fibers and is therefore transmitted at a high speed.

These nociceptors are activated when they detect very high or very low temperatures (more than 42°C or less than 5°C), as well as intense mechanical stimuli (more than 42°C or less than 5°C).as well as intense mechanical stimuli.

3. Chemical (chemoreceptors)

Chemical nociceptors respond to a variety of chemical compounds chemical compounds that are released by tissues upon damage, such as bradykininsuch as bradykinin and histamine. They also detect the presence of external toxic substances that can cause tissue injury, such as capsaicin from hot bell pepper and acrolein from tobacco.

4. Silent

This class of nociceptor is not characterized by the type of stimulus that activates it, but by the fact that it responds late, after the following has occurred inflammation of the tissue adjacent to the lesion..

5. Polymodal

Polymodal nociceptors respond to different types of stimulation: mechanical, thermal and chemical. They conduct pain signals through C-fibers, which are significantly slower than A-fibers. We can find this type of nociceptors in the dental pulp, among other parts of the body.

Pain pathways and their inhibition

Different spinal tracts transmit pain signals from the nociceptors to the cerebral cortex. In particular, the relevance of the relevance of the spinothalamic tractwhich connects the skin with the thalamus, a key structure in sending sensory afferents to the brain.

Nociceptive fibers are located in the dorsal (or posterior) horn of the spinal cord and are composed, as mentioned above, of A delta fibers and C fibers, as well as projection neurons and inhibitory interneurons.

There are three components of the pain experience: sensation, emotion and cognition.. The primary and secondary somatosensory cortex processes the discriminative-sensory dimension, while the associated negative emotion depends on the insula and anterior cingulate. The long-term feeling of pain is related to the prefrontal cortex.

The gating theory of Melzack and Wall proposes that the perception of non-painful stimuli blocks the transmission of pain signals to the central nervous system; thus, the experience of pain may be overridden if non-harmful tactile sensations predominate. Transcutaneous electrical stimulation therapy is based on this theory.

Pain inhibition can also occur downstream, from the brain to the nociceptive neurons. Endorphins from the periaqueductal gray matter, serotonin secreted by the raphe nuclei and noradrenaline from the locus ceruleus are of great importance in this regard.