Nociceptors (pain receptors): definition and types

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

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

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What are nociceptors? A definition

Nociceptors are sensory receptors that respond to stimuli that damage tissues or could do so, 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, that is, to the brain and spinal cord.

Nociceptors are located in different parts of the body, both in external and internal tissues. A) Yes,

the sensation of pain does not only occur on the skin or in the mucous membranes, but also in the muscles, the intestine or the bladder.

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

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

Types of nociceptor

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

We can divide the nociceptors depending on the kinds of stimuli to which they respond, although some of them react to various sensory modalities.

1. Mechanics (mechanoreceptors)

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

This type of nociception carries with it very fast responses because mechanical receptors transmit afferents through Delta A fibers, fast conducting myelinated nerves.

• Related article: "Myelin: definition, functions and characteristics"

2. Thermal (thermoreceptors)

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

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

3. Chemicals (chemoreceptors)

Chemical nociceptors respond to various chemical compounds released by tissues when damaged, such as bradykinin and histamine. They also detect the presence of external toxic substances that can cause tissue damage, such as capsaicin from hot peppers 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, once it has been produced. inflammation of the tissue adjacent to the injury.

5. Polymodal

Polymodal nociceptors respond to different types of stimulation: mechanical, thermal, and chemical. They conduct pain signals through C fibers, 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 spinothalamic tract, which connects the skin with the thalamus, a key structure in shipping sensory input to the brain.

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

There are three components to 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 the anterior cingulate. The long-term feeling of pain is related to the prefrontal cortex.

The Melzack and Wall gate theory proposes that the perception of non-painful stimuli blocks the transmission of pain signals to the central nervous system; thus, the experience of pain can be nullified if non-harmful tactile sensations predominate. Therapy by transcutaneous electrical stimulation is based on this theory.

Pain inhibition can also occur downstream, from the brain to the nociceptive neurons. In this sense, the endorphins of the periaqueductal gray matter, the serotonin secreted by the raphe nuclei and the noradrenaline of the cerulean locus.