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 nociceptor respond to mechanical, thermal or chemical stimuli, both external and self-induced.

In this article we will describe what the 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 marrow and brain, and the ways in which it can be inhibited.

What are nociceptors? A definition

Nociceptors are sensory receptors that respond to tissue-injurious or potentially tissue-injurious stimuli, and are located at the end of the axon of a sensory neuron. This response, which is known as nociception, consists of the emission of pain signals to the central nervous system, i.e. the brain and the spinal cord.

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

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

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

Types of nociceptor

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

We can divide the nociceptors according to the classes of stimuli to which they respond , although some of them react to various sensory modes.

1. Mechanics (mechanic-receptors)

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

This type of nociception leads to very fast responses because mechanical receptors transmit afferences through delta A fibers, fast conducting myelinated nerves .

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2. Thermal (thermoreceptors)

Conduction from thermal nociceptors also takes place through delta A-fibres 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 that tissues release when they are 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 characterised by the type of stimulus that activates it but by the fact that it responds late, once there has been inflammation of the tissue adjacent to the lesion .

5. Polymodals

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 nociceptor in the dental pulp, among other parts of the body.

The pain pathways and their inhibition

Different spinal tracts transmit the pain signals from the nociceptors to the cerebral cortex. In particular, the relevance of the spinothalamic tract , which connects the skin to the thalamus, a key structure in sending sensory input to the brain, has been highlighted.

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

There are three components of the experience of pain: 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 feeling of long-term pain is related to the prefrontal cortex.

Melzack and Wall’s 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 annulled if non-harmful tactile sensations predominate. Therapy using transcutaneous electrical stimulation is based on this theory.

Pain inhibition can also occur downstream, from the brain to the nociceptive neurons. The endorphins of the periaqueductal grey matter, serotonin secreted by the raphe nuclei and noradrenalin from the cerulean locus are of great importance in this respect.