Nociceptors (pain Receptors): Definition And Types
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 caused by the organism itself.
In this article we will describe What are nociceptors and how the 5 main types differ We will also briefly explain how the experience of pain works at the level of the spinal cord and brain, and the ways in which it can be inhibited.
Nociceptors are sensory receptors that respond to stimuli that injure tissues or that 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, the brain and spinal cord.
Nociceptors are located in different parts of the body, both in external and internal tissues. So, 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 nociceptors can occur through direct stimulation of tissues or indirectly, through the release of chemical substances in damaged tissue. These compounds include histamine, bradykinin potassium, serotonin, acetylcholine, substance P and ATP.
The axons of nociceptors 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.
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 depending on the types of stimuli to which they respond although some of them react to several sensory modalities.
1. Mechanical (mechanoreceptors)
Mechanical nociceptors are activated by intense tactile sensations, such as pricks, pressure or deformation; therefore, they respond to cuts and blows. Its response frequency is greater the more harmful the stimulus is.
This type of nociception entails very fast responses because the mechanical receptors transmit afferents through A delta fibers, fast-conducting myelinated nerves
2. Thermal (thermoreceptors)
Conduction from thermal nociceptors also takes place through A delta fibers and therefore transmits 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 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 pepper and acrolein from tobacco.
4. Silent
This type of nociceptor is not characterized by the type of stimulus that activates it but by the fact that it responds late, once it has occurred. 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.
Different spinal tracts transmit pain signals from the nociceptors to the cerebral cortex. In particular, it has been highlighted relevance of the spinothalamic tract which connects the skin with the thalamus, a key structure in sending sensory input to the brain.
Nociceptive fibers are located in the dorsal (or posterior) horn 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 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. Transcutaneous electrical stimulation therapy is based on this theory.
Pain inhibition can also occur downstream, from the brain to nociceptive neurons. In this sense, the endorphins of the periaqueductal gray matter, the serotonin secreted by the raphe nuclei and the norepinephrine of the locus coeruleus are very important.
