We call “nociceptors” the endings of cells 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 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 in the spinal cord and brain works, and the ways in which it can be inhibited.
What are nociceptors? a definition
Nociceptors are sensory receptors that they respond to stimuli that injure or could injure tissue, and are located at the end of the axon of a sensory neuron. This response, known as nociception, involves sending 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 the external and internal tissues. like that, 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.
Activation of nociceptors can occur through direct tissue stimulation or indirectly through the release of chemicals in damaged tissue. These compounds include histamine, bradykinin, Potassium, serotonin, acetylcholine, substance P and ATP.
Axons of nociceptors can be of two types: delta A fibers (Aδ) and C fibers. The former are myelinated, so that action potentials are transmitted at high speed through these fibers. For their part, the C fibers are much slower because these axons contain less myelin.
The transmission of nociceptive signals is triggered when tissues sense 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, While some of them react to various sensory modalities.
1. Mechanics (mechanoreceptors)
Mechanical nociceptors are activated from intense tactile sensations, such as punctures, pressure or deformation; therefore, they react to cuts and blows. Its frequency of response is all the greater as the stimulus is harmful.
This type of nociception involves very rapid responses because the mechanical receptors transmit afferents through A delta fibers, fast conduction myelinated nerves.
2. Thermal (thermoreceptors)
Conduction of thermal nociceptors also takes place through delta A fibers and are therefore transmitted at a high rate.
These nociceptors are activated when they detect very high or very low temperatures (over 42 ° C or less than 5 ° C), As well as intense mechanical stimuli.
3. Chemicals (chemoreceptors)
Chemical nociceptors respond to several chemicals that tissues release to be damaged, Such as bradykinin and histamine. They also detect the presence of external toxic substances that can damage tissue, such as capsaicin from spicy pepper and acrolein from tobacco.
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 occurred. inflammation of the tissue adjacent to the lesion.
Polymodal nociceptors respond to different types of stimulation: mechanical, thermal and chemical. They transmit pain signals through C fibers, which are significantly slower than A fibers. These types of nociceptors are found in dental pulp, among other parts of the body.
Pain pathways and their inhibition
Different spinal tracts transmit pain signals from nociceptors to the cerebral cortex. In particular the relevance of the spinothalamic tract, Which connects the skin to the thalamus, a key structure for sending sensory afferents to the brain.
Nociceptive fibers are located in the dorsal (or posterior) shaft of the spinal cord and are composed, as we have said, of delta A fibers and C fibers, as well as projection neurons and inhibitory interneurons.
There are three components to 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 previous coercion. Long-term pain sensation is related to the prefrontal cortex.
Melzack and Wall’s gate theory suggests that the perception of painless stimuli blocks the transmission of pain signals to the central nervous system; thus, the experience of pain can be canceled if non-harmful tactile sensations predominate. Transcutaneous electrical stimulation therapy is based on this theory.
Inhibition of pain can also occur in a downward direction, from the brain to nociceptive neurons. In this sense, the endorphins of the periaqueductal gray matter, the serotonin secreted by the nuclei of the raft and the norepinephrine of the locus coeruleus are very important.