A nociceptor () is a sensory neuron that responds to damaging or potentially damaging stimuli by sending "possible threat" signals to the spinal cord and the brain. The brain creates the sensation of pain to direct attention to the body part, so the threat can be mitigated; this process is called nociception.
Terminology
Nociception and pain are usually evoked only by pressures and temperatures that are potentially damaging to tissues. This barrier or threshold contrasts with the more sensitive visual, auditory, olfactory, taste, and somatosensory responses to stimuli. The experience of pain is individualistic and can be suppressed by stress or exacerbated by anticipation. Simple activation of a nociceptor does not always lead to perceived pain, because the latter also depends on the frequency of the action potentials, integration of pre- and postsynaptic signals, and influences from higher or central processes.
Nociceptors in peripheral tissues detect potentially harmful stimuli and send signals through peripheral nerves to the spinal cord, which conveys the information to the thalamus and then to the somatosensory cortex, where the location and intensity of pain are processed.
The insular cortex, another key region in pain processing networks, is involved in integrating sensory and emotional aspects of pain, and studies have reported disrupted function and reduced gray matter volume in this area among individuals with chronic pain.
Scientific investigation
Nociceptors were discovered by Charles Scott Sherrington in 1906. In earlier centuries, scientists believed that animals were like mechanical devices that transformed the energy of sensory stimuli into motor responses. Sherrington used many different experiments to demonstrate that different types of stimulation to an afferent nerve fiber's receptive field led to different responses. Some intense stimuli trigger reflex withdrawal, certain autonomic responses, and pain. The specific receptors for these intense stimuli were called nociceptors.
Studies of nociceptors have been conducted on conscious humans as well as surrogate animal models. The process is difficult due to invasive methods that could change the cellular activity of nociceptors being studied, the inability to record from small neuronal structures, and uncertainties in animal model systems as to whether a response should be attributed to pain or some other factor. The trigeminal ganglia are specialized nerves for the face, whereas the dorsal root ganglia are associated with the rest of the body. The axons extend into the peripheral nervous system and terminate in branches to form receptive fields.
Types and functions
Nociceptors are usually electrically silent when not stimulated. When the electrical energy reaches a threshold value, an action potential is induced and driven towards the central nervous system (CNS). This leads to the train of events that allows for the conscious awareness of pain. The sensory specificity of nociceptors is established by the high threshold only to particular features of stimuli. Only when the high threshold has been reached by either chemical, thermal, or mechanical environments are the nociceptors triggered.
In terms of their conduction velocity, nociceptors come in two groups. The Aδ fiber axons are myelinated and can allow an action potential to travel towards the CNS at speeds from 5 to 30 meters/second. The C fiber axons conduct more slowly at speeds from 0.4 to 2 meters/second due to their smaller diameters and little or no myelination of their axon.
Thermal
Thermal nociceptors are activated by noxious heat or cold at various temperatures. There are specific nociceptor transducers that are responsible for how and if the specific nerve ending responds to the thermal stimulus. The first to be discovered was TRPV1, and it has a threshold that coincides with the heat pain temperature of 43 °C. Other temperature in the warm–hot range is mediated by more than one TRP channel. Each of these channels express a particular C-terminal domain that corresponds to the warm–hot sensitivity. The interactions between all these channels and how the temperature level is determined to be above the pain threshold are unknown at this time. The cool stimuli are sensed by TRPM8 channels. Its C-terminal domain differs from the heat sensitive TRPs. Although this channel corresponds to cool stimuli, it is still unknown whether it also contributes in the detection of intense cold. An interesting finding related to cold stimuli is that tactile sensibility and motor function deteriorate while pain perception persists.
Mechanical
Mechanical nociceptors respond to excess pressure or mechanical deformation. They also respond to incisions that break the skin surface. The reaction to the stimulus is processed as pain by the cortex, just like chemical and thermal responses. These mechanical nociceptors frequently have polymodal characteristics. So it is possible that some of the transducers for thermal stimuli are the same for mechanical stimuli. The same is true for chemical stimuli, since TRPA1 appears to detect both mechanical and chemical changes. Some mechanical stimuli can cause release of intermediate chemicals, such as ATP, which can be detected by P2 purinergic receptors, or nerve growth factor, which can be detected by tropomyosin receptor kinase A (TrkA). Acid-sensing ion channels (ASIC) also detect acidity. They are the most common type of C-fiber nociceptors and express a rich repertoire of neurotransmitters. This effect of descending inhibition can be shown by electrically stimulating the periaqueductal grey area of the midbrain or the periventricular nucleus. They both in turn project to other areas involved in pain regulation, such as the nucleus raphe magnus which also receives similar afferents from the nucleus reticularis paragigantocellularis (NPG). In turn the nucleus raphe magnus projects to the substantia gelatinosa region of the dorsal horn and mediates the sensation of spinothalamic inputs. This is done first by the nucleus raphe magnus sending serotonergic neurons to neurons in the dorsal cord, that in turn secrete enkephalin to the interneurons that carry pain perception. Enkephalin functions by binding opioid receptors to cause inhibition of the post-synaptic neuron, thus inhibiting pain.
Following sensory neurogenesis, differentiation occurs, and two types of nociceptors are formed. They are classified as either peptidergic or nonpeptidergic nociceptors, each of which express a distinct repertoire of ion channels and receptors. Their specializations allow the receptors to innervate different central and peripheral targets. This differentiation occurs in both perinatal and postnatal periods. The nonpeptidergic nociceptors switch off the TrkA and begin expressing RET proto-oncogene, which is a transmembrane signaling component that allows the expression of glial cell line-derived neurotrophic factor (GDNF). This transition is assisted by runt-related transcription factor 1 (RUNX1) which is vital in the development of nonpeptidergic nociceptors. On the contrary, the peptidergic nociceptors continue to use TrkA, and they express a completely different type of growth factor. There currently is a lot of research about the differences between nociceptors. including fish and a wide range of invertebrates, including leeches, nematode worms, sea slugs, and larval fruit flies. Although these neurons may have pathways and relationships to the central nervous system that are different from those of mammalian nociceptors, nociceptive neurons in non-mammals often fire in response to similar stimuli as mammals, such as high temperature (40 degrees C or more), low pH, capsaicin, and tissue damage.
For example, in fruit flies, specific multidendritic sensory neurons play a role in nociception. In mollusks, nociceptive responses are mediated by pedal sensory neurons. Crustaceans, on the other hand, utilize a variety of sensory cell types, including chordotonal organs and mechanoreceptors, to detect potentially damaging stimuli (see also Pain in crustaceans).
See also
- Capsaicin and its mechanism of action in nociceptors
- Nociceptin and nociceptin receptor
- Piperine from black pepper
- TRPC ion channel