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Nociceptor
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==Types and functions== Nociceptors are usually electrically silent when not stimulated.<ref name="Nociceptors: the sensors of the pai"/> The peripheral terminal of the mature nociceptor is where the [[noxious stimuli]] are detected and transduced into electrical energy.<ref>Fein, A Nociceptors: the cells that sense pain http://cell.uchc.edu/pdf/fein/nociceptors_fein_2012.pdf</ref> 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 [[Nerve conduction velocity|conduction velocity]], nociceptors come in two groups. The [[A delta fiber|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.<ref>{{cite book |author1=Williams, S. J. |author2=Purves, Dale |title=Neuroscience |publisher=Sinauer Associates |location=Sunderland, Mass |year=2001 |isbn=978-0-87893-742-4 }}</ref><ref name="Nociceptors: the sensors of the pai"/> As a result, pain comes in two phases: an initial extremely sharp pain associated with the Aδ fibers and a second, more prolonged and slightly less intense feeling of pain from the C fibers. Massive or prolonged input to a C fiber results in a progressive build up in the [[Posterior grey column|dorsal horn]] of the spinal cord; this phenomenon called [[Pain wind-up|wind-up]] is similar to [[tetanus]] in muscles. Wind-up increases the probability of greater sensitivity to pain.<ref name = Fields>{{cite journal |vauthors=Fields HL, Rowbotham M, Baron R |title=Postherpetic neuralgia: irritable nociceptors and deafferentation |journal=Neurobiol. Dis. |volume=5 |issue=4 |pages=209–27 |date=October 1998 |pmid=9848092 |doi=10.1006/nbdi.1998.0204 |s2cid=13217293 }}</ref> ===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 [[Transient receptor potential channel|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 [[Adenosine triphosphate|ATP]], which can be detected by [[purine receptor|P2 purinergic receptors]], or [[nerve growth factor]], which can be detected by [[tropomyosin receptor kinase A]] (TrkA).<ref name="Ganong 2019"/> ===Chemical=== Chemical nociceptors have TRP channels that respond to a wide variety of spices. The one that sees the most response and is very widely tested is [[capsaicin]]. Other chemical stimulants are environmental irritants like [[acrolein]], a [[World War I]] [[chemical weapon]] and a component of cigarette smoke. Apart from these external stimulants, chemical nociceptors have the capacity to detect endogenous ligands, and certain fatty acid amines that arise from changes in internal tissues. Like in thermal nociceptors, TRPV1 can detect chemicals like capsaicin and spider toxins and acids.<ref name = Woolf/><ref name="Ganong 2019">{{cite book|title=Ganong's Review of Medical Physiology|year=2019|isbn=978-1-260-12240-4|last1=Yuan|first1=Jason|last2=Brooks|first2=Heddwen L.|last3=Barman|first3=Susan M.|last4=Barrett|first4=Kim E.|publisher=McGraw-Hill Education }}</ref> [[Acid-sensing ion channel|Acid-sensing ion channels (ASIC)]] also detect acidity.<ref name="Ganong 2019"/> ===Sleeping/silent=== Although each nociceptor can have a variety of possible threshold levels, some do not respond at all to chemical, thermal or mechanical stimuli unless injury actually has occurred. These are typically referred to as silent or sleeping nociceptors since their response comes only on the onset of inflammation to the surrounding tissue.<ref name = Jessell/> They were identified using electrical stimulation of their [[receptive field]].<ref name="Nociceptors: the sensors of the pai"/> ===Polymodal=== Nociceptors that respond to more than one type of stimuli are called ''polymodal''.<ref>{{cite book|last=Fein|first=Alan|title=Nociceptors: the cells that sense pain|url=https://books.google.com/books?id=hmOsHYsGdh0C&q=nociceptor&pg=SA1-PA5}}</ref> They are the most common type of C-fiber nociceptors and express a rich repertoire of [[neurotransmitters]].<ref name="Nociceptors: the sensors of the pai"/>
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