Nociception
Inphysiology,nociception(/ˌnəʊsɪˈsɛpʃ(ə)n/), alsonocioception;fromLatinnocere'toharm/hurt') is thesensory nervous system's process of encodingnoxious stimuli.It deals with a series of events and processes required for an organism to receive apainfulstimulus, convert it to a molecular signal, and recognize and characterize the signal to trigger an appropriate defensive response.
In nociception, intense chemical (e.g.,capsaicinpresent inchili pepperorcayenne pepper), mechanical (e.g., cutting, crushing), or thermal (heat and cold) stimulation ofsensory neuronscallednociceptorsproduces a signal that travels along a chain ofnerve fibersvia thespinal cordto thebrain.[1]Nociception triggers a variety of physiological and behavioral responses to protect the organism against an aggression, and usually results in a subjective experience, orperception,of pain insentientbeings.[2]
Detection of noxious stimuli
[edit]
Potentially damaging mechanical, thermal, and chemical stimuli are detected by nerve endings called nociceptors, which are found in theskin,on internal surfaces such as theperiosteum,jointsurfaces, and in some internalorgans.Some nociceptors are unspecializedfree nerve endingsthat have their cell bodies outside thespinal columnin thedorsal-root ganglia.[3]Others are specialised structures in the skin such as nociceptiveschwann cells.[4]Nociceptors are categorized according to theaxonswhich travel from the receptors to the spinal cord or brain. After nerve injury it is possible for touch fibres that normally carry non-noxious stimuli to be perceived as noxious.[5]
Nociceptive pain consists of an adaptive alarm system.[6]Nociceptors have a certain threshold; that is, they require a minimum intensity of stimulation before they trigger a signal. Once this threshold is reached, a signal is passed along the axon of the neuron into the spinal cord.
Nociceptive threshold testing deliberately applies a noxious stimulus to a human or animal subject to study pain. In animals, the technique is often used to study the efficacy of analgesic drugs and to establish dosing levels and period of effect. After establishing a baseline, the drug under test is given and the elevation in threshold recorded at specified times. When the drug wears off, the threshold should return to the baseline (pretreatment) value. In some conditions, excitation of pain fibers becomes greater as the pain stimulus continues, leading to a condition calledhyperalgesia.
Theory
[edit]Consequences
[edit]Nociception can also cause generalizedautonomic responsesbefore or without reaching consciousness to causepallor,sweating,tachycardia,hypertension,lightheadedness,nausea,andfainting.[7]
System overview
[edit]
This overview discussesproprioception,thermoception,chemoception, and nociception, as they are all integrally connected.
Mechanical
[edit]Proprioception is determined by using standard mechanoreceptors (especiallyruffini corpuscles(stretch) andtransient receptor potential channels(TRP channels). Proprioception is completely covered within thesomatosensory system,as the brain processes them together.
Thermoception refers to stimuli of moderate temperatures 24–28 °C (75–82 °F), as anything beyond that range is considered pain and moderated by nociceptors. TRP and potassium channels [TRPM (1-8), TRPV (1-6), TRAAK, and TREK] each respond to different temperatures (among other stimuli), which create action potentials in nerves that join the mechano (touch) system in the posterolateral tract. Thermoception, like proprioception, is then covered by the somatosensory system.[8][9][10][11][12]
TRP channels that detect noxious stimuli (mechanical, thermal, and chemical pain) relay that information to nociceptors that generate an action potential. Mechanical TRP channels react to depression of their cells (like touch), thermal TRPs change shape in different temperatures, and chemical TRPs act liketaste buds,signalling if their receptors bond to certain elements/chemicals.
Neural
[edit]- Laminae 3-5make upnucleus propriusin spinal grey matter.
- Lamina 2makes upsubstantia gelatinosa of Rolando,unmyelinated spinal grey matter. Substantia receives input from nucleus proprius and conveys intense, poorly localized pain.
- Lamina 1primarily project to theparabrachial areaandperiaqueductal grey,which begins the suppression of pain via neural and hormonal inhibition. Lamina 1 receive input from thermoreceptors via theposterolateral tract.Marginal nucleus of the spinal cord are the only unsuppressible pain signals.
- Theparabrachial areaintegrates taste and pain info, then relays it. Parabrachial checks if the pain is being received in normal temperatures and if thegustatory systemis active; if both are so the pain is assumed to be due to poison.
- Ao fiberssynapse on laminae 1 and 5 whileAbsynapses on 1, 3, 5, and C.C fibersexclusively synapse on lamina 2.[13][14]
- Theamygdalaandhippocampuscreate and encode the memory and emotion due to pain stimuli.
- Thehypothalamussignals for the release of hormones that make pain suppression more effective; some of these are sex hormones.
- Periaqueductal grey(with hypothalamic hormone aid) hormonally signalsreticular formation'sraphe nucleito produceserotoninthat inhibits laminae pain nuclei.[15]
- Lateral spinothalamic tractaids in localization of pain.
- Spinoreticularandspinotectal tractsare merely relay tracts to thethalamusthat aid in the perception of pain and alertness towards it. Fibers cross over (left becomes right) via the spinalanterior white commissure.
- Lateral lemniscusis the first point of integration of sound and pain information.[16]
- Inferior colliculus(IC) aids in sound orienting to pain stimuli.[17]
- Superior colliculusreceives IC's input, integrates visual orienting info, and uses the balance topographical map to orient the body to the pain stimuli.[18][19]
- Inferior cerebellar peduncleintegrates proprioceptive info and outputs to thevestibulocerebellum.The peduncle is not part of the lateral-spinothalamic-tract-pathway; the medulla receives the info and passes it onto the peduncle from elsewhere (seesomatosensory system).
- Thethalamusis where pain is thought to be brought intoperception;it also aids in pain suppression and modulation, acting like abouncer,allowing certain intensities through to the cerebrum and rejecting others.[20]
- Thesomatosensory cortexdecodes nociceptor info to determine the exact location of pain and is where proprioception is brought into consciousness; inferior cerebellar peduncle is all unconscious proprioception.
- Insulajudges the intensity of the pain and provides the ability to imagine pain.[21][22]
- Cingulate cortexis presumed to be the memory hub for pain.[23]
In non-mammals
[edit]Nociception has been documented in other animals, including fish[24]and a wide range ofinvertebrates,[25]including leeches,[26]nematode worms,[27]sea slugs,[28]and fruit flies.[29]As in mammals, nociceptive neurons in these species are typically characterized by responding preferentially to high temperature (40 °C or more), low pH, capsaicin, and tissue damage.
History of term
[edit]The term "nociception" was coined byCharles Scott Sherringtonto distinguish the physiological process (nervous activity) from pain (a subjective experience).[30]It is derived from the Latin verbnocēre,which means "to harm".
See also
[edit]- Electroreception– Biological electricity-related abilities
- Mechanoreceptor– Sensory receptor cell responding to mechanical pressure or strain
- Thermoception– Sensation and perception of temperature
- Proprioception– Sense of self-movement, force, and body position
References
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