Potentially damaging mechanical, thermal, and chemical stimuli are detected by nerve endings called nociceptors, which are found in the skin, on internal surfaces such as the periosteum, joint surfaces, and in some internal organs. Some nociceptors are unspecialized free nerve endings that have their cell bodies outside the spinal column in the dorsal root ganglia. Other nociceptors rely on specialised structures in the skin to transduce noxious information such as nociceptive schwann cells. Nociceptors are categorized according to the axons which 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. 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 in order 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 time points. When the drug wears off, the threshold should return to the baseline value. In some conditions, excitation of pain fibers becomes greater as the pain stimulus continues, leading to a condition called hyperalgesia.
This overview discusses proprioception, thermoception, chemoception and nociception as they are all integrally connected.
Mechanical
Proprioception is determined by using standard mechanoreceptors and transient receptor potential. Proprioception is completely covered within the somatosensory system as the brain processes them together. Thermoception refers to stimuli of moderate temperatures , as anything beyond that range is considered pain and moderated by nociceptors. TRP and potassium channels each respond to different temperatures which create action potentials in nerves which join the mechano system in the posterolateral tract. Thermoception, like proprioception, is then covered by the somatosensory system. TRP channels that detect noxious stimuli relay that info to nociceptors that generate an action potential. Mechanical TRP channels react to depression of their cells, thermal TRP change shape in different temperatures, and chemical TRP act like taste buds, signalling if their receptors bond to certain elements/chemicals.
Lamina 2 makes up substantia gelatinosa of Rolando, unmyelinated spinal grey matter. Substantia receives input from nucleus proprius and conveys intense, poorly localized pain.
The parabrachial area integrates taste and pain info, then relays it. Parabrachial checks if the pain is being received in normal temperatures and if the gustatory system is active; if both are so the pain is assumed to be due to poison.
Ao fibers synapse on laminae 1 and 5 while Ab synapses on 1, 3, 5, and C. C fibers exclusively synapse on lamina 2.
Superior colliculus receives IC’s input, integrates visual orienting info, and uses the balance topographical map to orient the body to the pain stimuli.
Inferior cerebellar peduncle integrates proprioceptive info and outputs to the vestibulocerebellum. The peduncle is not part of the lateral-spinothalamic-tract-pathway; the medulla receives the info and passes it onto the peduncle from elsewhere.
The thalamus is where pain is thought to be brought into perception; it also aids in pain suppression and modulation, acting like a bouncer, allowing certain intensities through to the cerebrum and rejecting others.
Nociception has been documented in non-mammalian animals, including fish and a wide range of invertebrates, including leeches, nematode worms, sea slugs, and fruit flies. As in mammals, nociceptive neurons in these species are typically characterized by responding preferentially to high temperature, low pH, capsaicin, and tissue damage.
History of term
The term "nociception" was coined by Charles Scott Sherrington to distinguish the physiological process from pain. It is derived from the Latin verb "nocēre", which means "to harm".
