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Nociception (also nocioception or nociperception) is the encoding and processing of harmful stimuli in the nervous system, and, therefore, the ability of a body to sense potential harm. It is the afferent activity in the peripheral and central nervous systems produced by stimulation of specialized free nerve endings called "nociceptors" or "pain receptors" that only respond to tissue damage caused by intense chemical (e.g., chilli powder in the eyes), mechanical (e.g., pinching, crushing) or thermal (heat and cold) stimulation. Once stimulated, a nociceptor sends a signal along a chain of nerve fibers via the spinal cord to the brain. Nociception triggers a variety of autonomic responses and may also result in a subjective experience of pain in sentient beings. Nociceptive neurons generate trains of action potentials in response to intense stimuli, and the frequency of firing determines the intensity of the pain.
The three types of pain receptors are cutaneous (skin), somatic (joints and bones), and visceral (body organs). It was previously believed that pain was simply the overloading of sensory receptors, but research in the first half of the 20th century indicated that pain is a distinct phenomenon that intertwines with all of the other senses, including touch. Pain was once considered a non-material experience, but recent studies show that pain is registered in specific parts of the brain. The main function of pain is to attract our attention to dangers and motivate us to avoid them.
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. The concentration of nociceptors varies throughout the body; they are found in greater numbers in the skin than in deep internal surfaces. The nociceptors are unspecialized free nerve endings that have their cell bodies outside the spinal column in the dorsal root ganglia. Nociceptors are categorized according to the axons which travel from the receptors to the spinal cord or brain.
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.
In some conditions, excitation of pain fibers becomes greater as the pain stimulus continues, leading to a condition called hyperalgesia.
Before reaching the brain, the spinothalamic tract splits into the lateral, "neospinothalamic" tract and the medial, "paleospinothalamic" tract.
Fast pain travels via type Aδ fibers to terminate in the dorsal horn of the spinal cord where they synapse on dendrites of the neospinothalamic tract. The axons of these neurons cross the midline (decussate) through the anterior white commissure and ascend contralaterally along the anterolateral system. These fibres terminate on the ventrobasal complex of the thalamus and synapse with the dendrites of the somatosensory cortex. Fast pain is felt within a tenth of a second of application of the pain stimulus and is a sharp, acute, prickling pain felt in response to mechanical and thermal stimulation. It can be localised easily if Aδ fibres are stimulated together with tactile receptors.
Slow pain is transmitted via slower type C fibers to laminae II and III of the dorsal horns, together known as the substantia gelatinosa. Impulses are then transmitted to nerve fibers that terminate in lamina V, also in the dorsal horn, synapsing with neurons that join fibers from the fast pathway, crossing to the opposite side via the anterior white commissure, and traveling upwards through the anterolateral pathway. These neurons terminate throughout the brain stem, with one tenth of fibres stopping in the thalamus and the rest stopping in the medulla, pons and periaqueductal grey of the midbrain tectum.
The body possesses an endogenous analgesia system, which can be supplemented with analgesic drugs to regulate nociception and pain. There is both an analgesia system in the central nervous system and peripheral receptors that decreases the grade in which nociception reaches the higher brain areas. The degree of pain can be modified by the periaqueductal gray before it reaches the thalamus and consciousness. According to gate control theory of pain, this area can also reduce pain when non-painful stimuli are received in conjunction with nociception.
The central analgesia system is mediated by three major components: the periaqueductal grey matter, the nucleus raphes magnus and the nociception inhibitory neurons within the dorsal horns of the spinal cord, which act to inhibit nociception-transmitting neurons also located in the spinal dorsal horn.
The peripheral regulation consists of several different types of opioid receptors that are activated in response to the binding of the body's endorphins. These receptors, which exist in a variety of areas in the body, inhibit firing of neurons that would otherwise be stimulated to do so by nociceptors.
The gate control theory of pain, proposed by Patrick David Wall and Ronald Melzack, postulates that nociception (pain) is "gated" by non-nociception stimuli such as vibration. Thus, rubbing a bumped knee seems to relieve pain by preventing its transmission to the brain. Pain is also "gated" by signals that descend from the brain to the spinal cord to suppress (and in other cases enhance) incoming nociception (pain) information.
If the signals are sent to the reticular formation and thalamus, the sensation of pain enters consciousness in a dull poorly localized manner. From the thalamus, the signal can travel to the somatosensory cortex in the cerebrum, when the pain is experienced as localized and having more specific qualities.
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 (40º Celsius or more), low pH, capsaicin, and tissue damage.