Nerve endings free#
Most tissues also contain free nerve endings that normally respond poorly to essentially all stimuli but become sensitive after tissue damage or in disease states. Unlike the majority of the sensory receptors and their ability to adapt, nociceptors can become sensitized, resulting in more activity and relaying of pain information to the CNS. Some individual fibers respond selectively to gentle brushing, to cooling the skin, to warming it, or to stimuli that are perceived as painful. Electrophysiological studies, however, have clearly differentiated among them. Nociceptors, thermoreceptors, and some mechanoreceptors are all free nerve endings no pronounced morphological differences are seen among them with presently available techniques. Vanderah PhD, in Nolte's The Human Brain, 2021 Nociceptors, Thermoreceptors, and Some Mechanoreceptors Have Free Nerve Endings In the rattlesnake, Crotalus horridus, the morphology of the pit organ promotes efficient thermoreception by a large number (several thousand) of intradermal myelinated axons 3–16 μm in diameter. The importance of FNEs for sensing prey is also evident in the infrared pit organs of snakes. Such vibrations are important signals of communication between moles. The primary function appears to be the tactile detection and recognition of invertebrate prey, but the presence of up to three paciniform corpuscles at the base of each Eimer’s organ suggests that it may also play a role in sensing seismic vibrations. The presence of three types of specialized receptor raises the possibility that Eimer’s organs have several important functions. Below the column are Merkel cell complexes and lamellated paciniform corpuscle. In this structure, the FNE extends through a column of thickened epidermal cells. Named after the German zoologist Gustav Heinrich Theodor Eimer (1843–1898), Eimer’s organs are a special form of FNE found in the rhinarium of fossorial mammals such as talpid moles.
Nerve endings skin#
The exploratory function of glabrous skin is exemplified by the nasal rays of the star-nosed mole, Condylura cristata, which are richly invested with Eimer’s organs and are the mole’s primary sense organs. This condition is hypothesized to enhance the function of glabrous skin as an exploratory surface. The lips and palms of primates feature an abundance of nonmyelinated C-fibers and a paucity of finely myelinated Aδ fibers, which together give glabrous skin a lower heat detection threshold and a higher heat pain threshold than hirsute skin. The FNEs of afferent fibers are mainly nociceptive in the nonmyelinated portion. Some FNEs respond to moderate cold or heat (thermoreception), to low-threshold mechanical touch (mechanoreception), or to damaging heat, cold, or deformation (nociception). Electrophysiological recordings of FNEs in the dermis show that they respond to several sensory modalities.
The terminal arborizations of myelinated afferent axons are nonmyelinated and devoid of Schwann cell investment. Afferent fibers are myelinated or nonmyelinated but are always of small diameter and low conduction velocity. FNEs are found in all connective tissues, including the epidermis and dermis of hirsute and glabrous skin. Dominy, in Encyclopedia of Neuroscience, 2009 Free Nerve EndingsįNEs are the terminal receptor structures of sensory C-fibers or Aδ fibers. The time course of nociceptor excitation suggests that during ischemic contractions, the receptors are first sensitized by as yet unknown factors, and then the sensitized nociceptors are activated by the mechanical force of the contractions. Other possible mediators are ATP, which is released from muscle fibers in patients with claudication ( Sutherland et al 2000), and NGF, which is released during ischemia and increases TRPV1 responsiveness in muscle afferent neurons ( Xing et al 2009). Lactic acid can be ruled out as a source because patients with McArdle’s disease-who are unable to produce lactic acid-exhibit intermittent claudication. One of the factors eliciting ischemic muscle pain may be low tissue pH. These receptors may be involved in intermittent claudication. McMahon FMedSci, FSB, in Wall & Melzack's Textbook of Pain, 2013 Responses of Muscle Free Nerve Endings to Contractions under IschemiaĪmong the group III/IV muscle receptors, a small subpopulation of the units did not respond to contractions without ischemia-and also not to ischemia alone-but were excited when contractions were performed under ischemic conditions ( Mense and Stahnke 1983, Kaufman et al 1984).
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