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In mammals, nociceptors are found in any area of the body that can sense noxious stimuli. External nociceptors are found in tissue such as the skin (cutaneous nociceptors), the corneas, and the mucosa. Internal nociceptors are found in a variety of organs, such as the muscles, the joints, the bladder, the
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.
Free nerve endings can detect temperature, mechanical stimuli (touch, pressure, stretch) or danger (nociception). Thus, different free nerve endings work as thermoreceptors, cutaneous mechanoreceptors and nociceptors. In other words, they express polymodality.
Nociceptors detect different kinds of damaging stimuli or actual damage. Those that only respond when tissues are damaged are known as "sleeping" or "silent" nociceptors. Thermal nociceptors are activated by noxious heat or cold at various temperatures. Mechanical nociceptors respond to excess pressure or mechanical deformation.
Pain receptors are known as nociceptors. Two main types of nociceptors exist, A-fiber nociceptors and C-fiber nociceptors. A-fiber receptors are myelinated and conduct currents rapidly. They are mainly used to conduct fast and sharp types of pain. Conversely, C-fiber receptors are unmyelinated and slowly transmit.
A noxious stimulus is a stimulus strong enough to threaten the body's integrity (i.e. cause damage to tissue).Noxious stimulation induces peripheral afferents responsible for transducing pain (including A-delta and C-nerve fibers, as well as free nerve endings) throughout the nervous system of an organism.
Type Aβ fibres from the skin are mostly dedicated to touch. However a small fraction of these fast fibres, termed "ultrafast nociceptors", also transmit pain. [6] Type Aδ fibers are the afferent fibers of nociceptors. Aδ fibers carry information from peripheral mechanoreceptors and thermoreceptors to the dorsal horn of the spinal cord.
External receptors that respond to stimuli from outside the body are called exteroreceptors. [4] Exteroreceptors include chemoreceptors such as olfactory receptors and taste receptors, photoreceptors (), thermoreceptors (temperature), nociceptors (), hair cells (hearing and balance), and a number of other different mechanoreceptors for touch and proprioception (stretch, distortion and stress).