During vision, rod and cone photoreceptors respond to light intensity and color. During hearing, mechanoreceptors in hair cells of the inner ear detect vibrations conducted from the eardrum. During taste, sensory neurons in our taste buds detect chemical qualities of our foods including sweetness, bitterness, sourness, saltiness, and umami savory taste. During smell, olfactory receptors recognize molecular features of wafting odors. During touch, mechanoreceptors in the skin and other tissues respond to variations in pressure.
Adequate stimulus can be used to classify sensory receptors. Somatic sensory receptors near the surface of the skin can usually be divided into two groups based on morphology:. A tonic receptor is a sensory receptor that adapts slowly to a stimulus, while a phasic receptor is a sensory receptor that adapts rapidly to a stimulus. As we exist in the world, our bodies are tasked with receiving, integrating, and interpreting environmental inputs that provide information about our internal and external environments.
Our brains commonly receive sensory stimuli from our visual, auditory, olfactory, gustatory, and somatosensory systems. Remarkably, specialized receptors have evolved to transmit sensory inputs from each of these sensory systems.
Sensory receptors code four aspects of a stimulus:. Receptors are sensitive to discrete stimuli and are often classified by both the systemic function and the location of the receptor.
Sensory receptors are found throughout our bodies, and sensory receptors that share a common location often share a common function. For example, sensory receptors in the retina are almost entirely photoreceptors. Our skin includes touch and temperature receptors, and our inner ears contain sensory mechanoreceptors designed for detecting vibrations caused by sound or used to maintain balance.
Force -sensitive mechanoreceptors provide an example of how the placement of a sensory receptor plays a role in how our brains process sensory inputs. Apply market research to generate audience insights.
Measure content performance. Develop and improve products. List of Partners vendors. Nociceptors often referred to as your "pain receptors," are free nerve endings located all over the body, including the skin, muscles, joints, bones, and internal organs. They play a pivotal role in how you feel and react to pain. The main purpose of a nociceptor is to respond to damage to the body by transmitting signals to the spinal cord and brain. Looking at this in more detail, if you stub your toe, the nociceptors on your skin are activated, causing them to send a signal to the brain, via the peripheral nerves to the spinal cord.
Pain resulting from any cause is messaged in this way. Keep in mind, these transmitted pain signals are complex, carrying information about both the location and intensity of the painful stimuli. That way your brain can fully process the pain and eventually send communication back to block further pain signals. In addition to the type of stimuli a nociceptor responds to, nociceptors are also classified by how fast they transmit pain signals. This speed of transmission is determined by the type of nerve fiber called an axon a nociceptor has.
There are two main types of nerve fibers. The first type is A fiber axon, which are fibers surrounded by a fatty, protective sheath called myelin. Myelin allows nerve signals called action potentials to travel rapidly. The second type is C fiber axons, which are not surrounded by myelin, and thus transmit action potentials at a slower speed. Due to the difference in transmission speed between the A and C fibers, the pain signals from the A fibers reach the spinal cord first.
As a result, after an acute injury, a person experiences pain in two phases, one from the A fibers and one from the C fibers. When an injury occurs such accidentally cutting your finger with a knife , the stimulated nociceptors activate the A fibers, causing a person to experience sharp, prickling pain. This is the first phase of pain, known as fast pain, because it is not especially intense but comes right after the painful stimulus. During the second phase of pain, the C fibers are activated, causing a person to experience an intense, burning pain that persists even after the stimulus has stopped.
The fact that burning pain is carried by the C fibers explains why upon touching a hot stove, there is a short delay before feeling the burn. No nociceptors are found inside the CNS. Figure 6. Nociceptors are not uniformly sensitive. Skin Nociceptors. Skin nociceptors may be divided into four categories based on function. The first type is termed high threshold mechanonociceptors or specific nociceptors. These nociceptors respond only to intense mechanical stimulation such as pinching, cutting or stretching.
The second type is the thermal nociceptors, which respond to the above stimuli as well as to thermal stimuli. The third type is chemical nociceptors, which respond only to chemical substances Figure 6.
A fourth type is known as polymodal nociceptors, which respond to high intensity stimuli such as mechanical, thermal and to chemical substances like the previous three types. A characteristic feature of nociceptors is their tendency to be sensitized by prolonged stimulation, making them respond to other sensations as well. Joint Nociceptors. The joint capsules and ligaments contain high-threshold mechanoreceptors, polymodal nociceptors, and "silent" nociceptors.
Many of the fibers innervating these endings in the joint capsule contain neuropeptides, such as substance P SP and calcitonin gene-related peptide CGRP. Liberation of such peptides is believed to play a role in the development of inflammatory arthritis. Visceral Nociceptors. Visceral organs contain mechanical pressure, temperature, chemical and silent nociceptors.
The visceral nociceptors are scattered, with several millimeters between them, and in some organs, there are several centimeters between each nociceptor Figure 6. Many of the visceral nociceptors are silent. The noxious information from visceral organs and skin are carried to the CNS in different pathways Figures 6.
Silent Nociceptors. In the skin and deep tissues there are additional nociceptors called "silent" or "sleep" nociceptors. One possible explanation of the "awakening" phenomenon is that continuous stimulation from the damaged tissue reduces the threshold of these nociceptors and causes them to begin to respond.
This activation of silent nociceptors may contribute to the induction of hyperalgesia, central sensitization, and allodynia see below. Many visceral nociceptors are silent nociceptors. Activation of the nociceptor initiates the process by which pain is experienced, e. These receptors relay information to the CNS about the intensity and location of the painful stimulus.
Nociceptors respond when a stimulus causes tissue damage, such as that resulting from cut strong mechanical pressure, extreme heat, etc. The damage of tissue results in a release of a variety of substances from lysed cells as well as from new substances synthesized at the site of the injury Figure 6.
Some of these substances activate the TRP channels which in turn initiate action potentials. These substances include:. The release of these substances sensitizes the nociceptors C fibers and reduces their threshold. This effect is referred to as peripheral sensitization in contrast to central sensitization that occurs in the dorsal horn. Within seconds after injury, an area of several cm around the injured site shows reddening caused by vasodilation called a flare.
This response inflammation becomes maximal after minutes Figure 6. Hyperalgesia is an increased painful sensation in response to additional noxious stimuli.
One explanation for hyperalgesia is that the threshold for pain in the area surrounding an inflamed or injured site is lowered.
These changes contribute to an amplification of pain or hyperalgesia, as well as an increased persistence of the pain. If one pricks normal skin with a sharp probe, it will elicit sharp pain followed by reddened skin. The reddened skin is an area of hyperalgesia. Allodynia is pain resulting from a stimulus that does not normally produce pain.
For example, light touch to sunburned skin produces pain because nociceptors in the skin have been sensitized as a result of reducing the threshold of the silent nociceptors.
Another explanation of allodynia is that when peripheral neurons are damaged, structural changes occur and the damaged neurons reroute and make connection also to sensory receptors i.
In conclusion, the several kinds of endogenous chemicals are produced with tissue damage and inflammation. These products have excitatory effects on nociceptors. However, it is not known whether nociceptors respond directly to the noxious stimulus or indirectly by means of one or more chemical intermediaries released from the traumatized tissue. Exposing the skin to controlled heat produced by heating element or laser makes it possible to measure the threshold for pain.
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