![]() In the skin and deep tissues there are additional nociceptors called "silent" or "sleep" nociceptors. The noxious information from visceral organs and skin are carried to the CNS in different pathways (Figures 6.3 and 6.4). Many of the visceral nociceptors are silent. The visceral nociceptors are scattered, with several millimeters between them, and in some organs, there are several centimeters between each nociceptor (Figure 6.3). Visceral organs contain mechanical pressure, temperature, chemical and silent nociceptors. Liberation of such peptides is believed to play a role in the development of inflammatory arthritis. Many of the fibers innervating these endings in the joint capsule contain neuropeptides, such as substance P (SP) and calcitonin gene-related peptide (CGRP). The joint capsules and ligaments contain high-threshold mechanoreceptors, polymodal nociceptors, and "silent" nociceptors. A characteristic feature of nociceptors is their tendency to be sensitized by prolonged stimulation, making them respond to other sensations as well. 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. The third type is chemical nociceptors, which respond only to chemical substances (Figure 6.2). The second type is the thermal nociceptors, which respond to the above stimuli as well as to thermal stimuli. These nociceptors respond only to intense mechanical stimulation such as pinching, cutting or stretching. The first type is termed high threshold mechanonociceptors or specific nociceptors. Skin nociceptors may be divided into four categories based on function. They fall into several categories, depending on their responses to mechanical, thermal, and/or chemical stimulation liberated by the damage, tumor, and/or inflammation. No nociceptors are found inside the CNS.ĭifferent nociceptors/free nerve endings, and the fibers carrying pain sensation from the nociceptors to the spinal cord. The cell bodies of nociceptors are mainly in the dorsal root and trigeminal ganglia. This action potential is transmitted to the spinal cord and makes a synaptic connection in lamina I and/or II. They transduce a variety of noxious stimuli into receptor potentials, which in turn initiate action potential in the pain nerve fibers. The TRP channels are similar to voltage-gated potassium channels or nucleotide-gated channels, having 6 transmembrane domains with a pore between domains 5 and 6. Recently, it was found that nerve endings contain transient receptor potential (TRP) channels that sense and detect damage. Nociceptors are free (bare) nerve endings found in the skin (Figure 6.2), muscle, joints, bone and viscera. Nociceptors are sensory receptors that detect signals from damaged tissue or the threat of damage and indirectly also respond to chemicals released from the damaged tissue. Noxious stimuli are stimuli that elicit tissue damage and activate nociceptors. Pain is termed nociceptive (nocer – to injure or to hurt in Latin), and nociceptive means sensitive to noxious stimuli. Recent discoveries about how the body detects, transmits and reacts to painful stimuli, have allowed physicians to relieve both acute and chronic pain. Individuals congenitally insensitive to pain are easily injured and most of them die at an early age.įor thousands of years, physicians have tried to treat pain without knowing the details of the ways in which pain is signaled from the injured part of the body to the brain, or the ways in which any of their remedies worked. Pain travels through redundant pathways, ensuring to inform the subject: “Get out of this situation immediately.” Without these attributes, the organism has no means to prevent or minimize tissue injury. Sensitivity and reactivity to noxious stimuli are essential to the well-being and survival of an organism. The ability to diagnose different diseases depends to a great extent on the knowledge of the different qualities and causes of pain. Pain information is transmitted to the CNS via three major pathways (Figure 6.1). Nature has made sure that pain is a signal we cannot ignore. The word "pain" is used to describe a wide range of unpleasant sensory and emotional experiences associated with actual or potential tissue damage. Pain is a submodality of somatic sensation. Pain differs from the classical senses (hearing, smell, taste, touch, and vision) because it is both a discriminative sensation and a graded emotional experience associated with actual or potential tissue damage. Most of the sensory and somatosensory modalities are primarily informative, whereas pain is a protective modality. Three pathways carrying pain sensation from the periphery to the central nervous system.
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