A nociceptor is the type of nerve ending that is responsible for the perception of pain in most animals. It is a sensory receptor that is present on nearly every region of an animal's body, both internally and externally. The chain of events by which a nociceptor detects pain and relays the signal to the brain is called nociception.
The central nervous system uses specific cells to send signals from the various areas of the body to the brain. These cells are called neurons. They send signals by means both electronic and chemical in nature. Specific neurons control, among other things, an animal's ability to sense itself and its environment. Others allow the body to respond to those stimuli with controlled movements. They are called sensory and motor neurons respectively.
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Within the category of sensory receptors lies the nociceptor. There are four kinds of nociceptors. Three of them react quickly to their various triggers.
A thermal nociceptor reacts when an animal's body is subject to extreme temperatures that can threaten the animal's well-being. Any temperature above 107° F (around 42° C) will prompt the thermal nociceptor. Pain that results from cuts or pressure to the body causes a response by a mechanical nociceptor. Chemical nociceptors recognize pain caused by chemicals, such as capsaicin. Capsaicin is found in some peppers and causes the chemical nociceptor to signal a burning sensation to the brain, similar to its thermal counterpart.
The fourth nociceptor, the sleeping nociceptor, becomes active only when the body has sustained damage. Sleeping nociceptors do not detect any of the pain sensations until activated by the inflammation that accompanies an injury.
A polymodal nociceptor is capable of detecting multiple types of pain. A mechanical pain receptor can operate as a polymodal nociceptor by identifying thermal stimuli along with pressure-induced pain. After an injury and inflammation has activated the sleeping nociceptor, it is not uncommon for this nociceptor to become polymodal as well.
When exposed to a triggering stimulus, the cell's action potential is activated, a process that is also referred to as a nerve impulse. During the nerve impulse the nociceptor converts the incoming stimulus into an electrical signal that is ultimately manifested as a physiological reaction. The central nervous system takes this nerve impulse and recognizes it as pain.