Nociceptors, or pain receptors, are located across the body. They are considered “free” nerve endings; they respond to noxious or harmful stimuli such as excessive heat, excessive cold, or chemicals and send a signal to the brain of pain. Nociceptors lie in both the cutaneous and subcutaneous layers of the skin as well as the cornea and a variety of internal organs. While nociceptors provide the signal to the brain indicating pain, the two are not mutually exclusive. It is possible to feel pain without a signal from a nociceptor, and similarly it is possible to feel no pain when they are firing signals. Nociception plays an integral role in everyday life, alerting you to dangers and signaling you to avoid them. However, the body does have some control over nociceptor signals, regulating the amount of pain that is felt at specific locations.
Types and Functions
Nociceptors can be classified into a few main groups depending on the stimuli that they respond to. These groups are polymodal nociceptors, mechanical nociceptors, thermal nociceptors, and chemical nociceptors. Polymodal nociceptors is the classification that most nociceptors fall into. These nociceptors respond to mechanical, thermal, and chemical stimuli. Other nociceptors show a high degree of selectivity for their stimuli, and it is important to note that the stimuli they respond to are excessive—the stimulus is strong enough to damage tissue.
There are also two different types of axons or fibers: unmyelinated C-fibers and lightly myelinated A delta fibers. Both of these fiber types conduct signals to the brain and spinal cord relatively slowly. Nevertheless, nociception does have a fast and a slow signal. A delta fibers conduct signals fairly quickly—approximately 20 meters per second (m/s)—whereas C-fibers conduct pain signals much more slowly—closer to 2 m/s. The more selective nociceptors typically conduct their signals along A delta fibers as these stimuli tend to be more intense and dangerous, thus requiring a faster response. C-fibers tend to conduct signals from polymodal nociceptors. Recent research is finding that C-fibers also have a large selectivity for histamine and are responsible for the perception of painful itch, like a bee sting. All nociceptors have a large receptive field because it is more important for the brain to get a signal of pain than to know the exact location of that pain.
There are two main pathways that signals from nociceptors travel down. These are the spinothalamic pathway and the trigeminal pathway. Within the spinothalamic pathway, the second-order neuron’s axon crosses over immediately and travels to the brain through the spinothalamic tract. It projects through the medulla, pons, and midbrain, eventually synapsing in the thalamus. This pathway ascends contralaterally. As is indicated in the name, the trigeminal pathway mainly conveys information about painful signals from the face and head (including the cornea). The first-order neurons in the trigeminal pathway synapse on second-order neurons in the spinal trigeminal nucleus of the brainstem. Eventually the axons from the second-order neurons cross over and ascend to the thalamus through the trigeminal lemniscus. The trigeminal lemniscus conveys information about the face’s orientation in space in addition to conveying information about pressure, pain, and temperature (partially through nociception).
Once the two pathways reach the thalamus, the signal is then sent to various regions of the cerebral cortex based on the location of the painful signal. This is where the signal is integrated and a potential motor reaction is created. Additionally, both pathways have many other neurons that will synapse at various levels within the brainstem and spinal cord. These synapses are important in playing a role in reflexes. Instead of your body waiting for the relatively slow signals to reach the brain, be processed, and a response to be sent back, it is more important to have a more immediate reaction. These reflexes are what tell you to pull your hand away from a hot stove before your brain registers that it is hot and that it burns. After this initial response, the brain has time to make a decision based on the information that it is receiving.
One theory proposed in the 1960s by Melzack and Wall introduced the idea that activity in nociceptors could be reduced with the addition of vibration, pressure, or touch sensations in the same location. These signals would override the feeling of pain coming from the nociceptor, causing the individual to feel less pain. This theory is called the gate theory of pain and was used to describe why rubbing the area around a wound would often help to alleviate some of the pain.
Another form of regulation includes endogenous opioids. Opioids are natural chemicals that the body uses to reduce sensations of pain. Modern opiates (manmade chemicals) perform a similar function and are often prescribed to help alleviate pain in patients. Both endogenous and exogenous opioids will bind to several types of receptors throughout the brain. These chemicals block pain signals through nociceptors to the brain, causing the individual to feel less pain.
Riannon C. Atwater
See also: Congenital Insensitivity to Pain; Nociception
Dafny, Nachum. (2015). Chapter 8: Pain modulation and mechanisms. Neuroscience Online. John H. Byrne (Ed.). Retrieved from http://neuroscience.uth.tmc.edu/s2/chapter08.html
Nociceptors. (2001). In D. Purves, G. J. Augustine, D. Fitzpatrick, W. C. Hall, A. LaMantia, J. O. McNamara, & S. M. Williams (Eds.), Neuroscience (2nd ed.). Sunderland, MA: Sinauer Associates. Retrieved from http://www.ncbi.nlm.nih.gov/books/NBK10965/