Nociception is the sense of pain. Although the feeling of pain seems related to the sense of touch, the two sensations operate very differently at a neurological level. Many think that pain is just the overstimulation of touch, but it is actually its own sense, using its own dedicated receptors and pathways to send its unique signal.
How the Body Detects Pain With Nociceptors
There are a number of sensory receptors throughout the human body, including photoreceptors in the eye that detect light and mechanoreceptors in the skin that detect pressure and movement. When detecting pain, the body uses nociceptors. These receptors are the bare axon terminals of neurons that emanate from the spinal nerve. They are sometimes referred to as free nerve endings.
Usually, neurons that have to bring a signal up to the brain are covered in a material called myelin,which allows the signal to be transported very quickly. The neurons that are associated with nociceptors, however, usually only have a little (if any) myelin, which means their signal is transported relatively slowly. There are two types of nociceptors: Aδ nociceptors and C-fiber nociceptors. The Aδ cells are faster and respond to dangerously intense stimuli, and C-fiber cells are slower and respond to thermal, mechanical, and chemical stimuli.
How the Brain Knows About the Pain
Once a pain receptor detects the painful stimulus, it sends a signal down its neuron to the spinal cord. When the signal gets there, it crosses over to the opposite side and shoots up to the brain. This means that an injury to the right side of the body sends a signal that climbs up the left side of the spinal cord.
In regular touch sensation, the signal is sent by neurons that go up the same side of the spine until they reach a part of the brain called the medulla. It is in the medulla that the touch sensation signal crosses to the other side.
This switching-sides organization gives rise to dissociated sensory loss. If a person experiences a spinal injury, it will affect different processes on either side of the body, depending on where the spine was injured. For example, if a person is injured on the right side of the spine, he or she will not be able to feel a touch on the right foot but will be able to feel pain on that foot. Likewise, the person will feel a touch on the left foot but will not be able to feel pain on that foot.
How the Brain Processes Pain
Once the pain signal gets to the brain, it is sent to a number of structures for processing. The location and intensity of the stimulus is deciphered by the primary and secondary somatosensory cortex. The emotions and automatic reactions to pain are processed in a number of location, including the hypothalamus, superior colliculus, and amygdala. The complete pain experience, however, arises from the co-operation of a network of brain areas.
Reducing Pain and Making it Worse
When pain is induced by tissue damage like cuts and bruises, nociceptors become even more sensitive to pain, a phenomenon known as hyperalgesia. When the tissue is damaged, a number of substances are released into the site of damage. These substances include histamine, serotonin, and prostaglandin, and their presence can modify the activity of nociceptive neurons, making them quicker to send signals to the brain.
To reduce the feeling of pain, one must limit the effect of these substances. Some medications, for example, like aspirin and ibuprofen, work by inhibiting the production of one of the substances (prostaglandin). In doing so, the feeling of pain will be reduced, but because it does not account for all of the substances, the pain persists, though to a smaller degree.
References
Purves, D., Augustine, G. J., Fitzpatrick, D., Hall, W., LaMantia, A., McNamara, J. O., & White, L. E. (Eds.). (2008). Neuroscience (4th ed.). Sunderland, MA: Sinauer Associates.
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