Mechanism of Pain

Pain acts as a warning system that protects you. For example, if you have your hand on a hot stove, pain tells you to stop touching the stove and remove your hand. In this way, pain protects your body from injury (or further injury if you have already hurt yourself). Pain also helps healing because when an injury hurts, you rest that part of the body more. Various type of stimuli can cause pain: mechanical (strong pressure, twist), thermal (hot or cold temperatures), electrical (shock), chemical (acid), or visceral (inside the body - heart attack, inflamed appendix).

When this stimuli damages your body's cells, they release a chemical called prostaglandin. This chemical activates your nervous system through specialized receptors in the skin and internal organs called "nociceptors" which are free nerve endings connected to small diameter myelinated A and unmyelinated C nerve fibers. When nociceptors detect any of the above types of pain-related stimuli or "nocicieption" through exposure to prostaglandin, they send messages of pain to the spinal cord.

A well-known theory for how pain works is called the Gate Control Theory which states that pain is a function of the balance between the information traveling into the spinal cord through large nerve fibers and information traveling into the spinal cord through small nerve fibers. Large nerve fibers carry non-nociceptive information and small nerve fibers carry nociceptive (pain stimulus) information. If the relative amount of activity is greater in large nerve fibers, there should be little or no pain. However, if there is more activity in small nerve fibers, then there will be pain:

1) Without any stimulation, both large and small nerve fibers are quiet and the inhibitory interneuron (I) blocks the signal in the projection neuron (P) that connects to the brain. The "gate is closed" and therefore NO PAIN.

2) With non-painful stimulation, large nerve fibers are activated primarily. This activates the projection neuron (P), BUT it ALSO activates the inhibitory interneuron (I) which then BLOCKS the signal in the projection neuron (P) that connects to the brain. The "gate is closed" and therefore NO PAIN.

3) With pain stimulation, small nerve fibers become active. They activate the projection neurons (P) and BLOCK the inhibitory interneuron (I). Because activity of the inhibitory interneuron is blocked, it CANNOT block the output of the projection neuron that connects with the brain. The "gate is open", therefore, PAIN!!

Pain can be acute or chronic. It is considered acute if it lasts for less than three months. It's considered chronic if it lasts for more than three months. In the case of chronic pain, pain is no longer considered a symptom. It is considered an illness.