Pain and Why It Hurts

You may not like it, but we need pain. Pain acts as a warning system that protects you. Pain says, "Warning, Warning....stop what you doing and do something else". 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 an injury hurts, you rest. There are some people who are born WITHOUT the sense of pain. These people have a rare condition called "congenital insensitivity to pain". Their nervous systems are not equipped to detect painful information. You may think this is a good thing....it is NOT. Without the ability to detect painful events, you would continue to cause injury to yourself. For example, if you broke a bone in your arm, you might continue using the arm because it did not hurt. You could cause further injury to your arm. People with congenital insensitivity to pain usually have many injuries like pressure sores, damaged joints and even missing or damaged fingers! So, what kind of things in the outside world can cause pain? Events that cause reactions are called stimuli. Stimuli are painful when they damage tissues or threaten to damage tissue. Pain is nature's way of telling the brain about injury to the body. Painful stimuli can be divided into several types:

Painful Stimuli
Energy	Example	Everyday Example	Possible result if untreated
	Mechanical *Strong Pressure *Pinch *Squeeze *Twist	Animal bite Knife cut Falling off a bike	Bruises Broken Bones Cuts
	Thermal (Temperature) *Hot *Cold	Fire Hot Chocolate Ice	Burns Frostbite
	Electrical	Electric Shock	Burns
	Chemical *Acid *Chili Peppers	Chemical burns	Broken Skin
	Visceral (Inside Your Body)	Heart Attack Inflamed appendix	Condition gets worse

I am sure that you could add to this list...what other things can cause pain? A toothache? A broken bone? Ok..now we know the "stimuli" that may cause pain. How do these stimuli activate the nervous system? There are specialized "receptors" in the skin and internal organs that are sensitive to stimuli that are painful. These receptors are called "nociceptors" and are free nerve endings connected to small diameter myelinated A and unmyelinated C nerve fibers - these are the nerve fibers that are LACKING in people with congenital insensitivity to pain (here is more information about nerve fibers). Nociception, then, is the response of the nervous system to painful stimulation. When the nociceptors detect a nociceptive stimulus, they send a message to the spinal cord. A famous theory concerning how pain works is called the Gate Control Theory devised by Patrick Wall and Ronald Melzack in 1965. This theory 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. Remember, large nerve fibers carry non-nociceptive information and small nerve fibers carry nociceptive 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. Here is what the gate control theory looks like:

Gate Control Theory I = "Inhibitory Interneuron"; P = "Projection Neuron" - = inhibition (blocking); + = excitation (activation) Let's go through the theory step by step: 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. 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. 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!!

(This image is courtesy National Library of Medicine, Bethesda, Maryland.)

Although the gate control theory has support from some experiments and does explain some observations seen in pain patients during therapy, it does not explain everything. However, think of this...what is one of the first things you do after you bump your head or pinch a finger by accident? You probably rub it and it feels better, right? Could this be explained by the gate control theory? Well, rubbing your bumped head or pinched finger would activate non-nociceptive touch signals carried into the spinal cord by large nerve fibers. According to the theory, the activity in the large nerve fibers would activate the inhibitory interneuron that would then block the projection neuron and therefore block the pain. From the spinal cord, the messages go directly to several places in the brain including the thalamus, midbrain and reticular formation. Some brain regions that receive nociceptive information are involved in perception and emotion. Also, some areas of the brain connect back to the spinal cord - these connections can change or modify information that is coming into the brain. In fact, this is one way that the brain can REDUCE pain. Two areas of the brain that are involved in reducing pain are the periaqueductal gray (PAG) and the nucleus raphe magnus. Neurosurgeons have implanted electrical stimulating electrodes near the PAG of some patients with severe pain - a small electrical shock through these electrodes can relieve pain in some patients!!

Other ways that pain can be controlled:
Method	Possible Mechanism(s)	Uses/Examples
Aspirin	Acts mostly in peripheral nervous system. Reduces inflammation.	Headache, musculoskeletal pain
Morphine	Acts in central nervous system (brain and spinal cord) to block pain messages. Activates pain-modulating systems in the brain that project to the spinal cord.	Post-operative pain; other pain conditions
Other pain reducing drugs	Act on a variety of neurotransmitter systems.	Variety of pain conditions
Hypnosis	May activate the pain-inhibitory pathway from the brain to the spinal cord (but not opioid pathways). May act somewhere in the brain (frontal lobe?) to shift a patient's attention away from the pain.	Dental procedures, childbirth, burns, headache
Acupuncture	Stimulation of large diameter nerve fibers that inhibit pain ("close the gate"). Could be placebo effect. Causes release of endorphins ("the body's own morphine-like substances"). Some types of acupuncture may stimulate small diameter nerve fibers and inhibit spinal cord pain mechanisms (this would not agree with the gate control theory)	Back pain, minor surgical operations
Placebo	Reduces anxiety. Causes release of endorphins	Headache, post-operative pain
Transcutaneous Electrical Nerve Stimulation (TENS)	Stimulation of large diameter nerve fibers which "close the gate" and reduce pain. Could be placebo effect.	Post-operative pain, arthritis, cancer pain
Neurosurgery	Removal or blockade of painful signals	Cancer pain
Stress	Activation of endogenous opiate system (endorphins) Activation of non-opiate pain inhibitory system	Football player continues to play regardless of injury. Soldier continues to fight regardless of wounds.

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