Ouch! Don't Touch Me!
Have you ever wondered why it actually hurts when people touch you? Why, when somebody taps you on the shoulder to get your attention, you jump in pain before turning round to glare at them? Other people don’t respond like that, so why do you?
Doctors and scientists explain these ‘extreme’ pain reactions by a phenomenon called ‘central sensitization.’ The pain pathways are hypersensitive, geared up to respond to the slightest touch, like a radio
that plays on full blast despite whatever volume you set it to.
PAIN PATHWAYS - HOW YOUR BODY SIGNALS AND MONITORS PAIN
The pain pathways are set in motion by receptors in the skin being sparked into action in response to a stimulus, such as a heavy object landing on your foot. These sparks ignite the ends of the nerves like a fuse and the pain signal begins to shoot along the nerve on its way to the spinal cord.
On reaching the spinal cord the incoming nerve has to pass the signal on to the next nerve in the pathway, as an athlete passes the baton in a relay race. However, there is a gap between the two nerves, scientifically referred to as the ‘synapse,’ over which the incoming signal has to cross. The incoming nerve succeeds in passing the baton by emptying packets of chemicals stored at its terminal end into the gap. The chemicals used in the pain pathways are calledglutamate and substance P. Glutamate and substance P move across the gap and open channels into the next nerve, acting as sparks to ignite the nerve and allowing the pain signal to race on its way up the spinal cord to the brain.
To ensure the sparks flying from one nerve to another are temporary, nerves have receptors on their terminal ends that act like vacuums to suck the glutamate and substance P back in, where they can be repackaged, ready to bridge the gap on arrival of another pain signal.
On arrival at the brain the pain signal reaches its end point and, like a fuse, ignites the dynamite, producing an explosion of pain. This is the moment when you are consciously aware of the pain and its location.
The Fine Tuning: Altering the Intensity of the Pain Signal
Increasing the Intensity of the Pain Signal
The nervous system needs to be able to communicate different levels of pain intensity to the brain. Not all the pain you feel seems the same magnitude. For example, dropping a heavy book on your toes tends to hurt more than dropping a small stone on them, even though you register both actions as painful.
One obvious reason would be that a book, being larger, would activate more nerves at the same time, leading to an increased number of simultaneous explosions in the brain. However, another more subtle reason involves the packets of chemicals stored in the nerve’s terminal end.
To communicate a weak pain signal the nerve releases only a few packets of substance P along with the packets of glutamate into gap. In order to increase the intensity of the pain signal, the nerve releases larger quantities of substance P. These larger quantities provide a greater number of sparks to open more channels and ignite the next nerve with more force.Hence the greater the amount of substance P released, the more intense the explosion of pain, when the pain signal reaches the brain.
Dampening Down the Pain Signal
The brain itself has the ability to dampen down the intensity of the incoming pain signals. Nerves in the brain send signals down the spinal cord to interrupt the transmission of the pain signals across the gap. The first nerve descending from the brain releases packets of the chemical serotonin to pass on the signal to the second nerve in this pathway. This second nerve is referred to as an inhibitory nerve because it inhibits pain by releasing packets of a chemical called enkephalin, one of the body’s natural painkillers. Enkephalin reduces the amount of substance P released by the incoming pain nerve, thus reducing the intensity of the pain signal.
Other nerves that are sparked into action at the skin also play a part in dampening the pain signal before it reaches the brain. These nerves carry information about ‘touch’ to the brain. They also release packets of enkephalin onto the incoming pain nerve as it reaches the spinal cord, once again decreasing the amount of substance P released. These ‘touch’ nerves can be activated by gently rubbing the skin and this is why gently rubbing a painful spot can reduce the intensity of the pain.
Abnormal Pain Responses in FM
People with FM are supersensitive to pain; their pain pathways have become hypersensitized. How has this happened?
The nervous system is not ‘hard wired,’ it possesses plasticity and is able to change its responses and adapt to new situations. If you suffer an injury or trauma, for example, the pain pathways will begin to send intense, repeated pain signals to the brain. These frequent, repeated signals cause large quantities of glutamate and substance P to be released into the gap. If these frequent signals continue over a period of time the large quantities of glutamate open new channels into the next nerve that had previously been blocked. The opening of these new channels greatly intensifies the pain signal. These new channels are scientifically referred to as NMDA channels and once open cause every pain signal from then on—no matter how weak—to be greatly intensified. This pain pathway has become hypersensitized.
The new intensity caused by the opening of the NMDA channels should only last as long as the injury or trauma. However, in FM these changes seem to persist, leaving the pain pathways chronically hypersensitized.
A lack of serotonin is believed to be intrinsic in FM. Serotonin has an important role in dampening the pain signals, as explained above. Reduced quantities of serotonin lead to smaller amounts of the natural painkiller enkephalin being released. This in turn has less impact on reducing the quantities of substance P. Hence the relationship between serotonin and substance P acts like a seesaw: as serotonin goes down, levels of substance P go up.
Levels of substance P have been shown to be three times higher than normal in patients with FM. These increased levels not only intensify the pain signals sent to the brain along the pain pathways, but also succeed in affecting neighboring nerve pathways. Substance P has the ability to move from where it has been released, some distance up and down the spinal cord—intensifying whatever signals are being sent to the brain by other nerves. Some of these other nerves could be those that communicate ‘touch’ to the brain. With the help of substance P these nerves become hypersensitized and begin to transmit very intensified touch signals now recognized by the brain as pain. The brain now translates ‘touch’ as pain, leaving the FM sufferer with great empathy towards the fairy tale "The Princess and the Pea."
