DEGREES OF PAIN [ THE NERVOUS SYSTEM ]

Similar pains don’t always register with the same intensity. Although nearly all humans-besides the very few who lack the ability to feel pain recognize extreme heat or a deep cut as painful, they can react differently. Some tolerate pain more easily, whereas others feel it more intensely. Physical, cultural, and psychological variables may also influence a person’s individual degree of pain tolerance.

Cultural and psychological influences on an individual’s tolerance of pain are more ethereal and hard to measure than physiological influences. During World War II, British soldiers injured in the brutal fighting at Anzio, Italy, in 1943 routinely refused morphine to kill their pain, while civilians who suffered far less serious wounds demanded it to ease their pain. The surgeon who noted the difference came to the conclusion that certain kinds of pain could be a matter of mind, not of the body.

Long-term, intense pain can create a different perception in the brain. This chronic sensation may confuse the central nervous system and result in hyperalgesia, or pain amplification. Such pain registers on the same kind of synaptic receptors that are activated during certain kinds of learning. Under the worst- case scenarios, the chronic pain causes the spinal cord to “learn” hyperalgesia, and pain’s sensitivity increases. Examples include the lingering pain of phantom limbs-the sensation of pain from an amputated arm or leg.

Neural networks that process stimuli from a limb remain primed to respond to signals even after it’s gone. Random signals may get misinterpreted as tingling, itching, pain, or some other sensation. Neuroscientist Vilayanur Ramachandran found he could create sensations in phantom limbs by applying pressure to various skin surfaces. His conclusion: The cerebral cortex relocated sensation pathways associated with the old limb. These pathways may always have existed in a weak state, but loss of the limb amplified them. Unfortunately, neural networks that continue to recognize “pain” signals from a missing limb become more strongly primed to repeat the mistake. Treatments for phantom pain range from drug therapy to acupuncture and deep brain stimulation. Newer treatments, using mirrors or virtual reality goggles, trick the brain into thinking it can control the amputated limb.