In 1995, the British Medical Journal published an astonishing report
about a 29-year-old builder. He accidentally jumped
onto a 15-centimeter nail, which pierced straight through
his steel-toed boot. He was in such agonizing pain that even
the smallest movement was unbearable. But when the doctors took off his boot,
they faced a surprising sight: the nail had never touched
his foot at all. For hundreds of years, scientists thought that pain was
a direct response to damage. By that logic, the more severe an injury
is, the more pain it should cause. But as we’ve learned more about
the science of pain, we’ve discovered that pain and tissue
damage don’t always go hand in hand, even when the body’s threat signaling
mechanisms are fully functioning. We’re capable of experiencing severe pain
out of proportion to an actual injury, and even pain without any injury, like the builder, or the well-documented
cases of male partners of pregnant women experiencing pain
during the pregnancy or labor. What’s going on here? There are actually two phenomena at play: the experience of pain, and a biological
process called nociception. Nociception is part of the nervous
system’s protective response to harmful or potentially harmful stimuli. Sensors in specialized nerve endings detect mechanical, thermal,
and chemical threats. If enough sensors are activated, electrical signals shoot up the nerve
to the spine and on to the brain. The brain weighs the importance
of these signals and produces pain if it decides
the body needs protection. Typically, pain helps the body
avoid further injury or damage. But there are a whole set of factors
besides nociception that can influence the experience of pain—
and make pain less useful. First, there are biological factors that
amplify nociceptive signals to the brain. If nerve fibers are activated repeatedly, the brain may decide they need
to be more sensitive to adequately protect the body
from threats. More stress sensors can be
added to nerve fibers until they become so sensitive that
even light touches to the skin spark intense electrical signals. In other cases, nerves adapt to send signals more
efficiently, amplifying the message. These forms of amplification are most common in people experiencing
chronic pain, which is defined as pain lasting
more than 3 months. When the nervous system is nudged
into an ongoing state of high alert, pain can outlast physical injury. This creates a vicious cycle in which
the longer pain persists, the more difficult it becomes to reverse. Psychological factors clearly
play a role in pain too, potentially by influencing nociception and
by influencing the brain directly. A person’s emotional state, memories, beliefs about pain and expectations
about treatment can all influence how much
pain they experience. In one study, children who reported believing they
had no control over pain actually experienced more intense pain than those who believed they
had some control. Features of the environment matter too: In one experiment, volunteers with a cold rod placed on
the back of their hand reported feeling more pain when they were
shown a red light than a blue one, even though the rod was the same
temperature each time. Finally, social factors like the
availability of family support can affect perception of pain. All of this means that a multi-pronged
approach to pain treatment that includes pain specialists, physical
therapists, clinical psychologists, nurses and other healthcare professionals
is often most effective. We’re only beginning to uncover the
mechanisms behind the experience of pain, but there are some promising
areas of research. Until recently, we thought the glial cells surrounding
neurons were just support structures, but now we know they have a huge role
in influencing nociception. Studies have shown that disabling certain
brain circuits in the amygdala can eliminate pain in rats. And genetic testing in people with
rare disorders that prevent them from feeling pain have pinpointed several other
possible targets for drugs and perhaps eventually gene therapy.
