Pain Perception
Pain is an unpleasant experience that involves both physical and psychological components. Feeling pain is quite adaptive because it makes us aware of an injury, and it motivates us to remove ourselves from the cause of that injury. In addition, experiencing pain also makes us less likely to suffer additional injury because we will be gentler with our injured body parts and will adapt to the way we interact with objects in our environment.
Generally speaking, pain can be considered to be neuropathic or inflammatory in nature.
neuropathic and inflammatory pain
Pain that signals some type of tissue damage is known as inflammatory pain.
In some situations, pain results from damage to neurons of either the peripheral or central nervous system. As a result, pain signals that are sent to the brain get exaggerated. This type of pain is known as neuropathic pain.
Multiple treatment options for pain relief range from relaxation therapy to the use of analgesic medications to deep brain stimulation. The most effective treatment option for a given individual will depend on a number of considerations, including the severity and persistence of the pain and any medical/psychological conditions.
Some individuals are born without the ability to feel pain. This very rare genetic disorder is known as congenital insensitivity to pain (or congenital analgesia).
congenital insensitivity to pain (congenital analgesia)
Those with congenital analgesia can detect differences in temperature and pressure, but they cannot experience pain. As a result, they often suffer significant injuries. Young children have serious mouth and tongue injuries because they have bitten themselves repeatedly. Not surprisingly, individuals suffering from this disorder have much shorter life expectancies due to their injuries and secondary infections of injured sites (U.S. National Library of Medicine, 2013).
Life Without Pain?
Imagine a life free of pain. How would it be—calm, fearless, serene? Would you feel invulnerable, invincible? Getting rid of pain is a popular quest—a quick search for “pain-free life” on Google returns well over 4 million hits—including links to various bestselling self-help guides promising a pain-free life in only 7 steps, 6 weeks, or 3 minutes. Pain management is a billion-dollar market, and involves much more than just pharmaceuticals. Surely a life with no pain would be a better one?
Well, consider one of the “lucky few”: 12-year-old “Thomas” has never felt deep pain. Not even when a fracture made him walk around with one leg shorter than the other, so that the bones of his healthy leg were slowly crushed to destruction underneath the knee joint. For Thomas and other members of a large Swedish family, life without pain is a harsh reality because of a mutated gene that affects the growth of the nerves conducting deep pain. Most of those affected suffer from joint damage and frequent fractures to bones in their feet and hands; some end up in wheelchairs even before they reach puberty (Minde et al., 2004). It turns out pain—generally—serves us well.
Action Potentials in the Receptor Cells Travel as Nerve Impulses with Different Speeds
When you step on a pin, this activates a host of mechanoreceptors, many of which are nociceptors. You may have noticed that the sensation changes over time. First you feel a sharp stab that propels you to remove your foot, and only then you feel a wave of more aching pain. The unpleasant ache you feel after the sharp pin stab is a separate, simultaneous signal sent from the nociceptors in your foot. The experience of stepping on a pin is, in other words, composed by two separate signals: one discriminatory signal allowing us to localize the touch stimulus and distinguish whether it’s a blunt or a sharp stab; and one affective signal that lets us know that stepping on the pin is bad.
It is common to divide pain into sensory–discriminatory and affective–motivational aspects (Auvray et al., 2010). This distinction corresponds, at least partly, to how this information travels from the peripheral to the central nervous system and how it is processed in the brain (Price, 2000).
How is it possible to do something so excruciatingly painful to yourself, as Aron Ralston did, and still manage to walk, talk, and think rationally afterward? The answer lies within the brain, where signals from the body are interpreted. When we perceive somatosensory and nociceptive signals from the body, the experience is highly subjective and malleable by motivation, attention, emotion, and context.
Motivation–Decision Model and Descending Modulation of Pain
motivation-decision model
According to the motivation–decision model, the brain automatically and continuously evaluates the pros and cons of any situation—weighing impending threats and available rewards (Fields, 2004, 2006).
Anything more important for survival than avoiding the pain activates the brain’s descending pain modulatory system—a top-down system involving several parts of the brain and brainstem, which inhibits nociceptive signaling so that the more important actions can be attended to.
In Ralston’s extreme case, his actions were likely based on such an unconscious decision process—taking into account his homeostatic state (his hunger, thirst, the inflammation and decay of his crushed hand slowly affecting the rest of his body), the sensory input available (the silence around him indicating his solitude), and his knowledge about the threats facing him (death, or excruciating pain that won’t kill him) versus the potential rewards (survival, seeing his family again). Ralston’s story illustrates the evolutionary advantage to being able to shut off pain: the descending pain modulatory system allows us to go through with potentially life-saving actions.
However, when one has reached safety or obtained the reward, healing is more important. The very same descending system can then “crank up” nociception from the body to promote healing and motivate us to avoid potentially painful actions. To facilitate or inhibit nociceptive signals from the body, the descending pain modulatory system uses a set of ON- or OFF-cells in the brainstem, which regulates how much of the nociceptive signal reaches the brain. The descending system is dependent on opioid signaling, and analgesics like morphine relieve pain via this circuit (Petrovic, Kalso, Petersson, & Ingvar, 2002).
Analgesic Power of Reward
Thinking about the good things, like his loved ones and the life ahead of him, was probably pivotal to Aron’s survival. The promise of a reward can be enough to relieve pain. Because expecting pain relief is a form of reward this can contribute to the placebo effect—where pain relief is due to the brain’s own opioid system (Eippert et al., 2009; Eippert et al.; Levine et al.). Eating tasty food, listening to good music, or feeling pleasant touch on your skin also decreases pain in both animals and humans, presumably through the same mechanism in the brain (Leknes & Tracey, 2008).
In a now classic experiment, Dum and Herz (1984) either fed rats normal rat food or let them feast on highly rewarding chocolate-covered candy (rats love sweets) while standing on a metal plate until they learned exactly what to expect when placed there. When the plate was heated up to a noxious/painful level, the rats that expected candy endured the temperature for twice as long as the rats expecting normal chow. Moreover, this effect was completely abolished when the rats’ opioid (endorphin) system was blocked with a drug, indicating that the analgesic effect of reward anticipation was caused by endorphin release.
For Aron the climber, both the stress from knowing that death was impending and the anticipation of the reward it would be to survive probably flooded his brain with endorphins, contributing to the wave of excitement and euphoria he experienced while he carried out the amputation “like a five-year-old unleashed on his Christmas presents” (Ralston, 2010). This altered his experience of the pain from the extreme tissue damage he was causing and enabled him to focus on freeing himself. Our brain, it turns out, can modulate the perception of how unpleasant pain is, while still retaining the ability to experience the intensity of the sensation (Rainville, Duncan, Price, Carrier, & Bushnell, 1997; Rainville, Feine, Bushnell, & Duncan, 1992).
Social rewards, like holding the hand of your significant other, have pain-reducing effects. Even looking at a picture of them can have similar effects—in fact, seeing a picture of a person we feel close to not only reduces subjective pain ratings, but also the activity in pain-related brain areas (Eisenberger et al., 2011). The most common things to do when wanting to help someone through a painful experience—being present and holding the person’s hand—thus seems to have a measurably positive effect.
Power of the Mind
The context of pain and touch has a great impact on how we interpret it. Just imagine how different it would feel to Aron if someone amputated his hand against his will and for no discernible reason. Prolonged pain from injuries can be easier to bear if the incident causing them provides a positive context—like a C-section scar that enabled a parent to bring their baby into the world—or phantom pain from a hand that was cut off to enable life to carry on.
The relative meaning of pain was illustrated by an experiment where the same moderate heat was rated as either painful or as pleasant when it provided relief from a more intense pain (Leknes et al., 2013). The interpretation of touch also varies, as knowing who or what is touching us affects our response. Additionally, we can experience pain and pleasure vicariously, as the same brain areas that process these sensations when we experience them ourselves are also active when we observe someone else experiencing them (Singer et al., 2004).