How Pain Works: Why Hurt and Harm Are Not the Same Thing

It seems obvious that pain means something is wrong in the body, and that more pain means more damage. Often that is close to true. Sometimes it is not, and that gap explains a lot of confusion for people whose pain outlasts an injury. This note explains in plain language how pain is defined, how short term and long term pain differ, and why the nervous system can keep producing pain after tissues have healed. It also flags where the science is unfinished.

What pain is, and what it is not

The International Association for the Study of Pain (IASP) defines pain as an unpleasant sensory and emotional experience associated with, or resembling that associated with, actual or potential tissue damage. Note the words "or resembling". The definition leaves room for pain without a visible cause.

The IASP adds several notes. Pain is always a personal experience, influenced to varying degrees by biological, psychological and social factors. Pain and nociception are different phenomena, and pain cannot be inferred solely from activity in sensory neurons. In other words, signals in sensory neurons and the experience of pain are not the same thing. The IASP also says a person's report of pain should be respected.

That last point is worth keeping in mind. Saying pain is produced by the nervous system does not mean it is imagined or exaggerated. Pain is real whenever someone feels it.

Acute and persistent pain

The US National Institute of Neurological Disorders and Stroke (NINDS) separates the two by time. Acute pain starts suddenly and ends when its cause is treated or has healed. It is usually sharp, because it acts as a warning about a threat to the body, such as an injury or overuse. Chronic pain lasts longer than three months or the expected healing time. In some cases an acute pain condition persists and becomes chronic. In other cases, chronic pain happens for no known reason.

The IASP notes that pain usually serves an adaptive role, which fits the warning idea. It may also have adverse effects on function and on social and psychological wellbeing. A useful alarm that will not switch off stops being useful.

Why pain can persist after tissues heal

NINDS explains that the brain systems involved in pain overlap with systems for emotions such as fear, anxiety and anger, for reward and motivation, and for attention and memory. There are also brain systems that contribute to relief from pain. Research shows that changes in these systems might contribute to the risk of chronic pain.

It also describes peripheral sensitization and central sensitization. These are changes in the nerves outside the brain and spinal cord, and in the brain and spinal cord themselves, that make a person more sensitive to pain and other sensations. NINDS says such changes are one way some people develop chronic pain after an injury has resolved.

The same idea shows up in back pain research. A 2018 paper from the Lancet Low Back Pain Series reports that for nearly all people with low back pain a specific nociceptive cause cannot be identified, and that increasing evidence shows central pain modulating mechanisms and beliefs about pain have important roles in persistent disabling back pain. Higher initial pain, psychological distress and pain in several body sites raised the risk that it would become persistent. Our note on back pain myths shows how this research feeds into guidelines.

Cautions against oversimplifying

Three cautions are in order. First, "the nervous system is involved" does not rule out a physical cause. Some pain does come from a clear problem, and some conditions need medical treatment. Our notes on when back or neck pain is urgent and neck pain and posture cover cases where a clinician should look.

Second, the research here is still developing. Terms such as sensitization describe changes that researchers observe, and what they mean for any one person is hard to say. Third, nobody can tell a reader from an article whether their own pain involves these mechanisms. A clinician who can examine and ask about the history is the right person for that question.

What the sources do support is a gentler framing. Hurt and harm are related but not identical, and pain that lingers is a recognised problem in its own right, not a sign that someone is exaggerating. Other notes on this theme are on the pain and posture shelf.

The short version

Pain is an experience built by the nervous system, shaped by biology, thinking and context, and it is not a direct meter of tissue damage. Short term pain is a warning that usually fades as the body heals, while long lasting pain can involve a more sensitive nervous system. Ask a clinician about pain that persists, and see our note on urgent warning signs for when to seek care quickly.