Sensitization

Sensitization is a state in which the threshold for nerves that transmit pain is lowered, making them respond more readily than usual. An easy way to understand it is to think of the nerves as temporarily raising their alert level.

Sensitization is broadly divided into two types, depending on where it occurs.

• Peripheral sensitization — Nerve endings at the site of an injury or inflammation become more sensitive. Touching that area feels more painful than usual.
• Central sensitization (central sensitization) — Pain-processing neurons in the spinal cord and brain that receive those signals become more sensitive. The International Association for the Study of Pain (IASP) defines this as “increased responsiveness of nociceptive neurons in the central nervous system to their normal or subthreshold afferent input.” In this case, pain may spread beyond the original injury site, or sensitivity may linger for a while after the stimulus has ended.

Sensitization brings changes such as hyperalgesia, in which the same stimulus feels more intense. It can also cause allodynia, in which light contact that would not normally hurt feels painful.

• Muscle soreness after exercise — Unfamiliar exercise causes small injuries within the muscles and the production of inflammatory substances. These substances make nerve endings in the muscles more sensitive, so movements or light pressure that would normally feel fine can become painful. This is covered in Delayed onset muscle soreness.
• Sensitivity after an injury — Even a light brush against a sprained ankle can hurt. This is the body's protective response, encouraging you to use the injured area carefully.
• A day after poor sleep — Lack of sleep lowers the pain threshold, making your body feel more painful than usual. You can read more in Sleep deprivation and pain sensitivity.
• Periods of high stress — When you are tense and exhausted, the nervous system as a whole becomes more sensitive, so even mild discomfort can feel more intense.

Acute sensitization is a normal response that protects the body. The nervous system temporarily raises its alert level so that you use the injured area carefully. As the injury heals and inflammation subsides, the nerves' threshold also returns to its usual level.

Sometimes sensitivity can remain even after the tissue has recovered. In these cases, the intensity of pain no longer matches the condition of the tissue exactly. The nervous system can change in response to experience (neuroplasticity), so a heightened state of sensitivity can also change again over time. There is also education that helps people learn how pain is produced (pain neuroscience education).

• Get enough sleep. Sleep is the most basic way to restore the pain threshold.
• Rather than avoiding all use of the injured area, move it a little at a time within a pain-free range. Movement within a comfortable range helps the nerves receive signals of safety.
• Increase exercise gradually. A sudden increase in intensity can make sensitivity linger along with muscle soreness.
• Ease tension. Activities that help your body feel comfortable, such as deep breathing, walking, or a warm shower, help lower the nerves' alert level.
• Try keeping a pain diary. Recording when pain worsens and when you feel more comfortable can help you see your own patterns.

• When the painful area keeps spreading even though you have not been injured
• When even light contact, such as clothing touching your skin, hurts enough to interfere with everyday life
• When sensitivity does not decrease after several weeks to several months
• When the painful area also shows unusual changes, such as swelling, a change in color, or warmth

Sensitization is a broad term for a phenomenon in which the response to a stimulus becomes greater than before. It is used not only in pain but also in fields such as immunology, learning, and sensory research. In pain, it refers to changes in which the same stimulus feels more intense, or a response occurs to a stimulus that previously did not bother you.

It helps to distinguish the scope of these terms. Central sensitization is a pain science term for a heightened state of sensitivity in the central nervous system. Sensitization is broader, encompassing changes at the level of peripheral nerves as well as increased responses in behavior and learning. A process in the opposite direction is habituation, in which the response to the same stimulus decreases. Both processes can work together in actual sensory experiences.

Saying that something has “become more sensitive” is one framework for describing a state. The intensity and repetition of stimuli, attention, expectations, sleep, stress, and the surrounding environment all influence sensory experience together. It therefore makes sense to consider tissue changes, the demands of movement, emotional state, and the environment together. Pain associated with increased sensitivity is still genuinely experienced pain.