A while back, lecturing about pain perception and enkephalin’s role as an endogenous pain killer, I remember bringing up the example of a bike rider who gets hit by a speeding car but doesn’t feel the pain until later on…I didn’t remember the exact term of this phenomenon back then and just stumbled upon an article with this interesting content:
Stress-Induced Analgesia (SIA)
Analgesia may be produced in certain stressful situations. Exposure to a variety of painful or stressful events produces an analgesic reaction. This phenomenon is called stress induced analgesia (SIA). For example, soldiers wounded in battle or athletes injured in sports events sometimes report that they do not feel pain during the battle or game; however, they will experience the pain later after the battle or game has ended. It has been demonstrated (in animals) that electrical shocks cause stress-induced analgesia. Based on these experiments, it is assumed that the stress the soldiers and the athletes experienced suppressed the pain which they would later experience.
It has been suggested that endogenous opiates (aka opiods, such as enkephalin) are released in response to stress and inhibit pain by activating the midbrain descending system. Moreover, some SIA exhibited cross tolerance with opiate analgesia, which indicates that this SIA is mediated via opiate receptors. Experiments using different parameters of electrical shock stimulation demonstrate that such stress produces analgesia and some of these stresses that produce analgesia could be blocked by the opioid antagonist naloxone, whereas others were not blocked by naloxone. These observations lead to the conclusion that both opiate and non-opiate forms of SIA exist.
The full text of this article can be found in the site of UT medical school at Houston.