Q: Why was shark-attack victim Bethany Hamilton able to keep her cool after losing a limb, while I wince in pain from a splinter?

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Because Bethany's brain kicked into survival mode. Several years ago I was in the Amazon preparing to operate on a boy with a partially amputated arm who had calmly found his way to me through a tangle of jungle trails. He said he felt no pain. But when I stuck him to inject novocaine, he screamed.

Pain is an alarm signal that alerts the mind to injury. But massive wounds, such as amputated arms, already have your full attention. A flood of pain signals would only interfere with other impulses going to your cerebral cortex (the center of consciousness) and could prevent you from developing a plan to save yourself. To avoid our panicking to death, the brain squelches our pain alarm.
So why did the boy cry out from a simple needle stick? Once he found help, pain had a different use: It alerted me to care for him—very gently.

Ken Kamler, M.D., is the author of Surviving the Extremes (Penguin).