Essentially what is happening is that your body is releasing endorphins, which are like a natural heroine, and actually is very similar in chemical structure. The body releases endorphins as one approaches their physical limit, and it allows one to continue of for a bit longer. The endorphins latch themselves to the brain's opioid receptors in the brain, which is the same place that heroin/morphine/opium latches to to get you high, as the name implies. I typically only feel the endorphin rush after I completely stop running, at which point it hits me all at once, and I usually have to sit down or at least lean on something. Prior to this point, I'm assuming that the endorphin release is just keeping me going rather than doping me up, but don't quote me on that - I have no sort of evidence to back this up.
|A nerve synapse. This is actually from a drug abuse site, showing how opiates travel through the nervous system. Just pretend those balls are endorphins rather than opiates.|