What causes cramping?

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I was curious as to the cause of my cramps that I experienced during the SCSM 2007. This is what I found:

(1) Cramping is also commonly seen as a symptom of ‘hitting the wall’ (or ‘bonking’), when an athlete completely uses up their muscle glycogen stores. To deplete fully stored muscles takes 2 to 2.5 hours of exercise.

Message: train harder! Or find a different strategy that delays muscle fatigue.

(2) Neural cramp: neural control of the muscle under fatigue.
"There is a disruption at spinal level which causes inhibition of the golgi tendon organ (GTO) nerve receptors, and thereby over- activation of the muscle spindles, resulting in the sustained contraction of the muscle. They also cite the fact that cramping occurs in biarticular (two-joint) muscles that are mostly shortened in the running cycle. It is in this position that tendons are under less tension and the GTOs are less active. A poor stretching regime during training is also considered a factor with neural related cramps, the argument being that an exaggerated reflex contraction due to a lack of, or irregular stretching, could increase muscle spindle activity."

Ok, I don't understand most of that. Just that cramps were due to inadequate stretching.

(3) Hyponatremia or low blood sodium
Thought mostly to affect ultra endurance competitors, such as the 54mile marathon runners or Ironman triathletes. Recently though, hyponatremia has been discussed more in the context of the 26-mile marathon. The body needs to maintain the right levels of blood sodium in order to draw the right amount of water into cells through the membranes. It doesn’t matter how much water you drink, if you have low sodium levels you can’t absorb any of it.

Takeaway message: don't just hydrate with water. Drink isotonic instead for the sodium.