There should be no physiological difference in terms of muscles used, except for the obvious difference between sitting and standing (which you are likely to vary more when climbing).

You obviously have a climber's build, which greatly helps you when climbing. It means, of course, that a heavier cyclist will beat you on the descents, however.

I am not sure why your heart rate should "explode" when descending. There is an interesting argument for saving energy on a descent. Since Air resistance becomes huge at the speeds encountered in a descent, a cyclist has to exert a lot to make even small differences in speed. Check out the following: www.sportsci.org/encyc/cyclingupdown/ cyclingupdown.html

Cheers,
Jamie