This is one of the greatest misconceptions in bodybuilding. Your receptors dont BURN OUT or its not like you make worse gains from cycle to cycle because your receptors aren't fresh.
Bill Roberts wrote an excellent article for meso-rx.com he basically stated
1) your body cant grow day in and day evne with anabolics, so two ten week cycles will probably be more productive than one twoenty week one.
2) as you get farther and further to or past your genetic limit gains come harder. This is simply a function of greater muscle mass and the greater inherent catabolic activity that comes from this. your body DOESNT want to maintain more muscle than it naturally can, and by taking drugs you are forcing your body to do so. This is why heavier cycles are needed to add the same weight.
A similar analogy could be seen as your max lift. Lets say your benc was 300 max. That is equivalent to lets say your GENETIC LIMIT. 180 lbs isnt hard to press. Add 20, lets say by a cycle, and 200 is a little harder, but still doable. More and more effort is needed (effort = required drugs) to lift the weight or add incrememnts of the same weight (20 lbs per effort) this is very similar to how your body works on cycles!