As I understand it, your body deals with muscular stress in three ways. When it sees that increased loads are expected of it, it can:
1) grow larger, which has several effects that are benefical to lifting more - mostly due to cell volume
2) it can increase the nerve signal density/strength
3) grow additional cells
The third is the most costly effort, and therefore the one that it is less likely to do (unless you happen to be genetically inclined to have less myostatin).
It is likely that actions 1 and 2 are what take first and foremost, and then when those no longer are enough, you get number 3 and then readjust the figures for 1 and 2.
When you take time off, the nerve connections stay there for more longer than the muscle does. The muscle size is the first to decrease, and then over time the number of cells will decrease.
When you then come back, you will have the nerve cells still denser in numbers there, so you will get a surprising amount of strength from what is there. Also, if the layoff wasn't especially long (weeks to a month), then you will have muscle tissues that are conditioned to respond well to the same actions as before, and will quickly respond to stimulus once you start up again.
In running, you can take up to 3 weeks off before you really start to see a decline in performance (a noticable decline that you can percieve on your own - if you are scientific about it, you can see results drop off after less than a week - this is in running, which I'm assuming isn't too different in the end since it is all just muscles).
There is a formula in running for the rate at which you can expect to come back and see returns in your gains without overexerting yourself and opening yourself up for increased chance of injury - but in order for me to look up that formula, I would have to get up off the couch right now.
If it really is important, I will get up and look.
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