Where do you draw the line, free-will adherents?

I would like to have a discussion about where the limits of free will are, and exactly why they are there. For example, I can choose not to eat, but I cannot choose not to starve; where is the demarcation of my control over the processes of my body? If the natural law that controls my digestion cannot be willed, then how can my neurons be willed? Without evidence to that effect, how can I reasonably conclude that I am in any way overcoming the natural processes that define me?

If you can, please be specific and as brief as possible, and thank you for your response!