I might be splitting hairs, but in a way, you can send the message to your brain to start the bodily waste removal process by walking into a bathroom. You might not have felt like you had to go to the bathroom, but surely you have walked into a bathroom for a purpose other than urinating or crapping and then had to do so. The brain affiliates the toilet with the bodily waste removal process and the sight of the toilet can trigger the brain to make you need to go to the bathroom. I guess you can consider this to be a voluntary body function if you intentionally entered the bathroom in an attempt to get the urge to do a 1 or 2. Usually it will trigger an urge to urinate just because there is always urine in the bladder that can be discharged. Again, I am splitting hairs, but here's an answer that you probably would never have thought about.