I too practice the slow plunge method. Here's what i at first assumed, then later found that a nerve follows along side a vein. The travel along veins to communicate to information as to transportation of chemicals to cells. They also function as nociceptors (communicate pain) whereas skeletal muscles function primarily to activate muscles have very little nociception. So, when you push the pin in, do so slowly. If you feel pain, you are likely very close to a vein. I've noticed that every time i feel pain, but go ahead and inject, a nice flow of blood comes when the pin is pulled out (unless i z-track). The sharp pain I'm talking about isn't the initial pain when piercing the skin, just to avoid confusion. That being said, depending upon how far the needle is into the muscle, i may push past the point of pain, although some times I'll reposition the pin to avoid a vein.