<qa>  What isotonic saline causes cell shrinkage?This study will last several hours during one study visit. Participants will receive both a painful and a painless injection while undergoing positron emission tomography PET brain imaging. The painful injection will consist of small amounts of hypertoninc saline concentrated saline that causes cell shrinkage in the jaw muscle over a 20-minute period. Several minutes after participants receive hypertonic saline, they will receive an injection with isotonic saline not associated with pain in the opposite jaw muscle. After participants receive the injections, they will either be told or not be told about a pain relief intervention. PET imaging will continue as participants either anticipate or do not anticipate pain relief. Participants will be asked about their pain levels repeatedly throughout the study; their responses will be entered into a computer-controlled system which will modulate rates of saline infusion. 