To the human body, acupuncture needles are a physical stimulus. In Western science, a stimulus is defined as a detectable change in either the external environment or within the body itself. When the body detects change, it produces a response. Although acupuncture is not yet fully understood by Western science, with modern technology scientists can now actually begin to "see" the body's response to acupuncture. For example, using an MRI (a very sophisticated x-ray), researchers have shown that when a needle is inserted at specific acupuncture points on the body, corresponding changes occur in the brain. In the West, acupuncture is most well-known for its ability to relieve pain so the majority of research thus far has been done in this area. Acupuncture points are now believed to stimulate the central nervous system (the brain and spinal cord) to release pain-relieving chemicals into the muscles, spinal cord and brain. Acupuncture may also stimulate other chemicals to be released by the brain, including hormones that influence the self-regulating system of the body.