Muscle spindles are sensorimotor organs located within skeletal muscle. Each muscle spindle is made up of three to five specialized muscle cells known as intrafusal cells. These cells, bundled together by a sheath of connective tissue, lay alongside the rest of the skeletal muscle fibers. When intrafusal cells detect a change in muscle length, they reflexively stimulate a muscle contraction to prevent over-stretching and muscle fiber damage. The above information is from. http://www.wisegeek.com/what-are-muscle-spindles.htm