Plasticity: ability to learn and inquire new skills. Result as to how the central nervous system lays down new connection to movements. The cns must learn to react in certain situation to gain new skills. Optogenetics: method used to control neurons of the brain to cause movement. Inject gene into specimen causing to move in reaction to different flashes of light. Technique can be used to help when people are disabled. Extrafusual muscle fibers: the muscle fibers used to move. Intrafusual muscle fibers: designed to detect stretch in a muscle. Stretch information is important as it tell the cns when the body part is in relation to the rest of the body. People who have problems in their cns cannot detect certain things such as where are their feet. This results in them needing to look at their feet as they walk. If not, will not know how to control and trip.