Muscle Fibers are a single cell within a muscle and they are also called the functional unit of a muscle. The number of muscle fibers you have is predetermined before you are even born. Muscle fibers have these cylinder shaped bundles of contractile proteins that are found in each muscle cell called myofibrils. The myofibrils have sarcomeres, which are the smallest functional sub-unit of a myofibril, and they are made up of these thick protein fillers known as actin and myosin. Those two proteins are responsible for the movement that we have in our bodies. There are two types of muscle fibers and they are slow-twitch muscle fibers and fast-twitch muscle fibers. They each have their own unique roles. Fast-twitch muscles are used when the muscles are preforming quick and intense activities like weight lifting. Slow-twitch muscles are used when the muscles are enduring a long period of activity like running or swimming. Each group of muscles contains over 100,000 muscles fibers in it. Motor neurons are responsible for muscle contraction. They send message like signals to the brain and spinal chord and then to all the muscles in the body. Every time you mover you leg or even just a finger, that’s the motor neurons sending a message to your brain telling it to move your leg or your finger. Each motor neuron is controlling anywhere between 2 and 2,000 muscles fibers. There are these things called motor units and they contain either all the of the slow-twitching muscles or all the fast-twitching muscles. Your body doesn’t just use one group of muscles fibers at a time. It can go from using fast-twitch muscles to using slow-twitch muscles.