Skeletal muscles contract rapidly in response to messages from the central nervous system. Each group of several |fibers| receives a ~nerve~ supply that allows voluntary |contraction| of the muscle. Muscles can move some body parts in several directions and others in only two directions. The direction the body part is moved depends largely on the shapes of the bones at the joints. The stimulus for the muscle |contraction| begins in the cerebral |cortex| and passes down the |spinal cord| and the ~nerve~ root to the junction between the |nerve fiber| and the muscle surface. This gap, called the end plate, acts as a kind of amplifier, increasing the effect of the tiny current coming down the |nerve fiber| to stimulate the much larger muscle |fiber|. On the arrival of the ~nerve~ impulse, a chemical called acetylcholine is released from the motor ~nerve~ ending and passes across the gap to stimulate the membrane of the muscle |fiber|. This stimulation is in the form of an electric current which passes along the surface of the muscle, causing it to contract. It takes one millisecond (1/1000th of a second) for the current to pass along the surface of the muscular |fiber|.
Cardiac muscle differs slightly from skeletal muscle because it has a built-in mechanism to maintain the necessary rhythmical |contraction| independently of any nervous connections. |Smooth muscles| react much more slowly to stimulation than skeletal muscles. The ~nerves~, when present, alter the activity of the muscle rather than initiating it. This action is somewhat similar to cardiac muscle. The |contractions| take place rhythmically without direct control from the central nervous system. The impulses for |contraction| come from within the muscle itself.
