The Muscles - Structure of the muscles

Each of the three kinds of muscle shares certain structural similarities with one or both of the others. All are made up of bundles of varying numbers of hair-thin fibers. In skeletal and smooth muscles, these fibers are lined up side by side in the bundle, while in cardiac muscle the fibers tend more to crisscross over one another. Skeletal muscle and cardiac muscle are both striped, that is, they show darker and lighter bands crossing over a group of adjacent fibers, while smooth muscle lacks these distinct cross-bands.

Both involuntary muscle types, smooth and cardiac, are controlled by signals carried by the autonomic nervous system. Signals that result in movements of the skeletal muscles are carried by a different nerve network, the central nervous system. The individual fibers in a muscle bundle with a particular function all react simultaneously to a signal from the nervous system; there is no apparent time lag from fiber to fiber.