Muscles in the human body have biochemical specialization and which makes them to perform many physiological functions. Animals use muscles to convert the chemical energy of ATP into mechanical work. Three different kinds of muscles are found in vertebrate animals: Heart muscle (muscle: makes up the wall of the heart). Smooth muscle is found in the walls of all the hollow organs of the body (except the heart). Skeletal muscle, attached to the skeleton. It is also called striated muscle. Skeletal muscle is made up of thousands of cylindrical muscle fibers. Each muscle fiber contains: an array of myofibrils that are stacked lengthwise and runs the entire length of the fiber, mitochondria, smooth endoplasmic reticulum and many nuclei. Sarcomeres are composed of thick (myosin) and thin (actin) filaments. The junction between the terminal of a motor neuron and a muscle fiber is called the neuromuscular junction. It is simply one kind of synapse. The terminals of motor axons contain thousands of vesicles filled with acetylcholine.