The three (
skeletal, cardiac and smooth) types of muscle have significant differences. However, all three use the movement
of actin against myosin to create contraction. In skeletal muscle, contraction is stimulated by electrical impulses transmitted by the nerves, the motor nerves and motoneurons in particular. Cardiac and smooth muscle
contractions are stimulated by internal pacemaker cells which regularly contract, and propogate contractions to other muscle cells they are in contact with. All skeletal muscle and many smooth muscle contractions are facilitated by the neurotransmitter acetylcholine.
Muscular activity accounts for much of the body''s energy consumption. All muscle cells produce adenosine triphosphate (ATP) molecules which are used to power the movement of the myosin heads.
Muscles contain ATP in the form of creatine phosphate which is generated from ATP and can regenerate ATP when needed with creatine kinase. Muscles also keep a storage form of glucose in the form of glycogen. Glycogen can be rapidly converted to glucose when energy is required for sustained, powerful contractions. Within the voluntary skeletal muscles, the glucose molecule is metabolized in a process called glycolysis which produces two ATP and two lactic acid molecules in the process. Muscle cells also contain globules of fat, which are used for energy during aerobic exercise. The aerobic energy systems take longer to produce the ATP and reach peak efficiency, and requires many more biochemical steps, but produces significantly more ATP than anaerobic glycolysis. Cardiac muscle on the other hand, can readily consume any of the three macronutrients (protein, glucose and fat) without a ''warm up'' period and always extracts the maximum ATP yield from any molecule involved. The heart and liver will also consume lactic acid produced and excreted by skeletal muscles during exercise.