Edward G. Moczydlowski
The primary function of muscle is to generate force or movement in response to a physiological stimulus. The human body contains three fundamentally different types of muscle adapted to specialized functions. Skeletal muscle is responsible for the voluntary movement of bones that underlies locomotion and work production. Skeletal muscle also controls the breathing cycle of the lungs via contraction of the diaphragm and functions as a pump assisting return of the venous blood supply to the heart. Cardiac muscle is specific to the heart as the biomechanical pump driving the delivery of blood to the lungs and tissues. Smooth muscle provides mechanical control of organ systems such as the digestive, urinary, and reproductive tracts as well as the blood vessels of the circulatory system and the airway passages of the respiratory system.
Contraction of muscles is initiated either by a chemical neurotransmitter or paracrine factor or by direct electrical excitation. All muscles transduce chemical energy released by hydrolysis of ATP into mechanical work. The unique physiological role of each of the three basic muscle types dictates inherent differences in the rate and duration of contraction, metabolism, fatigability, and ability to regulate contractile strength. For example, both skeletal and cardiac muscle must be capable of rapid force development and shortening. However, skeletal muscle must be able to maintain contractile force for relatively long periods. Cardiac muscle contracts only briefly with each heartbeat but must sustain this rhythmic activity for a lifetime. Smooth muscle, like skeletal muscle, must be able to regulate contraction over a wide range of force development and elastic changes in the size of organs such as the urinary bladder and uterus. In some tissues (e.g., sphincters), smooth muscle sustains contraction without fatigue for very long periods. Despite these differences, the trigger for muscle contraction is the same for all three types of muscle: a rise in the free cytosolic Ca2+ concentration ([Ca2+]i).
This chapter describes the fundamental physiology of muscle excitation, the coupling of excitation to contraction, the molecular mechanism of contraction, the regulation of contraction, and the related issues of muscle diversity. We describe general molecular mechanisms shared by all muscle cells and contrast the unique features of skeletal, cardiac, and smooth muscle. Because molecular mechanisms specific to cardiac myocytes are best understood in the unique context of the heart as a pump, we discuss details of cardiac muscle physiology at greater depth in Chapters 22.