Muscle tissue, which includes skeletal, cardiac, and smooth muscle, is responsible for virtually all movement and the maintenance of posture within the body. These different muscle types have distinct structures and locations, such as the striated appearance of skeletal and heart muscle or the involuntary control of smooth muscle lining internal organs. Despite these differences in form and function, all muscle cells share four fundamental physical properties that enable them to perform their mechanical work. These shared characteristics are not related to their specific organization but rather to the inherent capabilities of the muscle cell membrane and its internal proteins. These properties allow the muscle cell to respond to signals and generate force in a precisely controlled manner.
Responsiveness
Responsiveness, also termed excitability, is the ability of a muscle cell to receive and react to a stimulus from its environment. This reaction is the initiating step for all muscle activity and is typically triggered by a chemical signal, such as a neurotransmitter released from a nerve cell. The chemical signal binds to receptors on the muscle cell membrane, causing a change in its electrical state.
This change involves the movement of ions, like sodium, across the cell membrane, which generates an electrical impulse called an action potential. The action potential then travels along the entire surface of the muscle cell, acting as an internal signal to begin the process of contraction. Although the nervous system completely controls skeletal muscle, both cardiac and smooth muscle can also respond to other stimuli like hormones or local factors.
Contractility
Contractility is the ability of muscle tissue to shorten forcibly when an adequate stimulus is received. This is an active, energy-requiring process that generates tension within the muscle fibers. The active shortening occurs when protein filaments within the muscle cells, specifically actin and myosin, slide past one another.
This sliding filament mechanism is powered by the release of calcium ions within the cell and the splitting of adenosine triphosphate (ATP). The resulting tension allows the muscle to pull on its attachment points, such as bones or the walls of internal organs, to produce movement or maintain posture. However, muscle contraction does not always mean a physical shortening, as tension can be produced without a change in muscle length during what is known as isometric contraction.
Extensibility
Extensibility is a passive property that allows muscle tissue to be stretched or extended beyond its resting length. This characteristic is necessary for normal body movement and is the opposite of the active process of contraction. For instance, when a muscle group contracts, the opposing muscle group must be able to lengthen to allow the joint to move through its full range of motion.
A muscle is usually stretched by the contraction of an opposing muscle or by external forces, such as gravity or a therapist applying a passive stretch. Without sufficient extensibility, the range of motion around a joint would be severely restricted, hindering coordinated movement.
Elasticity
Elasticity is the ability of the muscle tissue to recoil and return to its original resting length after being stretched. This property is separate from extensibility; extensibility permits the stretch, while elasticity ensures the recovery of shape. The return to the original length is a passive process that does not require the cell to actively expend energy.
This recoil action is largely facilitated by specialized connective tissues and elastic proteins within the muscle structure, most notably the giant protein titin. Titin acts like a molecular spring within the muscle’s contractile units, the sarcomeres, providing passive tension that pulls the muscle back into shape after it has been stretched.