Chapter 27: Water and Salt Physiology: Introduction and Mechanisms

Skeletal muscle, the most studied type, is composed of multinucleated fibers containing repeating sarcomeres, the fundamental contractile units organized by actin and myosin filaments. The sliding filament theory is presented in detail, describing how cross-bridge cycling between myosin heads and actin filaments, fueled by ATP hydrolysis, leads to shortening of sarcomeres and whole-muscle contraction. Key molecular players include troponin, tropomyosin, and the regulatory role of intracellular calcium ions (Ca²⁺) released from the sarcoplasmic reticulum during excitation-contraction coupling. The process is initiated by action potentials traveling down motor neurons, triggering neurotransmitter release at the neuromuscular junction and depolarization of the muscle fiber membrane. The authors differentiate between fast-twitch (Type II) and slow-twitch (Type I) muscle fibers, each specialized for power and endurance, respectively. Differences in mitochondrial density, capillary supply, myoglobin content, and glycolytic enzyme activity explain the metabolic distinctions between fiber types. Motor unit recruitment and summation are discussed as mechanisms for controlling muscle force, along with isometric versus isotonic contractions and the length-tension relationship that governs optimal muscle performance. Smooth and cardiac muscle are examined for their unique properties. Smooth muscle, found in the walls of internal organs, operates involuntarily and lacks the organized sarcomeres of skeletal muscle. Cardiac muscle shares features of both skeletal and smooth muscle, including striations and involuntary control, and relies on gap junctions and pacemaker cells for synchronized contractions. The chapter also addresses the energetics of muscle contraction, including ATP regeneration through phosphagen systems, anaerobic glycolysis, and oxidative phosphorylation. The concept of fatigue is introduced, along with recovery mechanisms and muscle plasticity in response to activity and disuse. Finally, the authors touch on evolutionary and comparative aspects, such as asynchronous muscle contraction in insects and specialized muscle types in electric fish and sonic muscles in toadfish. Altogether, the chapter presents muscle as a dynamic and adaptable tissue central to animal function and performance.