Concept 33.1   Muscle Cells Develop Forces by Means of Cycles of Protein–Protein Interaction

• Skeletal muscle consists of bundles of muscle fibers. Each skeletal muscle fiber is a large, elongated cell containing multiple nuclei. Review Figure 33.2 and ACTIVITY 33.1
• A skeletal muscle fiber contains numerous myofibrils, which contain bundles of actin and myosin filaments. The regular, overlapping arrangement of the actin and myosin filaments into sarcomeres gives skeletal muscle its striated appearance. Review Figure 33.2
• Contraction is the development of force by a muscle. The molecular mechanism of contraction is described by the sliding-filament theory and involves the binding of the globular heads of myosin molecules to actin molecules to form cross-bridges. Upon binding to actin, a myosin head changes its conformation, causing the two filaments to move past each other. Release of the myosin heads from actin and their return to their original conformation requires ATP. Review Figure 33.3, Figure 33.4 and ANIMATED TUTORIAL 33.1
• Nerve cells make contact with skeletal muscle fibers at neuromuscular junctions. In general, each muscle fiber in skeletal muscle is innervated by a single nerve cell. Review Figure 33.5
• Excitation by a nerve impulse (action potential) stimulates a muscle fiber to contract by excitation–contraction coupling. An action potential spreads across the muscle fiber’s cell membrane and through the transverse tubules (T), causing Ca²⁺ to be released from the sarcoplasmic reticulum. Review Figure 33.6 and ACTIVITY 33.2
• Ca²⁺ binds to troponin and changes its conformation, pulling the tropomyosin strands away from the myosin-binding sites on the actin filaments. The muscle fiber continues to contract until the Ca²⁺ is returned to the sarcoplasmic reticulum. Review Figure 33.7

Concept 33.2   Skeletal Muscles Pull on Skeletal Elements to Produce Useful Movements

• Skeletal systems provide structures against which skeletal muscles can pull to produce useful movements. Endoskeletons are internal systems of rigid supports, consisting of bone and cartilage, to which muscles are attached. Review Figure 33.8
• Tendons connect muscles to bones. Muscles and bones work together around joints to produce movement. Review Figure 33.9 and Figure 33.10
• Exoskeletons are skeletons that enclose an animal, notably the hardened outer surfaces of arthropods. In arthropods muscles attach to apodemes, internal projections at the joints of the exoskeleton. Review Figure 33.11
• A hydrostatic skeleton is said to exist if an animal’s body or a part of its body becomes stiff and skeleton-like because of a high fluid pressure inside. Review Figure 33.12

Concept 33.3   Skeletal Muscle Performance Depends on ATP Supply, Cell Type, and Training

• Muscle contraction depends on a supply of ATP. Muscle cells have three systems for supplying ATP: the immediate system (preexisting ATP and creatine phosphate); the glycolytic system (anaerobic glycolysis); and the oxidative system (aerobic metabolism). The immediate system can fuel high muscle power output instantaneously but is exhausted within seconds. Glycolysis can regenerate ATP rapidly but is self-limiting. Oxidative metabolism delivers ATP more slowly but can continue to do so for a long time. Review Figure 33.13
• Slow oxidative muscle cells have cellular properties (e.g., abundant mitochondria) that facilitate extended, aerobic work; fast glycolytic muscle cells generate great forces for short periods of time. In vertebrates, slow oxidative cells contain the hemoglobin-like compound myoglobin, making these cells red. Review Figure 33.14
• Skeletal muscles contain varying proportions of slow oxidative cells and fast glycolytic cells, depending on genetic controls during muscle development and the demands placed on the muscles. The properties of muscles can be modified by training. Review Figure 33.15 and Figure 33.16

Concept 33.4   Many Distinctive Types of Muscle Have Evolved

• Cardiac muscle cells are striated, uninucleate, and electrically connected by gap junctions, so that action potentials spread rapidly throughout masses of cardiac muscle and cause coordinated contractions. In vertebrates some modified cardiac muscle cells serve as the pacemaker cells for rhythmic beating of the heart.
• Smooth muscle provides contractile force for internal organs such as the gut, blood vessels, and reproductive ducts. Some smooth muscle tissue (e.g., in the digestive tract) consists of sheets of cells that are electrically coupled through gap junctions, helping coordinate the contractions of adjacent cells. Review ANIMATED TUTORIAL 33.2
• Some insects drive flapping of their wings with asynchronous muscles, which unlike most muscles undergo multiple contractions with each excitation.
• Catch muscles, such as the adductor muscles of clams and scallops, can sustain strong contractions for long periods with little ATP.
• The electric organs of nearly all electric fish evolved from skeletal muscle and consist of modified, noncontractile muscle cells.