Chapter Summary

• Skeletal muscle consists of bundles of muscle fibers. Each skeletal muscle fiber is a large cell containing multiple nuclei.

• Skeletal muscles contain numerous myofibrils, which are 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 47.1, Activity 47.1

• Observations of the changes in the banding patterns of sarcomeres led to the development of sliding filament model of muscle contraction. Review Figure 47.2

• The molecular mechanism of muscle contraction involves the binding of the globular heads of myosin molecules to actin. Review Figure 47.3, Animation 47.1

• A single motor neuron and all the fibers it activates constitute a motor unit. Each nerve ending of the motor neuron forms a synapse with the muscle cell membrane. Action potentials spread across the muscle cell membrane and through the T tubules, causing Ca²⁺ to be released from the sarcoplasmic reticulum. Review Figure 47.5, Activity 47.2

• Ca²⁺ binds to troponin and changes its conformation, pulling the tropomyosin strands away from the myosin-binding sites on the actin filament. The muscle fiber continues to contract until the Ca²⁺ is returned to the sarcoplasmic reticulum. Review Focus: Key Figure 47.6

• Cardiac muscle cells are striated, uninucleate, branching, and electrically connected by gap junctions, so that action potentials spread rapidly throughout sheets of cardiac muscle and cause coordinated contractions.

• Smooth muscle provides contractile force for internal organs. Smooth muscle cells respond to stretch and to neurotransmitters from the autonomic nervous system. Review Figures 47.8, 47.9, Animation 47.2

• In skeletal muscle, a single action potential causes a minimum unit of contraction called a twitch. Twitches occurring in rapid succession can be summed to achieve sustained contraction, which is known as tetanus. Review Figure 47.10

• Slow-twitch fibers facilitate extended, aerobic work; fast-twitch fibers generate maximum forces for short periods of time. The ratio of slow-twitch to fast-twitch fibers in the muscles of an individual is largely genetically determined. Review Figure 47.11

• The force that a muscle fiber can produce depends on its initial state of extension or contraction. Review Figure 47.12, Investigating Life: What Is the Optimal Resting Position for the Jumping Muscle of the Frog?

• Anaerobic exercise stimulates the enlargement of muscle fibers through production of new microfilaments. Aerobic exercise stimulates greater oxidative capacity of muscle fibers.

• Muscle performance depends on a supply of ATP. Review Figure 47.13

• Skeletal systems provide supports against which muscles can pull.

• Hydrostatic skeletons are fluid-filled body cavities that can be squeezed by muscles. Review Figure 47.14

• Exoskeletons are hardened outer surfaces to which internal muscles are attached.

• Endoskeletons are internal systems of rigid rodlike, platelike, and tubelike supports, consisting of bone and cartilage to which muscles are attached. Review Figure 47.15

• Bone is continually remodeled by osteoblasts, which lay down new bone, and osteoclasts, which erode bone. Review Figure 47.16

• Bones develop from connective tissue membranes (membranous bone) or from cartilage (cartilage bone) through ossification. Review Figure 47.17

• Bone can be compact (solid and hard) or cancellous (containing numerous internal spaces). Most of the compact bone of mammals is composed of Haversian systems. Review Figure 47.18

• Joints enable muscles to power movements in different directions. Muscles and bones work together around joints as systems of levers. Review Figures 47.19, 47.21, Activity 47.3

• Tendons connect muscles to bones; ligaments connect bones to one another. Review Figure 47.20