Concept 36.1 Cycles of Protein–Protein Interactions Cause Muscles to Contract

• 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 Figures 36.1 and 36.2 and WEB ACTIVITY 36.1
   
• The molecular mechanism of muscle contraction involves the binding of the globular heads of myosin molecules to actin. Upon binding, the myosin head changes its conformation, causing the two filaments to move past each other (the sliding filament contractile mechanism). Release of the myosin heads from actin and their return to their original conformation requires ATP. Review ANIMATED TUTORIAL 36.1
   
• All the fibers activated by a single motor neuron constitute a motor unit. Each nerve ending of the motor neuron forms a synapse with the muscle cell membrane. When neurotransmitter is released at the synapse, the muscle cell membrane is depolarized and action potentials are generated. Action potentials spread across the plasma membrane and through the T tubules, causing Ca²⁺ to be released from the sarcoplasmic reticulum. Review Figure 36.5 and WEB ACTIVITY 36.2
   
• 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 Figure 36.6
   
• 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. Some cardiac muscle cells are pacemaker cells that generate and conduct electrical signals.
   
• Smooth muscle provides contractile force for internal organs. Smooth muscle cells respond to stretch, action potentials spreading from neighboring cells, hormones, or neurotransmitters from the autonomic nervous system. Review Figure 36.7 and ANIMATED TUTORIAL 36.2
   

Concept 36.2 The Characteristics of Muscle Cells Determine Muscle Performance

• In skeletal muscle, a single action potential causes a minimum unit of contraction called a twitch. Twitches occurring in rapid succession can be summed, thus increasing the strength of contraction. Maximum sustained tension is called tetanus. Review Figure 36.9
   
• Slow-twitch fibers facilitate extended, aerobic work; fast-twitch fibers generate maximum forces for short periods of time. Review Figure 36.10
   
• Muscle performance depends on a supply of ATP. Available ATP and creatine phosphate (CP) can fuel maximum tension instantaneously but are exhausted within seconds. Glycolysis can regenerate ATP rapidly but is limited by accumulation of lactic acid. Oxidative metabolism delivers ATP more slowly but can continue to do so for a long time. Review Figure 36.11
   

Concept 36.3 Muscles Pull on Skeletal Elements to Generate Force and Cause Movement

• Skeletal systems provide supports against which muscles can pull.
   
• Hydrostatic skeletons are fluid-filled body cavities that can be squeezed by muscles. Review Figure 36.12
   
• 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 36.13
   
• Bone is continually remodeled by osteoblasts, which lay down new bone, and osteoclasts, which erode bone. Review Figure 36.14
   
• Bones develop from connective tissue membranes (membranous bone) or from cartilage (cartilage bone) through ossification. Cartilage bone can grow until the centers of ossification meet. Review Figure 36.15
   
• Bone can be compact (solid and hard) or cancellous (containing numerous internal spaces).
   
• Joints enable muscles to power movements in different directions. Muscles and bones work together around joints as systems of levers. Review Figures 36.16 and 36.18 and WEB ACTIVITY 36.3
   
• Tendons connect muscle to bone; ligaments connect bones to each other. Review Figure 36.17