Vertebrate Skeletal Muscle Cells

• Thick and thin filaments are polarized polymers of individual protein molecules
• Muscles require ATP to contract
• Calcium and the regulatory proteins tropomyosin and troponin control contractions

Excitation–Contraction Coupling

Whole Skeletal Muscles

• Muscle contraction is the force generated by a muscle during cross-bridge activity
• A twitch is the mechanical response of a muscle to a single stimulus
• The velocity of shortening decreases as the load increases
• The frequency of action potentials determines the tension developed by a muscle
• A sustained high calcium concentration in the cytoplasm permits summation and tetanus
• The amount of tension developed by a muscle depends on the length of the muscle at the time it is stimulated
• In general, the amount of work a muscle can do depends on its volume
• BOX 20.1 Electric Fish Exploit Modified Skeletal Muscles to Generate Electric Shocks

Muscle Energetics

• ATP is the immediate source of energy for powering muscle contraction
• Vertebrate muscle fibers vary in their use of ATP
• Different animals employ different types of muscles that contribute to their achieving success
• BOX 20.2 Insect Flight

Neural Control of Skeletal Muscle

• The vertebrate plan is based on muscles organized into motor units
• The innervation of vertebrate tonic muscle is intermediate between the vertebrate and arthropod plans
• The arthropod plan employs multiterminal and polyneuronal innervation

Vertebrate Smooth (Unstriated) Muscle

• Smooth muscle cells are broadly classified
• Ca²⁺ availability controls smooth muscle contraction by myosin-linked regulation
• The autonomic nervous system (ANS) innervates smooth muscles

Vertebrate Cardiac Muscle