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Chapter 7: Muscles. Overview. The word muscle can refer to an organ or a tissue Muscles Make up 40-50% of body weight Can contract on conscious command Are responsible for movement. Functions of Muscles. Muscles convert chemical energy into mechanical force: Move body parts
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Overview • The word muscle can refer to an organ or a tissue • Muscles • Make up 40-50% of body weight • Can contract on conscious command • Are responsible for movement
Functions of Muscles • Muscles convert chemical energy into mechanical force: • Move body parts • Maintain body posture and stabilize joints • Adjust the volume of hollow structure (e.g., bladder) • Move substances within the body (e.g., pump blood) • Produce heat
Remember This! Adult muscle stem cells are called satellite cells; they produce myoblasts, which fuse to form skeletal muscle fibers.
Steps in Muscular Contraction An electrical signal in the somatic motor neuron A chemical signal (acetylcholine) in the synapse An electrical signal in the sarcolemma A chemical signal (calcium) in the sarcoplasm
Remember This! During muscle contraction sarcomeres and myofibrils shorten myofilaments do not change in length
Remember This! Levels of Skeletal Muscle Organization Muscle Fascicle (bundle of muscle fibers) Muscle fiber (muscle cell) Myofibril (bundle of myofilaments) Myofilament (stands of contractile proteins) Contractile protein smallest
Muscle Relaxation Relaxation is the reverse of contraction Acetylcholine release stops Nicotinic receptor channels close and sarcolemma repolarizes SR calcium channels close; Ca2+ ions taken into SR Tropomyosin covers binding sites Myosin no longer binds actin; sarcomere returns to resting length
Muscle Energy Production • ATP stores energy in chemical bond used by musclesATP + H2O ADP + H2O + PO4 + Energy • Glycolosis is the fastest method of generating ATP from nutrients. • Mitochondria generate ATP from glucose and fatty acid molecules.
Remember This! ATP binding causes the cross-bridge to release. The energy from ATP cleavage is necessary for the power stroke.
Remember This! The terms anaerobic metabolism and glycolysis are often used synonymously, but erroneously. Glycolysis is the necessary first step in both aerobic metabolism and anaerobic metabolism.
Muscle Fatigue • Muscle fatigue: loss of the ability to respond to nerve stimulation after vigorous exercise • Peripheral • Central
Remember This! In an everyday contraction at a given fiber length, contraction of individual muscle fibers is all-or-none, as the fiber contracts in incomplete tetanus.
Mechanics of Muscle Contraction • The force an individual muscle exerts depends on • The force exerted by each contracting fiber • The number of motor units contracting
Types of Contractions • Isotonic contraction: dynamic shortening of muscle tissue that maintains constant force; e.g., chewing food • Concentric contraction: myofilaments slide; sarcomeres, fibers, muscles shorten; movement occurs; e.g., biceps curl • Eccentric contraction: myosin heads grab actin filaments to slow movement; e.g., lowering weight • Isometric contraction: force is generated and muscle tenses, but myofibrils do not slide and length is unchanged; e.g., maintaining upright body posture
Remember This! The thing common to all muscle contraction is force, not movement.
Remember This! Intermediate filaments form the scaffolding of a smooth muscle cell, and myofilaments contract the cell. The myosin heads are regulated in smooth muscle; the binding sites on actin molecules are regulated in skeletal muscle.
