Chapter 12 a Muscles About this Chapter Skeletal

Chapter 12 a Muscles

About this Chapter • • Skeletal muscle Mechanics of body movement Smooth muscle Cardiac muscle

Three Types of Muscle Nucleus Muscle fiber (cell) Striations (a) Skeletal muscle Figure 12 -1 a

Three Types of Muscle Striations Muscle fiber Intercalated disk (b) Cardiac muscle Nucleus Figure 12 -1 b

Three Types of Muscle fiber Nucleus (c) Smooth muscle Figure 12 -1 c

Skeletal Muscle • • • Usually attached to bones by tendons Origin: closest to the trunk Insertion: more distal Flexor: brings bones together Extensor: moves bones away Antagonistic muscle groups: flexor-extensor pairs

Antagonistic Muscle Groups Triceps muscle relaxes Biceps muscle contracts (flexor) (a) Flexion Figure 12 -2 a

Antagonistic Muscle Groups Triceps muscle contracts (extensor) Biceps muscle relaxes (b) Extension Figure 12 -2 b

Organization of Skeletal Muscle Skeletal muscle Tendon Nerve and blood vessels Connective tissue Muscle fascicle: bundle of fibers Connective tissue Nucleus Muscle fiber (a) Figure 12 -3 a (1 of 2)

Organization of Skeletal Muscle Figure 12 -3 a (2 of 2)

Ultrastructure of Muscle ANATOMY SUMMARY ULTRASTRUCTURE OF MUSCLE Mitochondria Sarcoplasmic reticulum Thick filament Nucleus Thin filament T-tubules Myofibril Sarcolemma (b) A band Sarcomere Z disk Myofibril (c) M line I band H zone Titin (d) Z disk M line Myosin crossbridges M line Thick filaments Thin filaments Titin (e) Troponin Nebulin Myosin heads Myosin tail Hinge region Tropomyosin Myosin molecule (f) G-actin molecule Actin chain Figure 12 -3 b-f

Ultrastructure of Muscle ULTRASTRUCTURE OF MUSCLE Mitochondria Sarcoplasmic reticulum Thick Thin filament Nucleus T-tubules Myofibril Sarcolemma (b) Figure 12 -3 b

Ultrastructure of Muscle A band Sarcomere Z disk Myofibril (c) M line I band H zone Figure 12 -3 c

Ultrastructure of Muscle Titin (d) Z disk M line Myosin crossbridges Z disk Figure 12 -3 d

Ultrastructure of Muscle M line Thick filaments (e) Myosin heads Myosin tail Hinge region Myosin molecule Figure 12 -3 e

Ultrastructure of Muscle Thin filaments Titin Troponin Nebulin Tropomyosin G-actin molecule Actin chain (f) Figure 12 -3 f

Ultrastructure of Muscle A band Sarcomere Z disk Myofibril (c) M line I band H zone Titin (d) Z disk M line Thick filaments M line Myosin crossbridges Z disk Thin filaments Titin (e) Myosin heads Myosin tail Hinge region Myosin molecule Troponin Nebulin Tropomyosin G-actin molecule Actin chain (f) Figure 12 -3 c-f

T-Tubules and the Sarcoplasmic Reticulum T-tubule brings action potentials into interior of muscle fiber. Triad Thin filament Sarcolemma Sarcoplasmic reticulum stores Ca 2+ Thick filament Terminal cisterna Figure 12 -4

The Two- and Three-Dimensional Organization of a Sarcomere I band Sarcomere A band H zone I band Thin filament Thick filament (a) (b) M line Z disk I band thin filaments only (c) Z disk H zone thick filaments only M line thick filaments linked with accessory proteins Outer edge of A band thick and thin filaments overlap Figure 12 -5

Anatomy Review Animation PLAY Interactive Physiology® Animation: Muscular System: Anatomy Review: Skeletal Muscle Tissue

Muscle Contraction • • • Muscle tension: force created by muscle Load: weight that opposes contraction Contraction: creation of tension in muscle Relaxation: release of tension Steps leading up to muscle contraction: 1. Events at the neuromuscular junction 2. Excitation-contraction coupling 3. Contraction-relaxation cycle

Summary of Muscle Contraction Figure 12 -7

Events at the Neuromuscular Junction PLAY Events at the Neuromuscular Junction Interactive Physiology® Animation: Muscular System: Events at the Neuromuscular Junction

Changes in a Sarcomere During Contraction I band Myosin Z Actin Z A band Muscle relaxed Z Half of I band Sarcomere shortens with contraction Z M H zone H A band constant Z Half of I band Z A band Half of I band M line Z line Muscle contracted I H zone and I band both shorten Figure 12 -8

Sliding Filament Theory PLAY Interactive Physiology® Animation: Muscular System: Sliding Filament Theory

The Molecular Basis of Contraction Troponin G-Actin TN Tropomyosin blocks binding site on actin Myosin head Pi ADP (a) Relaxed state. Myosin head cocked. Figure 12 -9 a

The Molecular Basis of Contraction 1 Cytosolic Ca 2+ 3 Tropomyosin shifts, exposing binding site on actin 2 TN 5 ADP Power stroke 4 Pi (b) Initiation of contraction Actin moves 1 Ca 2+ levels increase in cytosol. 2 Ca 2+ binds to troponin (TN). 3 Troponin-Ca 2+ complex pulls tropomyosin away from actin’s myosin-binding site. 4 Myosin binds to actin and completes power stroke. 5 Actin filament moves. Figure 12 -9 b

The Molecular Basis of Contraction G-actin molecule Myosin binding sites Myosin filament 1 ATP binds to myosin. Myosin releases actin. Tight binding in the rigor state ATP ADP 2 Myosin hydrolyses ATP. Myosin head rotates and binds to actin. 4 Myosin releases ADP. Actin filament moves toward M line. Pi Contractionrelaxation Sliding filament Ca 2+ ADP Pi signal 3 Power stroke Relaxed state with myosin heads cocked Figure 12 -10

The Molecular Basis of Contraction G-actin molecule Myosin binding sites Myosin filament Tight binding in the rigor state Figure 12 -10, step 0

The Molecular Basis of Contraction G-actin molecule Myosin binding sites Myosin filament 1 ATP binds to myosin. Myosin releases actin. Tight binding in the rigor state ATP Figure 12 -10, steps 0– 1

The Molecular Basis of Contraction 1 ATP binds to myosin. Myosin releases actin. ATP 2 Myosin hydrolyses ATP. Myosin head rotates and binds to actin. ADP Pi Relaxed state with myosin heads cocked Figure 12 -10, steps 1– 2

The Molecular Basis of Contraction 2 Myosin hydrolyses ATP. Myosin head rotates and binds to actin. Ca 2+ signal 3 Power stroke Actin filament moves toward M line. ADP Pi Pi Relaxed state with myosin heads cocked Figure 12 -10, steps 2– 3

The Molecular Basis of Contraction 3 Power stroke 4 Myosin releases ADP. Actin filament moves toward M line. Pi ADP Figure 12 -10, steps 3– 4

Excitation-Contraction Coupling Axon terminal of somatic motor neuron 1 Muscle fiber 2 ACh 1 Somatic motor neuron releases ACh at neuromuscular junction. 2 Net entry of Na+ through ACh receptor-channel initiates a muscle action potential Na+ Motor end plate Ry. R T-tubule Sarcoplasmic reticulum Ca 2+ DHP Z disk Troponin Actin Tropomyosin M line Myosin head Myosin thick filament (a) Initiation of muscle action potential KEY DHP = dihydropyridine L-type calcium channel Ry. R = ryanodine receptor-channel Figure 12 -11 a

Excitation-Contraction Coupling Axon terminal of somatic motor neuron 1 Muscle fiber ACh 1 Somatic motor neuron releases ACh at neuromuscular junction. Motor end plate Ry. R T-tubule Sarcoplasmic reticulum Ca 2+ DHP Z disk Troponin Actin Tropomyosin M line Myosin head Myosin thick filament (a) Initiation of muscle action potential KEY DHP = dihydropyridine L-type calcium channel Ry. R = ryanodine receptor-channel Figure 12 -11 a, step 1

Excitation-Contraction Coupling Axon terminal of somatic motor neuron 1 Muscle fiber 2 ACh 1 Somatic motor neuron releases ACh at neuromuscular junction. 2 Net entry of Na+ through ACh receptor-channel initiates a muscle action potential Na+ Motor end plate Ry. R T-tubule Sarcoplasmic reticulum Ca 2+ DHP Z disk Troponin Actin Tropomyosin M line Myosin head Myosin thick filament (a) Initiation of muscle action potential KEY DHP = dihydropyridine L-type calcium channel Ry. R = ryanodine receptor-channel Figure 12 -11 a, steps 1– 2

Excitation-Contraction Coupling 3 3 4 DHP receptor opens Ry. R Ca 2+ release channels in sarcoplasmic reticulum and Ca 2+ enters cytoplasm. 4 5 7 5 Ca 2+ binds to troponin, allowing actin-myosin binding. Ca 2+ released 6 Myosin heads execute power stroke. 6 Myosin thick filament Distance actin moves (b) Excitation-contraction coupling Action potential in t-tubule alters conformation of DHP receptor. 7 Actin filament slides toward center of sarcomere. KEY DHP = dihydropyridine L-type calcium channel Ry. R = ryanodine receptor-channel Figure 12 -11 b

Electrical and Mechanical Events in Muscle Contraction • A twitch is a single contraction-relaxation cycle Muscle fiber +30 Action potential from CNS Neuron membrane potential in m. V -70 Motor Recording end plate electrodes +30 Axon Muscle fiber terminal membrane potential Muscle action in m. V potential -70 Time 2 msec Development of tension during one muscle twitch Contraction phase Relaxation phase Tension Latent period Time 10– 100 msec Time Figure 12 -12

Phosphocreatine 1. 2. 3. Creatine phosphate Glycolysis Krebs cycle Figure 12 -13

Locations and Possible Causes of Muscle Fatigue Figure 12 -14

Causes of Muscle Fatigue During Exercise • Extended submaximal exercise • Depletion of glycogen stores • Short-duration maximal exertion • Increased levels of inorganic phosphate • May slow Pi release from myosin • Decrease calcium release • Maximal exercise • Potassium (K+) leaves muscle fiber, leading to increased concentration that is believed to decrease Ca 2+

Skeletal Muscle Metabolism During Fatiguing Submaximal Exercise Question 12 -1

Fast-Twitch Glycolytic and Slow-Twitch Oxidative Muscle Fibers Figure 12 -15

Fast-Twitch Glycolytic and Slow-Twitch Oxidative Muscle Fibers Table 12 -2

Length-Tension Relationships in Contracting Skeletal Muscle Figure 12 -16