Muscle Tissue

Learning Objectives Describe the organization of muscle and the unique characteristics of skeletal muscle cells. Identify the structural components of the sarcomere. Summarize the events at the neuromuscular junction. Explain the key concepts involved in skeletal muscle contraction and tension production.

Learning Objectives Describe how muscle fibers obtain energy for contraction. Distinguish between aerobic and anaerobic contraction, muscle fiber types, and muscle performance. Identify the differences between skeletal, cardiac and smooth muscle.

SECTION 10-1 Skeletal muscle tissue and the Muscular System

Three types of muscle Skeletal – attached to bone Cardiac – found in the heart Smooth – lines hollow organs

Skeletal muscle functions Produce skeletal movement Maintain posture and body position Support soft tissues Guard entrances and exits Maintain body temperature

SECTION 10-2 Anatomy of Skeletal Muscle

Organization of connective tissues Epimysium surrounds muscle Perimysium sheathes bundles of muscle fibers Epimysium and perimysium contain blood vessels and nerves Endomysium covers individual muscle fibers Tendons or aponeuroses attach muscle to bone or muscle PLAY Animation: Gross anatomy of skeletal muscle

Figure 10.1 The Organization of Skeletal Muscles

Skeletal muscle fibers Sarcolemma (cell membrane) Sarcoplasm (muscle cell cytoplasm) Sarcoplasmic reticulum (modified ER) T-tubules and myofibrils aid in contraction Sarcomeres – regular arrangement of myofibrils

Figure 10.3 The Structure of a Skeletal Muscle Fiber

Figure 10.4 Sarcomere Structure, Part I

Myofibrils Thick and thin filaments Organized regularly

Figure 10.5 Sarcomere Structure, Part II

Figure 10.6 Levels of Functional Organization in Skeletal Muscle Fiber

Thin filaments F-actin Nebulin Tropomyosin Covers active sites on G-actin Troponin Binds to G-actin and holds tropomyosin in place

Thick filaments Bundles of myosin fibers around titan core Myosin molecules have elongate tail, globular head Heads form cross-bridges during contraction Interactions between G-actin and myosin prevented by tropomyosin during rest

Figure 10.7 Thick and Thin Filaments

Sliding filament theory Explains the relationship between thick and thin filaments as contraction proceeds Cyclic process beginning with calcium release from SR Calcium binds to troponin Trponin moves, moving tropomyosin and exposing actin active site Myosin head forms cross bridge and bends toward H zone ATP allows release of cross bridge

Figure 10.8 Changes in the appearance of a Sarcomere during the Contraction of a Skeletal Muscle Fiber Figure 10.8

SECTION 10-3 The Contraction of Skeletal Muscle

Tension Created when muscles contract Series of steps that begin with excitation at the neuromuscular junction Calcium release Thick/thin filament interaction Muscle fiber contraction Tension

Figure 10.9 An Overview of the Process of Skeletal Muscle

Control of skeletal muscle activity occurs at the neuromuscular junction Action potential arrives at synaptic terminal ACh released into synaptic cleft ACh binds to receptors on post-synaptic neuron Action potential in sarcolemma

Figure 10.10 Skeletal Muscle Innervation Figure 10.10a, b

Figure 10.10 Skeletal Muscle Innervation PLAY Animation: Neuromuscular junction Figure 10.10c

Excitation/contraction coupling Action potential along T-tubule causes release of calcium from cisternae of SR Initiates contraction cycle Attachment Pivot Detachment Return

Figure 10.12 The Contraction Cycle

Figure 10.12 The Contraction Cycle

Figure 10.12 The Contraction Cycle

Figure 10.12 The Contraction Cycle

Relaxation Acetylcholinesterase breaks down ACh Limits the duration of contraction PLAY Animation: Sliding filament theory

SECTION 10-4 Tension Production

Tension production by muscle fibers All or none principle Amount of tension depends on number of cross bridges formed Skeletal muscle contracts most forcefully over a narrow ranges of resting lengths

Figure 10.13 The Effect of Sarcomere Length on Tension

Twitch Cycle of contraction, relaxation produced by a single stimulus Treppe Repeated stimulation after relaxation phase has been completed

Summation Repeated stimulation before relaxation phase has been completed Wave summation = one twitch is added to another Incomplete tetanus = muscle never relaxes completely Complete tetanus = relaxation phase is eleminated

Figure 10.14 The Twitch and the Development of Tension

Figure 10.15 Effects of Repeated Stimulations

Tension production by skeletal muscles Internal tension generated inside contracting muscle fibers External tension generated in extracellular fibers

Figure 10.16 Internal and External Tension

Motor units All the muscle fibers innervated by one neuron Precise control of movement determined by number and size of motor unit Muscle tone Stabilizes bones and joints

Figure 10.17 The Arrangement of Motor Units in a Skeletal Muscle

Tension production by skeletal muscles Internal tension generated inside contracting muscle fibers External tension generated in extracellular fibers

Figure 10.16 Internal and External Tension

Motor units All the muscle fibers innervated by one neuron Precise control of movement determined by number and size of motor unit Muscle tone Stabilizes bones and joints

Figure 10.17 The Arrangement of Motor Units in a Skeletal Muscle

Contractions Isometric Tension rises, length of muscle remains constant Isotonic Tension rises, length of muscle changes Resistance and speed of contraction inversely related Return to resting lengths due to elastic components, contraction of opposing muscle groups, gravity PLAY Animation: Whole Muscle Contraction

Figure 10.18 Isotonic and Isometric Contractions

Figure 10.19 Resistance and Speed of Contraction PLAY Animation: Skeletal muscle contraction Figure 10.19

SECTION 10-5 Energy Use and Muscle Contraction

Muscle Contraction requires large amounts of energy Creatine phosphate releases stored energy to convert ADP to ATP Aerobic metabolism provides most ATP needed for contraction At peak activity, anaerobic glycolysis needed to generate ATP

Figure 10.20 Muscle Metabolism

Figure 10.20 Muscle Metabolism

Energy use and level of muscular activity Energy production and use patterns mirror muscle activity Fatigued muscle no longer contracts Build up of lactic acid Exhaustion of energy resources

Recovery period Begins immediately after activity ends Oxygen debt (excess post-exercise oxygen consumption) Amount of oxygen required during resting period to restore muscle to normal conditions

SECTION 10-6 Muscle Performance

Types of skeletal muscle fibers Fast fibers Slow fibers Intermediate fibers

Figure 10.21 Fast versus Slow Fibers

Fast fibers Large in diameter Contain densely packed myofibrils Large glycogen reserves Relatively few mitochondria Produce rapid, powerful contractions of short duration

Slow fibers Half the diameter of fast fibers Take three times as long to contract after stimulation Abundant mitochondria Extensive capillary supply High concentrations of myoglobin Can contract for long periods of time

Intermediate fibers Similar to fast fibers Greater resistance to fatigue

Muscle performance and the distribution of muscle fibers Pale muscles dominated by fast fibers are called white muscles Dark muscles dominated by slow fibers and myoglobin are called red muscles Training can lead to hypertrophy of stimulated muscle

Physical conditioning Anaerobic endurance Time over which muscular contractions are sustained by glycolysis and ATP/CP reserves Aerobic endurance Time over which muscle can continue to contract while supported by mitochondrial activities PLAY Animation: Muscle fatigue

SECTION 10-7 Cardiac Muscle Tissue

Structural characteristics of cardiac muscle Located only in heart Cardiac muscle cells are small One centrally located nucleus Short broad T-tubules Dependent on aerobic metabolism Intercalated discs where membranes contact one another

Figure 10.22 Cardiac Muscle Tissue

Functional characteristics of cardiac muscle tissue Automaticity Contractions last longer than skeletal muscle Do not exhibit wave summation No tetanic contractions possible

SECTION 10-8 Smooth Muscle Tissue

Structural characteristics of smooth muscle Nonstriated Lack sarcomeres Thin filaments anchored to dense bodies Involuntary

Figure 10.23 Smooth Muscle Tissue

Functional characteristics of smooth muscle Contract when calcium ions interact with calmodulin Activates myosin light chain kinase Functions over a wide range of lengths Plasticity Multi-unit smooth muscle cells are innervated by more than one motor neuron Visceral smooth muscle cells are not always innervated by motor neurons Neurons that innervate smooth muscle are not under voluntary control

You should now be familiar with: The organization of muscle and the unique characteristics of skeletal muscle cells. The structural components of the sarcomere. The events at the neuromuscular junction. The key concepts involved in skeletal muscle contraction and tension production. How muscle fibers obtain energy for contraction. Aerobic and anaerobic contraction, muscle fiber types, and muscle performance. The differences between skeletal, cardiac and smooth muscle