10 Muscle Tissue

10-1 An Introduction to Muscle Tissue Learning Outcomes 10-1 Specify the functions of skeletal muscle tissue. 10-2 Describe the organization of muscle at the tissue level. 10-3 Explain the characteristics of skeletal muscle fibers, and identify the structural components of a sarcomere. 10-4 Identify the components of the neuromuscular junction, and summarize the events involved in the neural control of skeletal muscle contraction and relaxation.

10-1 An Introduction to Muscle Tissue Learning Outcomes 10-5 Describe the mechanism responsible for tension production in a muscle fiber, and compare the different types of muscle contraction. 10-6 Describe the mechanisms by which muscle fibers obtain the energy to power contractions. 10-7 Relate the types of muscle fibers to muscle performance, and distinguish between aerobic and anaerobic endurance. 3

10-1 An Introduction to Muscle Tissue Learning Outcomes 10-8 Identify the structural and functional differences between skeletal muscle fibers and cardiac muscle cells. 10-9 Identify the structural and functional differences between skeletal muscle fibers and smooth muscle cells, and discuss the roles of smooth muscle tissue in systems throughout the body. 4

An Introduction to Muscle Tissue A primary tissue type, divided into: Skeletal muscle tissue Cardiac muscle tissue Smooth muscle tissue

10-1 Functions of Skeletal Muscle Tissue Skeletal Muscles Are attached to the skeletal system Allow us to move The muscular system Includes only skeletal muscles

10-1 Functions of Skeletal Muscle Tissue Six Functions of Skeletal Muscle Tissue Produce skeletal movement Maintain posture and body position Support soft tissues Guard entrances and exits Maintain body temperature Store nutrient reserves

10-2 Organization of Muscle Skeletal Muscle Muscle tissue (muscle cells or fibers) Connective tissues Nerves Blood vessels

10-2 Organization of Muscle Organization of Connective Tissues Muscles have three layers of connective tissues Epimysium Perimysium Endomysium

10-2 Organization of Muscle Epimysium Exterior collagen layer Connected to deep fascia Separates muscle from surrounding tissues 10

10-2 Organization of Muscle Perimysium Surrounds muscle fiber bundles (fascicles) Contains blood vessel and nerve supply to fascicles

10-2 Organization of Muscle Endomysium Surrounds individual muscle cells (muscle fibers) Contains capillaries and nerve fibers contacting muscle cells Contains myosatellite cells (stem cells) that repair damage

Figure 10-1 The Organization of Skeletal Muscles Skeletal Muscle (organ) Epimysium Perimysium Endomysium Nerve Muscle fascicle Muscle fibers Blood vessels Epimysium Blood vessels and nerves Tendon Endomysium Perimysium 13

Figure 10-1 The Organization of Skeletal Muscles Muscle Fascicle (bundle of fibers) Perimysium Muscle fiber Epimysium Blood vessels and nerves Endomysium Tendon Endomysium Perimysium 14

Figure 10-1 The Organization of Skeletal Muscles Muscle Fiber (cell) Capillary Myofibril Endomysium Sarcoplasm Epimysium Blood vessels and nerves Mitochondrion Myosatellite cell Sarcolemma Nucleus Tendon Axon of neuron Endomysium Perimysium 15

10-2 Organization of Muscle Organization of Connective Tissues Muscle Attachments Endomysium, perimysium, and epimysium come together: At ends of muscles To form connective tissue attachment to bone matrix I.e., tendon (bundle) or aponeurosis (sheet)

10-2 Organization of Muscle Blood Vessels and Nerves Muscles have extensive vascular systems that: Supply large amounts of oxygen Supply nutrients Carry away wastes Skeletal muscles are voluntary muscles, controlled by nerves of the central nervous system (brain and spinal cord)

10-3 Characteristics of Skeletal Muscle Fibers Skeletal Muscle Cells Are very long Develop through fusion of mesodermal cells (myoblasts) Become very large Contain hundreds of nuclei

Figure 10-2 The Formation of a Multinucleate Skeletal Muscle Fiber Muscle fibers develop through the fusion of mesodermal cells called myoblasts. Myoblasts A muscle fiber forms by the fusion of myoblasts. Muscle fiber LM  612 Nuclei Sarcolemma Myofibrils Myosatellite cell Nuclei Mitochondria Immature muscle fiber A diagrammatic view and a micrograph of one muscle fiber. Myosatellite cell Up to 30 cm in length Mature muscle fiber 19

Figure 10-2a The Formation of a Multinucleate Skeletal Muscle Fiber Muscle fibers develop through the fusion of mesodermal cells called myoblasts. Myoblasts A muscle fiber forms by the fusion of myoblasts. Myosatellite cell Nuclei Immature muscle fiber Myosatellite cell Up to 30 cm in length Mature muscle fiber 20

Figure 10-2b The Formation of a Multinucleate Skeletal Muscle Fiber Nuclei Sarcolemma Myofibrils Mitochondria A diagrammatic view and a micrograph of one muscle fiber. 21

10-3 Characteristics of Skeletal Muscle Fibers The Sarcolemma and Transverse Tubules The sarcolemma The cell membrane of a muscle fiber (cell) Surrounds the sarcoplasm (cytoplasm of muscle fiber) A change in transmembrane potential begins contractions

10-3 Characteristics of Skeletal Muscle Fibers The Sarcolemma and Transverse Tubules Transverse tubules (T tubules) Transmit action potential through cell Allow entire muscle fiber to contract simultaneously Have same properties as sarcolemma

10-3 Characteristics of Skeletal Muscle Fibers Myofibrils Lengthwise subdivisions within muscle fiber Made up of bundles of protein filaments (myofilaments) Myofilaments are responsible for muscle contraction Types of myofilaments: Thin filaments Made of the protein actin Thick filaments Made of the protein myosin

10-3 Characteristics of Skeletal Muscle Fibers The Sarcoplasmic Reticulum (SR) A membranous structure surrounding each myofibril Helps transmit action potential to myofibril Similar in structure to smooth endoplasmic reticulum Forms chambers (terminal cisternae) attached to T tubules

10-3 Characteristics of Skeletal Muscle Fibers The Sarcoplasmic Reticulum (SR) Triad Is formed by one T tubule and two terminal cisternae Cisternae Concentrate Ca2+ (via ion pumps) Release Ca2+ into sarcomeres to begin muscle contraction

Figure 10-3 The Structure of a Skeletal Muscle Fiber Myofibril Sarcolemma Nuclei Sarcoplasm MUSCLE FIBER Mitochondria Terminal cisterna Sarcolemma Sarcolemma Sarcoplasm Myofibril Myofibrils Thin filament Thick filament Triad Sarcoplasmic reticulum T tubules 27

Figure 10-3 The Structure of a Skeletal Muscle Fiber Myofibril Sarcolemma Nuclei Sarcoplasm MUSCLE FIBER 28

Figure 10-3 The Structure of a Skeletal Muscle Fiber Mitochondria Terminal cisterna Sarcolemma Sarcolemma Sarcoplasm Myofibril Myofibrils Thin filament Thick filament Triad Sarcoplasmic reticulum T tubules 29

Figure 10-3 The Structure of a Skeletal Muscle Fiber Mitochondria Sarcolemma Myofibril Thin filament Thick filament 30

Figure 10-3 The Structure of a Skeletal Muscle Fiber Terminal cisterna Sarcolemma Sarcoplasm Myofibrils Triad Sarcoplasmic reticulum T tubules 31

10-3 Structural Components of a Sarcomere Sarcomeres The contractile units of muscle Structural units of myofibrils Form visible patterns within myofibrils A striped or striated pattern within myofibrils Alternating dark, thick filaments (A bands) and light, thin filaments (I bands)

10-3 Structural Components of a Sarcomere Sarcomeres The A Band M line The center of the A band At midline of sarcomere The H Band The area around the M line Has thick filaments but no thin filaments Zone of overlap The densest, darkest area on a light micrograph Where thick and thin filaments overlap

10-3 Structural Components of a Sarcomere Sarcomeres The I Band Z lines The centers of the I bands At two ends of sarcomere Titin Are strands of protein Reach from tips of thick filaments to the Z line Stabilize the filaments

Figure 10-4a Sarcomere Structure, Part I I band A band H band Z line Titin A longitudinal section of a sarcomere, showing bands Zone of overlap M line Thin filament Thick filament Sarcomere 35

Figure 10-4b Sarcomere Structure, Part I I band A band H band Z line A corresponding view of a sarcomere in a myofibril from a muscle fiber in the gastrocnemius muscle of the calf Myofibril TEM  64,000 Z line Zone of overlap M line Sarcomere 36

Figure 10-5 Sarcomere Structure, Part II Myofibril A superficial view of a sarcomere Thin filament Thick filament Actinin filaments Titin filament Attachment of titin Z line I band M line H band Zone of overlap Cross-sectional views of different portions of a sarcomere 37

Figure 10-6 Levels of Functional Organization in a Skeletal Muscle Myofibril Surrounded by: Epimysium Surrounded by: Sarcoplasmic reticulum Epimysium Contains: Muscle fascicles Consists of: Sarcomeres (Z line to Z line) Sarcomere I band A band Muscle Fascicle Contains: Thick filaments Surrounded by: Perimysium Thin filaments Perimysium Contains: Muscle fibers Z line M line Titin Z line H band Muscle Fiber Surrounded by: Endomysium Endomysium Contains: Myofibrils 38

Figure 10-6 Levels of Functional Organization in a Skeletal Muscle Surrounded by: Epimysium Epimysium Contains: Muscle fascicles 39

Figure 10-6 Levels of Functional Organization in a Skeletal Muscle Muscle Fascicle Surrounded by: Perimysium Perimysium Contains: Muscle fibers 40

Figure 10-6 Levels of Functional Organization in a Skeletal Muscle Muscle Fiber Surrounded by: Endomysium Endomysium Contains: Myofibrils 41

Figure 10-6 Levels of Functional Organization in a Skeletal Muscle Myofibril Surrounded by: Sarcoplasmic reticulum Consists of: Sarcomeres (Z line to Z line) 42

Figure 10-6 Levels of Functional Organization in a Skeletal Muscle Sarcomere I band A band Contains: Thick filaments Thin filaments Z line M line Titin Z line H band 43

10-3 Structural Components of a Sarcomere Thin Filaments F-actin (filamentous actin) Is two twisted rows of globular G-actin The active sites on G-actin strands bind to myosin Nebulin Holds F-actin strands together

10-3 Structural Components of a Sarcomere Thin Filaments Tropomyosin Is a double strand Prevents actin–myosin interaction Troponin A globular protein Binds tropomyosin to G-actin Controlled by Ca2+

Figure 10-7ab Thick and Thin Filaments Sarcomere H band Actinin Z line Titin Myofibril The gross structure of a thin filament, showing the attachment at the Z line Troponin Active site Nebulin Tropomyosin G-actin molecules Z line M line F-actin strand The organization of G-actin subunits in an F-actin strand, and the position of the troponin–tropomyosin complex 46

Figure 10-7a Thick and Thin Filaments Actinin Z line Titin The gross structure of a thin filament, showing the attachment at the Z line 47

Figure 10-7b Thick and Thin Filaments Troponin Active site Nebulin Tropomyosin G-actin molecules F-actin strand The organization of G-actin subunits in an F-actin strand, and the position of the troponin–tropomyosin complex 48

10-3 Structural Components of a Sarcomere Initiating Contraction Ca2+ binds to receptor on troponin molecule Troponin–tropomyosin complex changes Exposes active site of F-actin

10-3 Structural Components of a Sarcomere Thick Filaments Contain about 300 twisted myosin subunits Contain titin strands that recoil after stretching The mysosin molecule Tail Binds to other myosin molecules Head Made of two globular protein subunits Reaches the nearest thin filament

Figure 10-7cd Thick and Thin Filaments Titin The structure of thick filaments, showing the orientation of the myosin molecules M line Myosin head Myosin tail Hinge The structure of a myosin molecule 51

10-3 Structural Components of a Sarcomere Myosin Action During contraction, myosin heads: Interact with actin filaments, forming cross-bridges Pivot, producing motion

10-3 Structural Components of a Sarcomere Sliding Filaments and Muscle Contraction Sliding filament theory Thin filaments of sarcomere slide toward M line, alongside thick filaments The width of A zone stays the same Z lines move closer together

Figure 10-8a Changes in the Appearance of a Sarcomere during the Contraction of a Skeletal Muscle Fiber I band A band Z line H band Z line A relaxed sarcomere showing location of the A band, Z lines, and I band. 54

Figure 10-8b Changes in the Appearance of a Sarcomere during the Contraction of a Skeletal Muscle Fiber I band A band Z line H band Z line During a contraction, the A band stays the same width, but the Z lines move closer together and the I band gets smaller. When the ends of a myofibril are free to move, the sarcomeres shorten simultaneously and the ends of the myofibril are pulled toward its center. 55

10-3 Structural Components of a Sarcomere Skeletal Muscle Contraction The process of contraction Neural stimulation of sarcolemma Causes excitation–contraction coupling Muscle fiber contraction Interaction of thick and thin filaments Tension production

Figure 10-9 An Overview of Skeletal Muscle Contraction Neural control Excitation–contraction coupling Excitation Calcium release triggers ATP Thick-thin filament interaction Muscle fiber contraction leads to Tension production 57

Figure 10-9 An Overview of Skeletal Muscle Contraction Neural control 58

Figure 10-9 An Overview of Skeletal Muscle Contraction Excitation Calcium release ATP triggers Thick-thin filament interaction 59

Figure 10-9 An Overview of Skeletal Muscle Contraction Muscle fiber contraction leads to Tension production 60

10-4 Components of the Neuromuscular Junction The Control of Skeletal Muscle Activity The neuromuscular junction (NMJ) Special intercellular connection between the nervous system and skeletal muscle fiber Controls calcium ion release into the sarcoplasm A&P FLIX Events at the Neuromuscular Junction

Figure 10-11 Skeletal Muscle Innervation Motor neuron Path of electrical impulse (action potential) Axon Neuromuscular junction Synaptic terminal SEE BELOW Sarcoplasmic reticulum Motor end plate Myofibril Motor end plate 62

Figure 10-11 Skeletal Muscle Innervation The cytoplasm of the synaptic terminal contains vesicles filled with molecules of acetylcholine, or ACh. Acetylcholine is a neurotransmitter, a chemical released by a neuron to change the permeability or other properties of another cell’s plasma membrane. The synaptic cleft and the motor end plate contain molecules of the enzyme acetylcholinesterase (AChE), which breaks down ACh. Vesicles ACh The synaptic cleft, a narrow space, separates the synaptic terminal of the neuron from the opposing motor end plate. Junctional fold of motor end plate AChE 63

Figure 10-11 Skeletal Muscle Innervation The stimulus for ACh release is the arrival of an electrical impulse, or action potential, at the synaptic terminal. An action potential is a sudden change in the transmembrane potential that travels along the length of the axon. Arriving action potential 64

Figure 10-11 Skeletal Muscle Innervation When the action potential reaches the neuron’s synaptic terminal, permeability changes in the membrane trigger the exocytosis of ACh into the synaptic cleft. Exocytosis occurs as vesicles fuse with the neuron’s plasma membrane. Motor end plate 65

Figure 10-11 Skeletal Muscle Innervation ACh molecules diffuse across the synatpic cleft and bind to ACh receptors on the surface of the motor end plate. ACh binding alters the membrane’s permeability to sodium ions. Because the extracellular fluid contains a high concentration of sodium ions, and sodium ion concentration inside the cell is very low, sodium ions rush into the sarcoplasm. ACh receptor site 66

Figure 10-11 Skeletal Muscle Innervation The sudden inrush of sodium ions results in the generation of an action potential in the sarcolemma. AChE quickly breaks down the ACh on the motor end plate and in the synaptic cleft, thus inactivating the ACh receptor sites. Action potential AChE 67

10-4 Components of the Neuromuscular Junction Excitation–Contraction Coupling Action potential reaches a triad Releasing Ca2+ Triggering contraction Requires myosin heads to be in “cocked” position Loaded by ATP energy A&P FLIX Excitation-Contraction Coupling

Figure 10-10 The Exposure of Active Sites SARCOPLASMIC RETICULUM Calcium channels open Myosin tail (thick filament) Tropomyosin strand Troponin G-actin (thin filament) Active site Nebulin In a resting sarcomere, the tropomyosin strands cover the active sites on the thin filaments, preventing cross-bridge formation. When calcium ions enter the sarcomere, they bind to troponin, which rotates and swings the tropomyosin away from the active sites. Cross-bridge formation then occurs, and the contraction cycle begins. 69

10-4 Skeletal Muscle Contraction The Contraction Cycle Contraction Cycle Begins Active-Site Exposure Cross-Bridge Formation Myosin Head Pivoting Cross-Bridge Detachment Myosin Reactivation A&P FLIX The Cross Bridge Cycle

Figure 10-12 The Contraction Cycle Contraction Cycle Begins The contraction cycle, which involves a series of interrelated steps, begins with the arrival of calcium ions within the zone of overlap. Myosin head Troponin Tropomyosin Actin 71

Figure 10-12 The Contraction Cycle Active-Site Exposure Calcium ions bind to troponin, weakening the bond between actin and the troponin– tropomyosin complex. The troponin molecule then changes position, rolling the tropomyosin molecule away from the active sites on actin and allowing interaction with the energized myosin heads. Sarcoplasm Active site 72

Figure 10-12 The Contraction Cycle Cross-Bridge Formation Once the active sites are exposed, the energized myosin heads bind to them, forming cross-bridges. 73

Figure 10-12 The Contraction Cycle Myosin Head Pivoting After cross-bridge formation, the energy that was stored in the resting state is released as the myosin head pivots toward the M line. This action is called the power stroke; when it occurs, the bound ADP and phosphate group are released. 74

Figure 10-12 The Contraction Cycle Cross-Bridge Detachment When another ATP binds to the myosin head, the link between the myosin head and the active site on the actin molecule is broken. The active site is now exposed and able to form another cross-bridge. 75

Figure 10-12 The Contraction Cycle Myosin Reactivation Myosin reactivation occurs when the free myosin head splits ATP into ADP and P. The energy released is used to recock the myosin head. 76

Figure 10-12 The Contraction Cycle Resting Sarcomere Zone of overlap (shown in sequence above) 77

Figure 10-12 The Contraction Cycle Contracted Sarcomere 78

10-4 Skeletal Muscle Contraction Fiber Shortening As sarcomeres shorten, muscle pulls together, producing tension Muscle shortening can occur at both ends of the muscle, or at only one end of the muscle This depends on the way the muscle is attached at the ends

Figure 10-13 Shortening during a Contraction When both ends are free to move, the ends of a contracting muscle fiber move toward the center of the muscle fiber. When one end of a myofibril is fixed in position, and the other end free to move, the free end is pulled toward the fixed end. 80

10-4 Skeletal Muscle Relaxation Contraction Duration Depends on: Duration of neural stimulus Number of free calcium ions in sarcoplasm Availability of ATP

10-4 Skeletal Muscle Relaxation Ca2+ concentrations fall Ca2+ detaches from troponin Active sites are re-covered by tropomyosin Rigor Mortis A fixed muscular contraction after death Caused when: Ion pumps cease to function; ran out of ATP Calcium builds up in the sarcoplasm

10-4 Skeletal Muscle Contraction and Relaxation Summary Skeletal muscle fibers shorten as thin filaments slide between thick filaments Free Ca2+ in the sarcoplasm triggers contraction SR releases Ca2+ when a motor neuron stimulates the muscle fiber Contraction is an active process Relaxation and return to resting length are passive

Table 10-1 Steps Involved in Skeletal Muscle Contraction and Relaxation Steps in Initiating Muscle Contraction Steps in Muscle Relaxation Synaptic terminal Motor end plate T tubule Sarcolemma Action potential reaches T tubule ACh released, binding to receptors ACh broken down by AChE Sarcoplasmic reticulum releases Ca2 Sarcoplasmic reticulum recaptures Ca2 Ca2 Active site exposure, cross-bridge formation Actin Active sites covered, no cross-bridge interaction Myosin Contraction ends Contraction begins Relaxation occurs, passive return to resting length 84