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An Introduction to Muscle Tissue • 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 1. 2. 3. 4. 5. 6. 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 1. Epimysium 2. Perimysium 3. Endomysium 10-2 Organization of Muscle • Epimysium – Exterior collagen layer – Connected to deep fascia – Separates muscle from surrounding tissues Figure 10-1 The Organization of Skeletal Muscles Skeletal Muscle (organ) Epimysium Perimysium Endomysium Nerve Muscle fascicle Muscle Blood fibers vessels Epimysium Blood vessels and nerves Tendon Endomysium Perimysium 10-2 Organization of Muscle • Perimysium – Surrounds muscle fiber bundles (fascicles) – Contains blood vessel and nerve supply to fascicles Figure 10-1 The Organization of Skeletal Muscles Skeletal Muscle (organ) Epimysium Perimysium Endomysium Nerve Muscle fascicle Muscle Blood fibers vessels Epimysium Blood vessels and nerves Tendon Endomysium Perimysium 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 Blood fibers vessels Epimysium Blood vessels and nerves Tendon Endomysium Perimysium 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 Figure 10-1 The Organization of Skeletal Muscles Muscle Fiber (cell) Capillary Myofibril Endomysium Sarcoplasm Epimysium Mitochondrion Blood vessels and nerves Tendon Myosatellite cell Sarcolemma Nucleus Axon of neuron Endomysium Perimysium 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. LM 612 Muscle fiber Sarcolemma Nuclei Myofibrils Myosatellite cell Nuclei Mitochondria Immature muscle fiber Myosatellite cell A diagrammatic view and a micrograph of one muscle fiber. Up to 30 cm in length Mature muscle fiber 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 Figure 10-2b The Formation of a Multinucleate Skeletal Muscle Fiber LM 612 Muscle fiber Sarcolemma Nuclei Myofibrils Mitochondria A diagrammatic view and a micrograph of one muscle fiber. 10-3 Characteristics of Skeletal Muscle Fibers • The Sarcolemma and Transverse Tubules – The sarcolemma • The cell membrane of a muscle fiber (cell) • Under the endomysium • Surrounds the sarcoplasm (cytoplasm of muscle fiber) • A change in transmembrane potential begins contractions 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 T tubules reticulum 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 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 T tubules reticulum 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 Figure 10-3 The Structure of a Skeletal Muscle Fiber Mitochondria Terminal cisterna Sarcolemma Sarcolemma Sarcoplasm Myofibril Myofibrils Thin filament Thick filament Triad Sarcoplasmic T tubules reticulum 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 Figure 10-3 The Structure of a Skeletal Muscle Fiber Terminal cisterna Sarcolemma Sarcoplasm Myofibrils Triad Sarcoplasmic T tubules reticulum 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 Terminal cisterna Sarcolemma Sarcoplasm Myofibrils Triad Sarcoplasmic T tubules reticulum Figure 10-1 The Organization of Skeletal Muscles Skeletal Muscle (organ) Epimysium Perimysium Endomysium Nerve Muscle fascicle Muscle Blood fibers vessels Epimysium Blood vessels and nerves Tendon Endomysium Perimysium 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 Figure 10-1 The Organization of Skeletal Muscles Muscle Fiber (cell) Capillary Myofibril Endomysium Sarcoplasm Epimysium Mitochondrion Blood vessels and nerves Tendon Myosatellite cell Sarcolemma Nucleus Axon of neuron Endomysium Perimysium 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 T tubules reticulum Figure 10-3 The Structure of a Skeletal Muscle Fiber Myofibril Nuclei Sarcolemma Sarcoplasm MUSCLE FIBER Figure 10-3 The Structure of a Skeletal Muscle Fiber Mitochondria Terminal cisterna Sarcolemma Sarcolemma Sarcoplasm Myofibril Myofibrils Thin filament Thick filament Triad Sarcoplasmic T tubules reticulum Figure 10-3 The Structure of a Skeletal Muscle Fiber Mitochondria Sarcolemma Myofibril Thin filament Thick filament Figure 10-3 The Structure of a Skeletal Muscle Fiber Terminal cisterna Sarcolemma Sarcoplasm Myofibrils Triad Sarcoplasmic T tubules reticulum 1. 2. 3. 4. 5. 6. 7. Perimysium Muscle Epimysium Muscle fiber Endomysium Muscle fascicle Tendon Figure 10-3 The Structure of a Skeletal Muscle Fiber 1. Myofibrils 2. Sarcoplasmic reticulum 3. Myofibril 4. T-tubules 5. Sarcomere 6. Triad 7. Sarcolemma 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) 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 Sarcomere Thin Thick filament filament 10-3 Structural Components of a Sarcomere • Sarcomeres – The A Band • M line – At midline of sarcomere • The H Band – Has thick filaments but no thin filaments • Zone of overlap – The densest, darkest area on a light micrograph – Where thick and thin filaments overlap 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 Sarcomere Thin Thick filament filament 10-3 Structural Components of a Sarcomere • Sarcomeres – The I Band • Z lines – At two ends of sarcomere • Titin – 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 Sarcomere Thin Thick filament filament Figure 10-4b Sarcomere Structure, Part I I band A band H band A corresponding view of a sarcomere in a myofibril from a muscle fiber in the Myofibril gastrocnemius Z line muscle of the calf Z line TEM 64,000 Zone of overlap M line Sarcomere Figure 10-5 Sarcomere Structure, Part II Sarcomere Myofibril A superficial view of a sarcomere Thin filament Actinin filaments Thick filament Titin filament Attachment of titin Z line Cross-sectional views of different portions of a sarcomere I band M line H band Zone of overlap Figure 10-6 Levels of Functional Organization in a Skeletal Muscle Skeletal Muscle Myofibril Surrounded by: Sarcoplasmic reticulum Surrounded by: Epimysium 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 Perimysium Thin filaments Contains: Muscle fibers Z line M line H band Muscle Fiber Endomysium Surrounded by: Endomysium Contains: Myofibrils Titin Z line Figure 10-6 Levels of Functional Organization in a Skeletal Muscle Skeletal Muscle Surrounded by: Epimysium Epimysium Contains: Muscle fascicles Figure 10-6 Levels of Functional Organization in a Skeletal Muscle Muscle Fascicle Perimysium Surrounded by: Perimysium Contains: Muscle fibers Figure 10-6 Levels of Functional Organization in a Skeletal Muscle Muscle Fiber Endomysium Surrounded by: Endomysium Contains: Myofibrils Figure 10-6 Levels of Functional Organization in a Skeletal Muscle Myofibril Surrounded by: Sarcoplasmic reticulum Consists of: Sarcomeres (Z line to Z line) 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 H band Titin Z line 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 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 10-3 Structural Components of a Sarcomere • Thin Filament – Nebulin • The center of the thin filament – G-actin • The active sites on actin strands bind to myosin 10-3 Structural Components of a Sarcomere • Thin Filaments – Tropomyosin • Prevents actin–myosin interaction – Troponin • 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 Z line M line 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 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 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 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 10-3 Structural Components of a Sarcomere • Initiating Contraction – Ca2+ binds to receptor on troponin molecule – Troponin pulls tropomyosin away from f-actin – 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 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 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 Z line A band H band Z line A relaxed sarcomere showing location of the A band, Z lines, and I band. 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. 10-3 Structural Components of a Sarcomere • Skeletal Muscle Contraction – The process of contraction • Neural stimulation of sarcolemma • Muscle fiber contraction • Tension production Figure 10-9 An Overview of Skeletal Muscle Contraction Neural control Excitation–contraction coupling Excitation Calcium release triggers Thick-thin filament interaction Muscle fiber contraction leads to Tension production ATP Figure 10-9 An Overview of Skeletal Muscle Contraction Neural control Figure 10-9 An Overview of Skeletal Muscle Contraction Excitation Calcium release triggers Thick-thin filament interaction ATP Figure 10-9 An Overview of Skeletal Muscle Contraction Muscle fiber contraction leads to Tension production 10-4 Components of the Neuromuscular Junction • The Control of Skeletal Muscle Activity – The neuromuscular junction (NMJ) • Between nervous system and skeletal muscle fiber • Stimulates release of calcium ions into sarcoplasm 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 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 The synaptic cleft, a narrow space, separates the synaptic terminal of the neuron from the opposing motor end plate. Junctional AChE fold of motor end plate ACh 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 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 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 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 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 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. 10-4 Skeletal Muscle Contraction • The Contraction Cycle 1. 2. 3. 4. 5. 6. Contraction Cycle Begins Active-Site Exposure Cross-Bridge Formation Myosin Head Pivoting Cross-Bridge Detachment Myosin Reactivation 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 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 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. 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. 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. 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.
