Biol 2401 Biol 2401 Fundamentals of Anatomy and Physiology Mrs. Willie Grant wgrant4@alamo.edu (210) 643-8968

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. 10-5 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. 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.

An Introduction to Muscle Tissue A primary tissue type, divided into: Skeletal muscle tissue Cardiac muscle tissue Smooth muscle tissue Skeletal Muscles Are attached to the skeletal system Allow us to move

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 Skeletal Muscle Organization 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 Exterior collagen layer Connected to deep fascia Separates muscle from surrounding tiss Perimysium Separates muscle from surrounding tissue Endomysium Surrounds individual muscle cells (muscle fibers) Contains capillaries and nerve fibers contacting muscle cells Contains myosatellite cells (stem cells) that repair damage

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 tendon (bundle) or aponeurosis (sheet) 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)

Figure 10-2 The Formation of a Multinucleate Skeletal Muscle Fiber Muscle fibers develop through the fusion of mesodermal cells called myoblasts. Cells are long Cells develop from myoblasts Cells become very large Cells contain hundreds of nuclei Myoblasts A muscle fiber forms by the fusion of myoblasts. Muscle fiber LM  612 Nuclei Sarcolemma Myofibrils Myosatellite cell Nuclei Mitochondria Immature muscle fiber 1 Which structure shown releases calcium ions to trigger muscle contraction? A diagrammatic view and a micrograph of one muscle fiber. Myosatellite cell Up to 30 cm in length Mature muscle fiber 9

10-3 Characteristics of Skeletal Muscle Fibers The Sarcolemma and Transverse Tubules The sarcolemma—cell membrane of a muscle fiber Surrounds the sarcoplasm (cytoplasm of muscle fiber) A change in transmembrane potential begins contractions 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 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 13

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 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 3 Which proteins are present in the A band? In the I band?

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 2 Which of the following is the smallest: muscle fiber, thick filament, myofibril? Which is the largest? Surrounded by: Endomysium Endomysium Contains: Myofibrils 17

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 Tropomyosin Is a double strand Prevents actin–myosin interaction Troponin A globular protein Binds tropomyosin to G-actin Controlled by Ca2+

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 Myosin Action During contraction, myosin heads: Interact with actin filaments, forming cross-bridges Pivot, producing motion 21

10-3 Structural Components of a Sarcomere Sliding Filaments and Muscle Contraction Sliding Filament Theory 1 Thin filaments of sarcomere slide toward M line, alongside thick filaments 2 The width of A zone stays the same 3 Z lines move closer together

4 What happens to the I band and the H zone as muscles contract 4 What happens to the I band and the H zone as muscles contract? Do the lengths of the thick and thin filaments change?

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 (the “pull” when a muscle fiber contracts) production

Figure 10-9 An Overview of Skeletal Muscle Contraction Neural control Contraction occurs when skeletal muscle Fibers are activated by neurons. Excitation–contraction coupling Calcium ions are released from SR. Excitation Calcium release Calcium ions trigger interaction between thick/thin filaments. triggers ATP Thick-thin filament interaction Muscle fiber contraction Mulscle fiber contracts. leads to Tension is produced. Tension production 25

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 single axon may branch to control more than one skeletal muscle fiber, but each muscle fiber has only one neuromuscular junction (NMJ). At the NMJ, the synaptic terminal of the neuronlies near the motor end plate of the muscle fiber. 5 What part of the sarcolemma contains acetylcholine receptors?

10-4 Components of the Neuromuscular Junction Excitation–Contraction Coupling (The link between the generation of an action potential in the sarcolemma and the start of a muscle contraction) (1) Action potential reaches a triad (pair of terminal cisternae and one T tubule) (2) Releasing Ca2+ (3) Triggering contraction Requires myosin heads to be in “cocked” position Loaded by ATP energy

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

Figure 10-12 The Contraction Cycle Resting Sarcomere Zone of overlap (shown in sequence above) 32

Figure 10-12 The Contraction Cycle Contracted Sarcomere 33

10-4 Skeletal Muscle Contraction Fiber Shortening As sarcomeres shorten, muscle pulls together, producing tension (the “pull” when a muscle fiber contractts) 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. 35

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