The Muscular System Muscle Contractions

The Sarcolemma and Transverse Tubules Specific name of muscle fiber plasma membrane Has openings across the surface that lead into a network of transverse tubules, or T tubules T tubules allow for electrical stimuli to reach deep into each fiber The sarcoplasm Specific name for muscle fiber cytoplasm

Myofibrils in Muscle Fiber Hundreds to thousands in each fiber Are encircled by T tubules and are as long as the entire muscle fiber Are bundles of thick and thin myofilaments Actin molecules are found in thin filaments Myosin molecules are found in thick filaments Are the contractile proteins that shorten and are responsible for contraction

The Sarcoplasmic Reticulum Or SR Specialized smooth endoplasmic reticulum Expanded end that is next to the T tubule is the terminal cisternae Contain high concentrations of calcium ions Triad A combination of two terminal cisternae and one T tubule

Sarcomeres Smallest functional unit of skeletal muscle fiber Formed by repeating myofilament arrangements Each myofibril has about 10,000 sarcomeres Thick and thin filament arrangements are what produce the striated appearance of the fiber Overlapping filaments define lines and bands

Sarcomere Lines Z lines Thin filaments at both ends of the sarcomere Another protein connects the Z lines to the thick filament to maintain alignment M line Made of connections between the thick filaments

Sarcomere Bands A band I band Contains the thick filaments Contains the thin filaments, including the Z line .

Figure 7-2 The Organization of a Skeletal Muscle Fiber. Terminal cisterna Sarcoplasmic reticulum T tubules Triad Sarcolemma Mitochondria Thick filament Thin filament Myofilaments MYOFIBRIL The structure of a skeletal muscle fiber. SARCOMERE Z line Zone of overlap M line Myofibril I band H band Zone of overlap A band The organization of a sarcomere, part of a single myofibril. Z line M line Z line A stretched out sarcomere. Z line M line Z line and thin filaments Myosin head Thick filaments Active site Actin molecules Myosin tail ACTIN STRAND MYOSIN MOLECULE Hinge Troponin Tropomyosin The structure of a thick filament. Thin filament The structure of a thin filament.

Figure 7-2a The Organization of a Skeletal Muscle Fiber. Terminal cisterna Sarcoplasmic reticulum T tubules Triad Sarcolemma Mitochondria Thick filament Myofilaments Thin filament MYOFIBRIL The structure of a skeletal muscle fiber.

Figure 7-2b The Organization of a Skeletal Muscle Fiber. SARCOMERE Zone of overlap Z line M line Myofibril I band H band Zone of overlap A band The organization of a sarcomere, part of a single myofibril.

Figure 7-2c The Organization of a Skeletal Muscle Fiber. Z line M line Z line A stretched out sarcomere. M line Z line Z line and thin filaments Thick filaments

Figure 7-2d The Organization of a Skeletal Muscle Fiber. Active site Actin molecules ACTIN STRAND Troponin Tropomyosin Thin filament The structure of a thin filament.

Figure 7-2e The Organization of a Skeletal Muscle Fiber. Myosin head Myosin tail Hinge MYOSIN MOLECULE The structure of a thick filament.

Thin and Thick Filaments Actin A thin twisted protein, with specific active sites for myosin to bind to At rest, active sites are covered by strands of tropomyosin, held in position by troponin Myosin A thick filament with tail and globular head that attaches to actin active sites during contraction

Motor Unit A single motor neuron and all the muscle fibers it innervates Motor units are dispersed throughout the muscle Fine control movements Use motor units with very few fibers per neuron Gross movements Motor units have a high fiber-to-neuron ratio

Recruitment A mechanism for increasing tension to create more movement A graded addition of more and more motor units to produce adequate tension

Muscle Tone and Atrophy Some muscles at rest will still have a little tension Primary function is stabilization of joints and posture Atrophy Occurs in a muscle that is not regularly stimulated Muscle becomes small and weak Can be observed after a cast comes off a fracture