11/2/14 6:03 PM Characteristics of Muscle • Responsiveness (Excitability)—To chemical signals, stretch, and electrical changes across the plasma membrane • Conductivity—Local electrical charge triggers a wave of excitation that travels along the muscle fiber • Contractility—Shortens when stimulated • Extensibility—Capable of being stretched between contractions • Elasticity—Returns to its original resting length after being stretched Structure of Muscle Fiber • Epimysium: Connective tissue that covers entire muscle • Perimysium: Connective tissue around muscle fasciles o Fasciles: Surrounded by perimysium • Endomysium: Connective tissue around muscle cells o Continuous with collagen fibers of tendons and connective tissue of bone matrix o Collagen—Somewhat extensible and elastic (stretches slightly under tension and recoils when released)  Resists excessive stretching and protects muscle from injury  Returns muscle to its resting length  Contribute to power output and muscle efficiency • Muscle Fibers (Cells): Surrounded by endomysium • Myofibrils • Myofilaments: Thick and thin filaments Skeletal Muscle: Voluntary, striated muscle attached to one or more bones • Multinucleated • Striated: Alternating light and dark transverse bands as a result of overlapping of internal contractile proteins • Voluntary: Usually subject to conscious control Muscle Fiber (Muscle Cell) • Sarcolemma: Plasma membrane of muscle cell • Sarcoplasm: Cytoplasm of a muscle fiber • Myofibrils: Long protein bundles that occupy the main portion of the sarcoplasmo Glycogen: Storage form of glucose in abundance to provide energy with heightened exercise  Ready for action o Myoglobin: Red pigment that carries oxygen needed for muscle activity  Equivalent of hemoglobin in muscle cells • Multiple Nuclei—Flattened nuclei pressed against the inside of the sarcolemma o Myoblasts: Stem cells that fuse together to form each muscle fiber o Satellite Cells: Unspecialized myoblasts remaining between the muscle fiber and endomysium that are important for muscle repair  May multiply and produce new muscle fibers to some degree • Repair by fibrosis (scar formation) rather than regeneration of functional muscle • Mitochondria: Packed in spaces between myofibrils o Very active so need a lot of these • Sarcoplasmic Reticulum (SR): Smooth endoplasmic reticulum of muscle cells that forms a network around each myofibril **Calcium reservoir** o Calcium activates the muscle contraction process • Terminal Cisternae: Dilated end-sacs of SR which cross muscle fiber from one side to the other • Transverse Tubules (T-Tubules): Tubular infoldings of the sarcolemma which penetrate through the cell and emerge on the other side • Triad: 2 Terminal Cisternae + 1 T-Tubule Myofibril: Bundle of parallel myofilaments • Myosin: Thick filament basic molecule o Several myosin molecules bond together to form a thick filament  Looks like two golf clubs intertwined o Myosin Head: Sticks off of thick filament • Actin: Thin filament basic moleculeo Thin filaments consist of two intertwined strands of fibrous actin (F-actin) o String of globular actin (G-actin)  Think of two curly hairs wound together --> Each strand = Individual curl o Thin filaments associated with regulatory proteins—tropomyosin and troponin  Contractile Proteins: Do work  Tropomyosin: Covers the myosin binding sites on each actin bead  Troponin: Located on myosin binding site when muscle is relaxed  When calcium is released, it binds to troponin and causes a shift in the tropomyosin complex that causes it to roll away  Calcium is KEY—Makes binding sites free and no longer covered by tropomyosin complex  Not everywhere on tropomyosin, just a few spots **Think gate at front of mansion to regulate who comes in—Regulatory proteins guard actin and myosin to let them know when to contract • Titin (Connectin): Elastic filament that anchors myosin to Z disc o Think of springs/slinky—Only let muscles stretch a certain amount Sarcomere • Two Z discs at ends—Distance between them decreases when muscle contracts • Anisotropic Band (A-Band): Full length of myosin filament o Find thick, thin and elastic filament o Stays the same during muscle contraction • H-Band: Middle of A-Band o Find thick filament and elastic filament o Shortens during muscle contraction • M-Line: Complex proteins that bisect the H-Band to provide structural support • I-Band: Next to Z disc on both sideso Find thin filament and elastic filament o Shortens during muscle contraction Striations—Made by thick and thin filaments • When muscle contracts, only amount of OVERLAP changes in filaments • Sarcomere Shortening: Dystrophin and other linking proteins transfer pull to extracellular tissues o Duchenne Muscular Dystrophy: Linking protein missing  Mutation or absence of dystrophin The Nerve-Muscle Relationship • Denervation Atrophy: Cut nerve and get shrinkage of muscle because of failure to receive nerve signal • Somatic Motor Neurons: Neurons that innervate skeletal muscles • Motor Unit: One nerve fiber and all the muscle fibers innervated by it o Average motor unit—Ex: Breathing and most other movements o Small motor unit: Fine control—Ex: Picking up safety pin, moving eyes o Large motor unit—Ex: Gastrocnemius (when walking need more strength than control) **1 nerve can innervate multiple muscle fibers, but a muscle fiber can only be innervated by one nerve** Neuromuscular Junction (Synapse) • Junctional Folds: Acetylcholine receptors on muscle membrane • Synaptic Knob: Terminal swelling at the end of the nerve • Synaptic Cleft: Space between nerve and muscle o Pre-synaptic membrane (nerve) o Post-synaptic membrane (muscle) • Schwann Cell: Covers nerve • Exocytosis of acetylcholine occurs when nerve signal travels down nerve fibers and reaches synaptic knob11/2/14 6:03 PM The Neuromuscular Junction • 50 million ACh (acetylcholine) receptors • Junctional Folds: Increase the surface area for the acetylcholine receptors Myasthenia Gravis: Autoimmune disease of women ages 20-40 • Very few receptors available for ACh • Diagnosed by asking someone to gaze up—after 60 seconds one eyelid will start to droop, after 90 seconds that one eyelid will completely close o Unable to