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 sarcoplasm o 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 molecule o 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 sides o 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 knob 11 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 contract muscles that open the eyes Symptoms Double vision drooping of eyelids weakness of skeletal muscles esp in limbs Treatments o 1 Cholinesterase Inhibitors Antibodies sitting on receptor making very few available for ACh makes sure ACh stays around longer by inhibiting enzyme that breaks down ACh to make it more likely to access the few receptors that are available o 2 Immunosuppressive Agents Inhibit ability to produce antibodies Risk for infection with decreased