Chs. 7-11, 19, 22-24

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Biological Sciences
Catherine Nash

FINAL EXAM REVIEW Lecture 17 Skeletal Muscles o attached to bones of the skeleton by tendons (collagen) o positions and moves the skeleton o 40% total body weight o contracts in response to a signal from a somatic motor neuron o cannot initiate own contraction o contraction not influenced by hormones Smooth Muscle o primary muscle of internal organs (except heart) o influence the movement of materials around and in and out of the body o multiple levels of control o primary extrinsic control arises from autonomic nervous system (ANS) o some can contract spontaneously i.e without signals from CNS o modulated by endocrine system Cardiac Muscle o only found in the heart o moves blood through circulatory system o multiple levels of control o primary extrinsic control arises from autonomic nervous system (ANS) o some can contract spontaneously i.e without signals from CNS Skeletal Muscle o Origin: - point of muscle bone attachment closest to the trunk. - usually more stationary. o Insertion: - The point of muscle bone attachment more distal (distant) from the trunk. - usually more mobile Flexor and Extensor muscles o Flexor muscles: brings bones closer together e.g bicep o Extensor muscles: moves bones away from each other e.g tricep o Most joints have pairs of muscles: 1 flexor, 1 extensor o Antagonistic muscle groups: - flexor-extensor pairs exert opposite effects - when one contracts, the other relaxes Skeletal muscles are composed of muscle fibres o Muscle cells are the largest cells in body o Groups of muscle cells function together as a unit o Long, cylindrical with up to several 100 nuclei each muscle fibre (or cell) o Fibres arranged in parallel o Each muscle fibre is covered in connective tissue o Groups of adjacent fibres are contained in fasicles o Collagen, elastic fibres, nerves and blood vessels are found between the fasicles o Entire muscle is enclosed in connective tissue and tendons Components of muscle fibres o Muscle fibre: comprised of myofibrils, which contain thick and thin filaments - the contractile machinery of the muscle o Sarcolemma = Cell membrane o Transverse tubules (T-tubules) = Invaginations of the sarcolemma o Sarcoplasm = Cytoplasm (contains mitochondria and glycogen for muscle contraction) Sarcoplasmic reticulum (SR) = Modified endoplasmic reticulum - Wrapped around myofibrils - releases Ca during muscle contraction o Mitochondria : provides ATP for muscle contraction T Tubules allow conduction of the Action Potential Transverse tubules (T-tubules): invaginations of the sarcolemma, closely associated with terminal cisternae to form a triad. - membrane of t tubules is continuous with muscle fibre - lumen of t-tubules are continuous with extracellular fluid - allow depolarisations of sarcolemma to quickly reach interior of a muscle fibre. - transport Action Potentials from NMJ to interior of fibre 2+ Sarcoplasmic reticulum (SR): releases Ca ions from the terminal cisternae - terminal cisternae gathers and stores Ca 2+ Myofibrils are the contractile units of muscle fibres - highly organised bundles of contractile and elastic proteins that carry out work for contraction - one muscle fibre contains a 1000 or more myofibrils Components of myofibrils Sarcomere: fundamental contractile unit of myofibril that repeats throughout the length of the myofibril Sarcomeres contain: Myosin (thick filaments) Actin (thin filaments) Two Z lines per sarcomere Actin (thin) and Myosin (thick) filaments are sandwiched between Z lines Z lines: Zig-zag protein structures Attachment site (anchor) for thin (actin) filaments M line: proteins that form attachment sites for thick (myosin) filaments - equivalent to Z disk for thin filaments A Band: Darkest band Encompasses entire length of thick filament H zone: Central region of A band lighter Only contains thick filaments I band: brightest in colour on fibre Contains only thin filaments Z disk runs through middle of I band Occupies space between A bands of adjacent sarcomeres Structure of Myofibrils: thick filaments o Myosin heads are clustered at end of filament o The hinge region allows the heads to swivel at point of attachment o Myosin is the motor protein of the myofibril o Composed of protein chains that intertwine o Form a long tail and pair of tadpole-like heads 250 myosin molecules join to form a thick filament Structure of Myofibrils: thin filaments o Single actin molecules = g actin globular protein o Polymerise to form f actin (fibrous). 2 polymers twist creating thin filament Tropomyosin and troponin: start and stop contraction Tropomyosin: long fibrous molecule - extends over many actin molecules - blocks myosin binding when muscle at rest Troponin complex: actin, tropomyosin, Ca2+
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