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ANP1105 (101)
Chapter 9

ANP1105 Chapter 9: Muscles and Muscle Tissue notes

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University of Ottawa
Anatomy and Physiology
Jacqueline Carnegie

Muscle Types of Muscle Tissue: • Skeletal and smooth muscle are elongated and therefore called muscle fibers • “myo”, “mys” and “sarco” refer to muscle Skeletal Muscle: • Skeletal muscle: organs that attach to and cover the skeleton • Longest muscle cells • Striated • Voluntary muscle: subject to voluntary control • Responsible for body mobility • Can contract rapidly, but tires easily (requires rest after short periods of activity) • Powerful and adaptable Cardiac Muscle: • Only in the heart- cardiac • Striated • Involuntary • Pacemaker sets rate of contraction • Contracts at steady rate, but neural input can increase heart rate Smooth Muscle: • Found in the walls of hollow visceral organs, ex: stomach and bladder • Role: force fluids through internal channels • Elongated, but non-striated • Involuntary • Contractions are slow and sustained Muscle Functions: Movement: almost all bodily movement results from muscle contraction. Skeletal muscles are responsible for locomotion and manipulation. Cardiac muscle pumps blood through the body and smooth muscle found in the walls of blood vessels maintains blood pressure. Smooth muscles found in the organs also squeeze substances through the organ and along the tracts. Posture and Position: skeletal muscles maintain posture constantly working against gravity Joints: stabilize and strengthen joints. Heat: muscles generate heat as they contract. Heat is important in maintaining body temperature. Accounts for 40% of the body’s mass, so accounts for the most heat generation. Skeletal muscles also protect fragile internal organs by enclosing them. Smooth muscle forms valves to regulate passage of substances through the body, dilates and constricts the pupils, and forms arrector muscles attached to hair follicles. Functional Characteristics of Muscle: 1) Excitability (Irritability): ability to receive and respond to a stimulus; stimulus usually a chemical (NT, hormone, pH) response = AP along sarcolemma + muscle contraction 2) Contractility: ability to shorten forcibly when adequately stimulated 3) Extensibility: ability to be stretched or extended 4) Elasticity: ability to resume resting length after being stretched Anatomy of a Skeletal Muscle • Each skeletal muscle is an organ made of several kinds of tissues • Skeletal muscle fibers dominate, but blood vessels, nerves, and CT are also present • One nerve, one artery, and one or more veins serve each muscle • Every skeletal muscle fiber has a nerve ending that controls its activity • Rich blood supply: need large amounts of energy and continuous supply of oxygen, they also make large amounts of wastes Connective Tissue Sheaths: • CT’s wrap individual muscle fibers • Reinforce and hold together muscles during contraction • Contribute to natural elasticity of muscle tissue • Provide exit and entry routes for blood vessels and nerves Epimysium: overcoat of dense irregular CT that surrounds the whole muscle Perimysium and Fascicles: • Within each skeletal muscle, muscle fibers are grouped into fascicles (bundles) • Surrounding each fascicle is a layer of fibrous CT called perimysium. Endomysium: • Thin CT that surrounds each individual muscle fiber • Consists of areolar CT MicroscopicAnatomy of a Skeletal Muscle Fiber • Each skeletal muscle fiber: long cylindrical cell with multiple oval nuclei located in the sarcolemma (PM of muscle fiber) • Huge cells: diameter ranges from 10-100μm and up to 30cm long • Is a syncytium: hundreds of embryonic cells fuse to produce each fiber (explains large size and multiple nuclei) Sarcoplasm: • Cytoplasm of a muscle cell • Contains high amounts of glycosomes (stored glycogen that provides glucose during muscle cell activity) and myoglobin (red pigment that stores oxygen) Myofibrils: • Each muscle cell consists of parallel myofibrils • Densely packed • 80% of cell volume (100’s to 1000’s per cell) Striations: a repeating series of light and dark bands. Dark Abands (myosin and actin) and light I bands (actin) are perfectly aligned, giving it a striated appearance • Each DarkA band has a lighter section in the middle called the H zone o H zone: area with no thin filaments (relaxed muscle); only myosin • Each H zone has a vertical, dark M line formed by myomesin proteins o M line: connects adjacent thick filaments (desmin) • Each light I band has a darker area called the Z disc • The region between two Z discs is called the sarcomere o Smallest functional unit of the skeletal muscle o Contains an Aband flanked by an I band making skeletal muscle look striated Myofilaments: • Thick filaments, containing myosin, extend entire length ofAband • Thick filaments are connected in the middle by the M line • Thin filaments, containing actin, extend across the I band and partly into theAband • Z disc anchors the thin filaments and connects all myofibrils of a cell Molecular Structure of Myofilaments Thick Filaments: • 16nm in diameter • Primarily composed of myosin protein • Each myosin molecule = rod-like tail (2 heavy polypeptide chains) and 2 globular heads (end of heavy chains + 2 light chains) • Globular heads: o Business end of myosin o During contraction, link thick and thin filaments together forming crossbridges • Each thick filament contains 300 myosin molecules o Arrayed with their tails forming the central part (smooth) and heads facing outward at the end of each thick filament (studded) • The myosin heads contain actin,ATP-binding sites andATPase which splits ATP to generate energy for muscle contraction Thin Filaments: • 7-8nm in diameter • Primarily composed of actin protein • Has protein-shaped polypeptide subunits called G-actin (globular actin) o G-actin: active sites to which myosin heads attach during contraction • G-actin subunits are polymerized into long actin filaments called F-actin (filamentous actin) • 2 intertwined actin filaments form each thin filament • Also contain several regulatory proteins: o Tropomyosin:  Rod-like protein  Spiral around actin and help stiffen and stabilize it  Tropomyosin are arranged end to end along actin filaments  In relaxed muscle fiber, blocks myosin-binding sites on actin o Troponin:  Globular 3-polypeptide complex  TnI: inhibitory subunit that binds to actin  TnT: binds to tropomyosin and helps position actin  TnC: binds calcium ions • Troponin and tropomyosin help control the myosin-actin interactions involved in contraction Other Proteins that Form the Structure of Myofilaments: Elastic filament: • Composed of titin: extends from Z disc to the thick filament, running through the thick filament (forming its core) attaching to the M line. • Holds the thick filament in place maintaining the organization of theAband • Helps the muscle spring back to shape after stretching • Titin stiffens when it recoils, helping the muscle resist excessive stretching, which might pull the sarcomeres apart Dystrophin: • Protein which links the thin filaments to integral proteins of the sarcolemma (anchored to extracellular matrix) Sarcoplasmic Reticulum and T-tubules: Skeletal muscle fibers contain two sets of tubules that help regulate muscle contraction: (1) sarcoplasmic reticulum and (2) T tubules (1) Sarcoplasmic Reticulum: • Smooth endoplasmic reticulum • Site of Ca storage • Surround each myofibril • Most run longitudinally connecting at the H zone • Terminal cisterns form perpendicular cross channels at theAband-I band junctions; always occur in pairs • Regulates intracellular levels of calcium: stores Ca and releases it when muscle is stimulated to contract (2) T tubules: • At eachA band-I band junction, sarcolemma protrudes deep into the cell forming a tube called T tubule (bringsAP as far as possible into muscle fiber) • T tubule = Transverse tubule • Increase the muscle’s surface area • Runs between the two terminal cisterns, forming triads • Encircle each sarcomere • Electrical impulses are conducted to the deepest regions of the muscle cell and every sarcomere (electrical impulses run along sarcolemma which continues into T tubules) • Impulses signal for release of Ca from terminal cisterns • Ensures that each muscle fiber contracts simultaneously Triad: • Trio of 2 terminal cisternae and 1 T-tubule • T system: thousands of T-tubules (2/sarcomere) in a single muscle cell At the triad, where T tubules and SR are in close contact, integral proteins (from T tubules and SR) protrude into the intermembrane spaces which act as voltage sensors. Those from the SR form gated channels through which those of the terminal cisterns release Ca . 2+ Sliding Filament Mechanism • In relaxed muscle fiber, thin and thick filaments overlap only at ends of theAband • During contraction, thin filaments slide past thick filaments so that myosin and actin filaments overlap more • During contraction, Sarcomere shortens, but filaments remain same length • Relaxed: thin and thick filaments only slightly overlap • Contracted: thin filaments penetrate deeper intoAband and Z discs pull toward thick filaments Steps: I. Nerve impulse stimulates fiber (increases Ca ) 2+ II. Myosin heads on thick filaments latch onto myosin binding sites on actin of thin filaments and sliding begins III. Cross bridge attachments form (requires Ca ) and break many times which propel thin filaments toward the center of the sarcomere (cross bridges reach out and push actin) IV. Event occurs simultaneously and muscle cell shortens Overall: • Z discs are pulled toward M lines • I bands shorten • Distance between Z discs shortens • H zones disappear • Abands move closer but stay same length Details: • Intracellular Ca is low: myosin binding sites on actin is blocked by tropomyosin 2+ • Ca elevated: o Ca binds to troponin o Troponin alters shape and position of tropomyosin, moving it away from the binding site o Binding site is exposed • ATP allows for cross bridge detachment and when hydrolyzed, allows for myosin head to cock up • Single power stroke of all crossbridges = 1% shortening • About 30% muscle shortening occurs on average • Only half of the myosin heads are actively pulling at one time (to allow the other half to rest) 2+ • Relaxation occurs when sarcoplasmic reticulum reclaims the Ca • Rigormortis: muscle stiffens and does not relax because there is noATP for the cross bridge to detach o Muscles eventually go soft from degeneration Cross bridge cycle: Aseries of events during which the myosin heads pull thin filaments toward the center of the sarcomere I. Cross bridge formation: energized myosin head attaches to an actin myofilament, forming a cross bridge. II. Power stroke:ADP and inorganic Phosphorus are released and the myosin head pivots and bends to its low-energy state.As a result, it pulls the actin toward the M line III. Cross bridge detachment:AfterATP attaches to myosin, the link be
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