L16: Muscle Tissue Flashcards
Types of muscle. General characteristics of each
1.) Skeletal muscle (striated): voluntary movement, quick and forceful contraction 2.) Cardiac muscle (striated): involuntary movement of heart, rigorous and rhythmic contraction 3.) Smooth muscle (non-striated): involuntary movement, pupils, accommodation of lens, pilorection, peristalsis, blood vessel lumen diameter changes, contraction slow
Name the myofilaments of skeletal and cardiac muscle and their components
1.) Thin filaments: actin, tropomyosin and troponin 2.) Thick filaments: myosin (myosin II)
Structure of actin
- composed of globular actin (G-actin) monomers that assemble to form a polymer - 2 polymers become twisted to form a double-strand helix called a filamentous actin (F-actin)
Subunits of troponin. Function of each?
- TnT: attaches to tropomyosin, binding entire troponin complex - TnC: binds Calcium ions - TnI: binds troponin complex to actin molecules, inhibiting actin-myosin interactions
Structure of tropomyosin
- Consists of two polypeptide chains which form an alpha-helix - Each alpha-helix rests on 7 G-actin molecules and covers their active site for myosin - Tropomyosin alpha-helices line up in a head-to-tail arrangement to forms long tropomyosin filaments
Discuss composition of myosin
- 2 heavy chains - 4 light chains - Heavy meromyosin refers to degradation product of trypsin, which are the larger components of the 2 heavy chains (tails with heads) and the 4 light chains - Light meromyosin refers to the other degradation product, which are the smaller components of the 2 heavy chains (tails)
What is a sarcomere?
- Contractile unit of striated muscle
Sarcomere, myofibril, muscle fiber arrangement
- Overlapping thin and thick filaments form a sarcomere - Long cylindrical series end-to-end of sarcomeres form a myofibril - Many parallel myofibrils are enclosed within a muscle fiber (aka cell)
Picture of sarcomere with defined regions
Regions of sarcomere in relaxed state
1.) A-band = dark band of thick and thin filaments 2.) H-band = thick filaments only, bisects H-band 3.) I-band = light band, thin filaments only, made up on adjacent sarcomeres 4.) Z-line/disk = attachment for thin filaments, bisects I-band, alpha-actinin here 5.) M-line = protein structures lying between thick filaments, holding them in register, CK found here, bisects H-band
List accessory proteins of sarcomere. What is the function of each?
- Titin: anchors each thick filament to Z-disk - Alpha-actinin: anchors thin filament to Z-disk - Nebulin: wraps around each thin filament and anchors them to Z-disk - Myomesin: secures thick filaments in register at M-line - C protein: same function as myomesin - Dystrophin: cytoplasmic protein that binds thin filaments to laminin (external lamina component) outside cell
What is the sarcolemma?
- Muscle cell membrane with external and reticular lamina
What are sarcosomes?
- Mitochondria of muscle cells
Describe CT coverings found in muscle
1.) Endomysium: reticular fibers covering each muscle fiber 2.) Perimysium: thin collagenous CT that cover fascicle (bundle of many fibers) 3.) Epimysium (aka deep fascia): dense irregular collagenous CT enclosing gross muscle – contains neurovascular bundle – sends septa within the muscle so that vessels and nerve fibers may reach deeper aspects
Skeletal muscle specific features
- Multinucleated (syncytium), striated - T-tubules: invaginations of sarcolemma that function to carry nerve impulses deep into cell - Sarcoplasmic reticulum: sER that form sleeve around each myofibril - Terminal cisternae: feature of SR that store calcium in their lumen - Triad = 1 T-tubule + 2 terminal cisternae, found at A-I junction of sarcomere - Satellite cells: regenerative cells that lie bw muscle cell and external lamina
Discuss innervation of skeletal muscle
- Motor end plate (NMJ or myoneuronal junction): one per muscle fiber, consisting of axon terminal containing synaptic vesicles with ACh at pre-synaptic membrane, synaptic cleft (space) and postsynaptic membrane, which is muscle cell sarcolemma
Define motor unit?
- Lower motor neuron and all muscle fibers it innervates
What is myasthenia gravis?
- Autoimmune disease causing muscle weakness - Individuals produce Abs that bind to ACh receptor protein on sarcolemma, which block ACh from binding, preventing muscle contraction, causing weakness.
What is muscular dystrophy? DMD?
- Genetic degenerative disease affecting striated muscle - DMD is most common and most severe, affects primarily skeletal muscle - Dystrophin that is found in cytoplasm binds thin filaments to external lamina is absent - Disease characterized by weakness and atrophy of shoulder, hip, pelvic and thigh muscles. Can eventually affect heart and breathing muscles.
Cardiac muscle specific features/characteristics
- Weakly striated - Cells are short and branched - Glycogen and lipid droplets in cytoplasm, FAs are major fuel of heart - Lipofuscin = aging pigment, degradation product - 1-2 nuclei per cell - Atrial granules (ANP and BNP), most numerous in right atrium-diuretic hormones that lower BP - Mitochondria make up 40% of cytoplasmic volume - Intercalated discs: intercellular junctions that transmit contractile forces from cell to cell and also permit flow of ions from cell to cell, so that muscle contracts as syncytium - Dyad at Z-disk: 1 T-tubule + 1 SR - Purkinje fibers: specialized cells in endocardium, relay electrical fibers. With SA node, AV node, bundle of His, these make up impulse-generating and conducting sytem
Structure of intercalated disc of cardiac muscle
- Transverse portion: fascia adherens and desmosomes – perpendicular to long axis. Fascia adherens=anchor actin filaments of terminal sarcomere to plasma membrane, transmits contractile forces from cell-to-cell. Desmosomes: binds cells together. - Lateral portion: gap junctions – parallel to longitudinal axis. Permit flow of ions from cell to cell so that muscle contracts as syncytium.
What is the troponin test? Clinical significance
- Troponin I or T test = blood test ordered for chest pain pts - Specific for cardiac muscle tissue damage
Smooth muscle specific features/characteristics
- Sheets - No striations (no sarcomeres), still do contain thick and thin (except troponin) filaments - Mononuclear - Gap junctions - Myofilament bundles criss-cross and form dense bodies at intersections, which attach to internal surface of cell membrane - Dense bodies contain alpha-actinin - No T-tubules - SR not well developed - Contraction by sliding filament mechanism - Caveolae = pinocytotic vesicles/bubbles on inner surface of membrane containing calcium, released during contraction
Provide brief overview of steps of striated muscle contraction
- Impulse transmitted down axon, reaches pre-synaptic membrane - Calcium enters - Synaptic vesicles containing ACh attach to inner surface and are exocytosed - ACh binds to ACh receptors on post-synaptic membrane (sarcolemma) - Na enters, K exits, depolarization - Depolarization spreads, into T-tubules - T-tubules carry impulses into cell, trigger change in permeability at SR - Calsequestrin stores calcium in SR, liberates calcium into cytoplasm - Calcium attach to TnC, changing conformation of troponin - TnT pushes tropomyosin into spiral groove of actin, exposing actins active sites - Myosin head associates with actin - With ATP, myosin head pulls actin filament over myosin filament - Note, lengths of filaments remain same, overlapping increases
Note changes to regions on sarcomeres as a result of contraction
- A band remains same size - H band almost disappears - I band becomes smaller - Z-discs come closer to midline
What is rigor mortis?
- Muscle stiffness after death, result from actin-myosin complexes remaining in bound form as no ATP is available
