L16: Muscle Tissue

Question 1

Types of muscle. General characteristics of each

Answer 1

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

Question 2

Name the myofilaments of skeletal and cardiac muscle and their components

Answer 2

1.) Thin filaments: actin, tropomyosin and troponin 2.) Thick filaments: myosin (myosin II)

Question 3

Structure of actin

Answer 3

• 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)

Question 4

Subunits of troponin. Function of each?

Answer 4

• TnT: attaches to tropomyosin, binding entire troponin complex - TnC: binds Calcium ions - TnI: binds troponin complex to actin molecules, inhibiting actin-myosin interactions

Question 5

Structure of tropomyosin

Answer 5

• 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

Question 6

Discuss composition of myosin

Answer 6

• 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)

Question 7

What is a sarcomere?

Answer 7

• Contractile unit of striated muscle

Question 8

Sarcomere, myofibril, muscle fiber arrangement

Answer 8

• 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)

Question 9

Picture of sarcomere with defined regions

Answer 9

Question 10

Regions of sarcomere in relaxed state

Answer 10

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

Question 11

List accessory proteins of sarcomere. What is the function of each?

Answer 11

• 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

Question 12

What is the sarcolemma?

Answer 12

• Muscle cell membrane with external and reticular lamina

Question 13

What are sarcosomes?

Answer 13

• Mitochondria of muscle cells

Question 14

Describe CT coverings found in muscle

Answer 14

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

Question 15

Skeletal muscle specific features

Answer 15

• 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

Question 16

Discuss innervation of skeletal muscle

Answer 16

• 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

Question 17

Define motor unit?

Answer 17

• Lower motor neuron and all muscle fibers it innervates

Question 18

What is myasthenia gravis?

Answer 18

• 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.

Question 19

What is muscular dystrophy? DMD?

Answer 19

• 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.

Question 20

Cardiac muscle specific features/characteristics

Answer 20

• 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

Question 21

Structure of intercalated disc of cardiac muscle

Answer 21

• 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.

Question 22

What is the troponin test? Clinical significance

Answer 22

• Troponin I or T test = blood test ordered for chest pain pts - Specific for cardiac muscle tissue damage

Question 23

Smooth muscle specific features/characteristics

Answer 23

• 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

Question 24

Provide brief overview of steps of striated muscle contraction

Answer 24

• 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

Question 25

Note changes to regions on sarcomeres as a result of contraction

Answer 25

• A band remains same size - H band almost disappears - I band becomes smaller - Z-discs come closer to midline

Question 26

What is rigor mortis?

Answer 26

• Muscle stiffness after death, result from actin-myosin complexes remaining in bound form as no ATP is available