Muscles

1. Muscles Chapter 9

2. Types of Muscle Tissue • Derived from mesoderm • Skeletal • Skeletal, striated, voluntary, multinucleated • Fast, fatigued contractions • Adaptable (paperclip vs textbook) • Smooth • Visceral, nonstriated, involuntary, uninucleated • Slow, sustained contractions • Cardiac • Cardiac, striated, involuntary, uninucleated • Contraction rate stabilized by pacemaker cells • Neural control can alter • Intercalated discs

3. Functional Characteristics • Excitability • Receive and respond to a stimulus (pH or NT) • Electrical impulse along the sarcolemma • Contractibility • Shorten and thicken w/ appropriate stimulation • Extensibility • Stretch or extend without damage • Elasticity • Return to normal shape after a stretch

4. Muscle Function • Produce movement • Sk: locomotion, manipulation, and response • Sm: squeeze substances through • Car: keep blood moving • Maintain posture and position • Adjustments to stay erect or seated despite gravity • Protection • Encloses viscera and forms valves (control) • Generate heat • Contractions keep body temp at 98.6 • Stabilize joints • Pull on bones for movement, but strengthen joints

5. Gross Anatomy of Skeletal Muscle • Discrete organs of all 4 tissue types • Nerves and blood • 1 nerve, 1 artery, & 1+ vein per muscle • Enter centrally; 1 nerve ending per muscle fiber (cell) • Constant O2 and nutrients b/c contractions are high E demand • Connective tissue • Support and reinforce • 3 layers (internal to external) • Endomysium: cover muscle fiber • Perimysium: cover fasicle • Epimysium: cover muscle • Attachments • Direct: epimysium fused to periosteum • Indirect: epimysium beyond muscle = tendon

6. Microscopic Anatomy • Sarcolemma • Sacroplasm • Glycogen, myoglobin, and mitochondria • Myofibrils • Actin (thin) and myosin (thick) proteins arranged into repeating sarcomeres • Sarcoplasmic reticulum (SR) • Smooth ER surrounding myofibrils • Triads • Terminal cisternae • SR is enlarged and joins with T tubules; occur in pairs • Transverse (T) tubules • Deep indentions of sarcolemma into sarcoplasm; conduct Ca2+ into cell

7. Sarcomere Organization • Smallest functional unit of skeletal muscle fiber • A bands dark b/c contain thick and thin filaments • H zone is lighter middle because it lacks thin filaments • M line created by a protein that link thick filaments • I bands light b/c contain thin filaments only • Z line connect thin filaments together in a zigzag pattern • Marks end of sarcomere • Zone of overlap • 6 thin surround 1 thick; 3 thick surround 1 thin

8. Myofilament Structure • Thick filaments • Bundles of myosin proteins • Composed of a rod-like tail and globular head • Head forms cross bridges; attach to site on actin; contain ATPases • Thin filaments • Twisted strands of F actin, composed of G actin • G actin contains ‘active sites’ where myosin can attach • Tropomyosin forms stiffening chains that cover ‘active sites’ • Troponin holds the tropomyosin in place • Changes shape to expose active sites • G-actin = pearl, F-actin = strand, tropomysin = strands together

9. Sliding Filament Theory • During contraction, sarcomeres (not filaments) shorten • Z lines closer, shortening sarcomere • H band and I band narrow • Zone of overlap increases • A band doesn’t change

10. Sliding Mechanism • Cross bridges detached • Tropomyosin blocks ‘active sites’ • Active site exposed  cross bridge attach • Ca2+ binds troponin  shape change • Myosin head pivots toward M line  thin filaments to center • Cross bridges detach and mysoin reactivated • ATP binds  ATPase resets

11. Neuromuscular Junction (NMJ) • Innervation of muscle fiber by an axon terminal • 1 NMJ per muscle fiber • Motor unit: motor neuron and all muscle fibers innervated • Fewer fibers = more precise • Number determines strength of muscle • Separated by a synaptic cleft • Axon terminal houses synaptic vesicles filled with acetylcholine (Ach) • Impulse opens Ca2+ channels to release • Motor end plate is depression in the sarcolemma for the axon • Contains Ach receptors • Propogates an action potential (AP) http://www.colorado.edu/intphys/Class/IPHY3430-200/image/figure7m.jpg

12. Introduction to Action Potentials • Resting membrane is polarized (charge separation) • NT binds = opens gated ion channels (Na + and K +) • Depolarize cell (less ‘–’ or more ‘+’) locally • Spreads throughout plasma membrane in waves • Initiates AP • Adjacent Na + open  more depolarization to threshold • Na + close, K + open = repolarization • Refractory period because no stimuli can initiate • Resets electrical condition to resting state • Na + /K + pump restore ionic condition • All or none response, b/c unstoppable once started

13. Excitation – Contraction Coupling • Stimuli releases ACh, depolarizes end plate • AP propagated down T tubules • Termianlcisternae of SR release Ca 2+ • Electrical signal raise Ca2+ levels by opening Ca2+ channels • Ca2+ binds troponin, removing tropomyosin block • Contraction occurs (see earlier) • Ca2+ levels decrease, tropomyosin replaced = relaxation • ATP dependent Ca2+ pump into SR • Repeat with stimulation

14. Skeletal Muscle Contractions • Muscle Tone • Alternating active motor units while muscle at rest • No active movements produced • Stabilize joints and maintain posture • Ensure response ready • Isotonic • Tension constant as muscle length changes • Isometric • Tension increases to peak, but muscle length unchanged • Moving a load greater than developable tension • Concentric : force w/shortening • Eccentric : force w/lengthening (gastrocnemius & hills)

15. Muscle Twitch • Response to a single stimulation • Quick contract, relax cycle in 3 phases • Latent period • Excitation – coupling is occurring • Muscle tension increases, but no contraction • Contraction period • Cross bridges active • Peak tension, muscle shortens • Relaxation period • Reentry Ca2+ into SR • Muscle tension to zero, resting • Varies between muscle types • Strength depends on # of motor units, recruitment

16. Graded Muscle Responses • Contraction varies depending on circumstance • Wave summation (time) • 2 stimuli in rapid succession = larger contraction 2nd time • Refractory period unaltered • Tetanus (speed) • Sustained contraction w/ or w/o partial relaxation • Unfused • Fused • Treppe • Increase tension with repeated contractions • Warming up  stronger later to same stimulus

17. Muscle Metabolism

18. Muscle Disorders • Myasthenia gravis: autoimmune disease, loss of Ach receptors • Rigor mortis: ATP depletion prevents cross bridge detachment • Atrophy: degeneration of muscle from disuse • Duchenne muscular dystrophy: sex-linked disease that destroys muscle • Hernia: organ protrudes through abdominal wall • Myalgia: muscle pain • Fibromyositis: inflammation of a muscle and CT coverings • Strain: excessive stretching and tearing of muscle or tendon