Understanding Skeletal Muscle: Anatomy, Organization, and Classification

1. Exercise 14 Microscopic Anatomy, Organization, and Classification of Skeletal Muscle

2. SKELETAL MUSCLE • Voluntary or involuntary? • Striated or not? • Multinucleate or uninucleate?

3. Terminology • “Myo-” or “-Mys-” = muscle • “Sarco-” = flesh

4. Microscopic Anatomy • Muscle Fiber = long, slender, cylindrical, MULTInucleate cell • Nuclei located just under sarcolemma (cell membrane) Fig. 10-2

5. Muscle Fiber Anatomy • Sarcolemma • Plasma membrane of muscle fiber • Trans-membrane potential: charge differences on either side • Sarcoplasm • Cytoplasm of muscle fiber • Stored glycogen, mitochondria, myoglobin, actin, myosin

6. Fig. 10-3

7. Transverse tubules • T tubules • Narrow tubes, continuous with sarcolemma • Extend into sarcoplasm, fluid-filled • Myofibrils • Parallel, extend length of muscle fiber • Each muscle fiber contains 100s-1000s • Composed of myofilaments • Actin (thin filaments) & myosin (thick filaments)

8. Fig. 10-3

9. Sarcoplasmic Reticulum (SR) • Smooth ER • Tubular network around each myofibril • Terminal cisternae: enlarged portion of SR on either side of T tubule • Storage/release of Ca • Triad = T tubule + 2 terminal cisternae Fig. 10-3

10. Muscle contraction begins when Calcium ions are released • from the terminal cisternae • Into the sarcoplasm • ADAM cd-rom

11. Sarcomeres • Organized group of myofilaments • Contractile units, smallest functional unit of muscle fiber • Z-line to Z-line Fig. 10-4

12. Sarcomeres • A band: DARK • Thick and thin filaments • M line is in the middle of the sarcomere/A band • I band: LIGHT • midline is Z line/disc • Thin filaments only Fig. 10-4

13. Myofilaments • Thick filaments: myosin • Rod-like tail bound to other myosin • Head projects outward • has binding sites for actin (thin filaments), ATP • “cross-bridges” Contraction: The myosin head changes shape & will grab onto the actin…and pull it toward the M-line Fig. 10-7

14. Myofilaments • Thin filaments: 3 types of protein molecules • actin • Double helix (2 strands) coil around each other • has binding sites for myosin • Site is blocked by troponin-tropomyosin complex when muscle’s NOT contracting • Tropomyosin • Rod-like, coils around actin to stiffen it • Covers binding sites • Troponin • 3 protein-complex • One of each binds to: actin, tropomyosin, calcium ions (this one’s empty in a resting cell…see ADAM cd)

15. Fig. 10-7

16. Connective Tissue Wrappings:3 Layers • Epimysium = outer • Perimysium = central • Endomysium = inner

17. Connective Tissue Wrappings • Epimysium • Epi = on (outside layer) • Surrounds entire muscle • Dense layer of collagen fibers, connected to deep fascia • Separates muscle from other tissues/organs Fig. 10-1

18. Connective Tissue Wrappings • Perimysium • Peri = around (central layer) • Divides muscle into compartments: • Fascicle = bundle of muscle fibers (cells) • Collagen, elastic fibers • Blood vessels, nerves to each fascicle Fig. 10-1

19. Connective Tissue Wrappings • Endomysium • Endo = inside (inner layer) • Surrounds individual skeletal muscle cells (fibers) • Interconnects adjacent muscle fibers • Capillary networks to supply blood to cells, nerve fibers to control muscle Fig. 10-1

20. Connective Tissues, cont. • Layers are continuous & interwoven – blend into one another • Muscle’s ends: layers unite to form • Bundle: TENDON • Attach skeletal muscles to bones • Contraction pulls the bone(s) • Broad sheet: APONEUROSIS • Attach skeletal muscles to bones or other muscles

21. Connective tissue layers! • Muscle—covered by epimysium • Fascicles—covered by perimysium • Fiber (cell)—covered by endomysium • Myofibril—covered by sarcoplasmic reticulum • Sarcomere—contains thick and thin filaments

22. Fig. 10-6

23. Blood & Nerve Supply • Muscles need lots of energy, lots of oxygen • Blood vessels supply these • Each fiber (cell) has capillary blood supply • Contraction stimulated by nerve impulses • Axons (nerve fibers) penetrate connective tissue layers, & innervate individual muscle fibers (cells)

24. How Muscles Contract: ADAM cd-rom

25. Neuromuscular Junction • Junction of motor neuron & a muscle fiber • Motor neuron sends a signal to cause a movement of a skeletal muscle • Neuron—elongated portion = axon • Ends in expanded branches (synaptic terminals) • synaptic cleft: space between synaptic terminal & muscle fiber’s sarcolemma

26. Neuromuscular Junction • Synaptic terminals • Contain synaptic vesicles • Filled with acetylcholine (ACh) • Neurotransmitter: chemical released by neuron • Release of ACh triggers muscle contraction • Sarcolemma of motor end plate has receptors to bind ACh

27. Neuromuscular Junction “Motor end plate” Sarcolemma highly folded • Increases surface area • Sites of ACh binding “Action Potential” Fig. 10-10

28. Neuromuscular Junction • Motor unit = A single motor neuron & ALL the muscle fibers it controls Fig. 10-17

29. Fig. 10-10

30. Fig. 10-10