1. MUSCULAR SYSTEM Structure and Function

2. Skeletal Muscle Properties 1. Excitability = ability to receive and respond to a stimulus  Also called irritability or responsiveness 2. Contractility = ability to shorten when stimulated 3. Extensibility = ability to be stretched 4. Elasticity = ability to recoil and resume resting length

3.  muscle cell = muscle fiber  A “muscle” is composed of many muscle fibers arranged in bundles called fascicles  Fascia = connective tissue that separates muscles; also forms tendons Organization of Skeletal Muscle

4.  Three layers of connective tissue: epimysium perimysium endomysium Muscle fiber

5. Organization of Skeletal Muscle  Three layers of connective tissue: 1. Epimysium outermost layer; surrounds the entire muscle 2. Perimysium surrounds a bundle of muscle fibers (fascicle) 3. Endomysium surrounds individual muscle fibers  All three layers of connective tissue come together to form a tendon which attaches the muscle to bone

6. epimysium perimysium endomysium Organization of Skeletal Muscle

7.  Hierarchy of organization: Muscle  fascicle  fiber  myofibrils  myofilaments (actin/myosin)

8. Skeletal Muscle Anatomy  Sarcolemma = plasma membrane of muscle cell  Sarcoplasm = cytoplasm of muscle cell  Sarcoplasmic reticulum = stores calcium for contraction  Transverse tubules = network of narrow tubes that form passageways through a muscle fiber  Myofibrils = protein filaments (myofilaments)  Actin and myosin

9. Skeletal Muscle Anatomy sarcolemma mitochondria sarcoplasm myofibril nucleus

10. Skeletal Muscle Anatomy  Sarcomere = functional unit of skeletal muscle  There may be thousands of sarcomeres in one fiber aligned end to end

11. Myofilaments  Form dark and light bands which gives skeletal muscle a striped appearance  Interaction between these myofilaments within each sarcomere causes the sarcomere to shorten during contraction

12. Actin (thin filament)  The most abundant protein in eukaryotic cells  Forms the light band of the sarcomere  Each actin molecule has an active site capable of interacting with myosin  At rest, active sites are blocked

13. Myosin (thick filament)  Motor protein  Forms the dark band of the sarcomere  Each myosin molecule has a globular head and a long fibrous tail  Myosin head interacts with actin active site during contraction

14. LABEL THIS DIAGRAM IN YOUR NOTES!!!

16. Sarcomere Structure  DArk band = A band = Myosin (thick)  LIght band = I band = actin (thin)

17. Sarcomere structure  Z line = marks the boundaries of each sarcomere  M line = middle of the sarcomere

18. Sarcomere structure  H zone = region of sarcomere where thick filaments do not overlap with thin filaments  Zone of overlap = region of sarcomere where thick filaments overlap with thin filaments

19. Sarcomere Structure sarcomere Z line Thick filament Thin filament I band A band M line H zone Animation

20. Microscopes…Skeletal Muscle  In your lab notebook, create a new Lab titled “Skeletal Muscle”  Part I. Microscopic observations  Draw a picture of skeletal muscle under HIGH POWER  Identify and label multiple nuclei, muscle fibers, banding patterns

22. Label…A, I, M, Z, H animation

23.  Changes to the sarcomere  I band gets smaller  Z lines move closer together  H zone decrease  Zones of overlap get larger  Width of A bands doesn’t change Label…A, I, M, Z, H animation

24. Sliding Filament Theory (model)  The theory of how muscle contracts  Contraction occurs as the thin filament “slides” past the thick filament  Involves 5 different molecules and calcium  Myosin  Actin  Tropomyosin  Troponin  ATP

25. Role of Nervous System in Muscle Contraction  Skeletal muscles contract only under nervous system control  Neuromuscular junction = association of nerve fiber with muscle fiber  Acetylcholine = the neurotransmitter that acts on skeletal muscle by triggering calcium release  Calcium is the “key” to allowing actin and myosin to interact during contraction

29. Before contraction begins…  Muscle at rest  Troponin is bound to actin filaments  Tropomyosin binds to the troponin Blocks active site preventing contraction from occuring  Myosin heads (cross bridges) are “primed” for contraction  Signal from nervous system (impulse) triggers the release of calcium

30. animation

31. What do you know about a muscle at rest?

32. The contraction cycle 1. Calcium binds to troponin, shifting tropomysoin, exposing actin active site

33. The contraction cycle 2. Myosin head binds to actin active site (cross-bridge forms)

34. The contraction cycle 3. Attached myosin head pivots toward center of sarcomere  This step requires energy that was stored in myosin head prior to contraction

35. The contraction cycle 4. Cross-bridges detach; myosin binds another ATP

36. The contraction cycle 5. Myosin is reactivated. The entire cycle can now be repeated

38. The contraction cycle  Ends when calcium ion concentration returns to normal Accomplished by active transport of calcium into sarcoplasmic reticulum Contraction animation

39. Sketches in your lab book 1. Skeletal muscle  If you do not, get a microscope and do this first! 2. Actin Filament  Label actin molecules, troponin, tropomyosin and an active site 3. Myosin Filament  Should contain multiple myosin molecules  Label head and tail 4. Sarcomere (at rest vs. contracted)  Without detail of individual filaments  Label A band, I band, M line, Z line, H zone  Describe changes to these structures during contraction

40. Sketches

42. The Contraction Cycle BIG PICTURE  Skeletal muscle fibers shorten as thin filaments interact with thick filaments and sliding occurs  The trigger for contraction is calcium  Calcium ions are released when the muscle fiber is stimulated by an impulse from a motor neuron  Contraction is an active process  Relaxation and return to normal are passive processes

43. Rigor Mortis  Upon death, circulation stops and skeletal muscles are deprived of nutrients  Muscle fibers run out of ATP  Calcium ions cannot be cleared  Sustained contraction occurs  All body muscles are involved  Individual becomes “stiff”