Muscles and Muscle Tissue

1. Muscles and Muscle Tissue Karen Marshall, Associate Professor Montgomery College Takoma Park Campus

2. Muscle Tissues • highly cellular • well-vascularized • three types • differ in structure, body location, function, means of activation (contraction) • skeletal • cardiac • smooth

3. Similarities (Skeletal & Smooth Muscle) • 1) cells • elongated • called muscle fibers • 2) muscle contraction • dependent on two kinds of myofilaments • actin • myosin • 3) muscle terms • myo, mys, sarco • sarcolemma ~ PM of muscle • sarcoplasm ~ muscle fiber cytoplasm

4. Skeletal Muscle Tissue • voluntary, striated • packed by connective tissue sheets into skeletal muscles • fibers are longest • stripes (striations) • location • attached to and covers bony skeleton • controlled voluntarily

5. Skeletal Muscle Tissue • contracts rapidly • tires easily • must rest after short periods of activity • function • responsible for overall body mobility • *see Table 9.4 for comparison of muscle types

6. Cardiac Muscle Tissue • striated, involuntary • location • found only in heart • constitutes the bulk of the heart wall • controlled involuntarily • neural control allows a shift into high gear for short periods

7. Cardiac Muscle Tissue • cells • uninucleate • branch & fit tightly together at junctions (intercalated discs) • contracts at a steady rate • pacemaker of heart (SA node)

8. Smooth Muscle Tissue • nonstriated, involuntary • location • walls of hollow visceral organs • stomach, urinary bladder, respiratory passages • function • force fluids & other substances through internal body channels • controlled involuntarily • contractions are slow & sustained

9. Functions of Muscle • 1) produce movement • 2) maintain posture • 3) stabilize joints • 4) generate heat • during contraction

10. Functional Characteristics of Muscle • 1) excitability • irritability • ability to receive & respond to a stimulus • stimulus • environmental change that arises inside or outside the body • usually a chemical • NT, hormone, change in pH • response • generation of electrical impulse • passes along the sarcolemma contraction of muscle cells

11. Functional Characteristics of Muscle • 2) contractility • ability of a muscle to shorten forcibly when adequately stimulated • 3) extensibility • ability of a muscle to be stretched or extended • even beyond its resting length when relaxed • 4) elasticity • ability of a muscle fiber to recoil & resume its resting length • after being stretched

12. Gross Anatomy of Skeletal Muscle • each skeletal muscle is a discrete organ • consists of several kinds of tissues • muscle fibers predominate • blood vessels • nerve fibers • connective tissue

13. Connective Tissue Wrappings • muscle fibers are wrapped & held together by several different layers • three layers • external to internal • epimysium • perimysium & fascicles • endomysium

14. Connective Tissue Wrappings • epimysium • overcoat • dense irregular connective tissue • surrounds the entire muscle

15. Connective Tissue Wrappings • perimysium & fascicles • muscle fibers are grouped into bundles of sticks • fascicle • each fascicle surrounded by a layer of fibrous connective tissue • perimysium

16. Connective Tissue Wrappings • endomysium • surrounds each muscle fiber w/in a fascicle • consists of connective tissue & mostly reticular fibers

17. Connective Tissue Wrappings • all connective sheaths continuous • one another • tendons (join muscles to bones) • w/ contraction • fibers pull on the sheath movement

18. Connective Tissue Wrappings • functions • support each cell • reinforce muscle as a whole • contribute to elasticity of tissue

19. Connective Tissue Wrappings (fig 9.1)

20. Nerve & Blood Supply • each muscle • one nerve • an artery • one or more veins • enter or exit near the central part of the muscle • branch throughout the connective tissue • provide for normal muscle activity

21. Attachments • direct (fleshy) • epimysium of muscle is fused to • periosteum of a bone • connective tissue covering • perichondrium of a cartilage • fibrous connective tissue covering

22. Attachments • indirect • muscle’s connective tissue wrapping extend beyond muscle • ropelike tendon • connects muscle to bone • sheetlike aponeurosis • connects muscle to other muscles or to bones • tendon or aponeurosis • anchors the muscle to connective tissue covering • bone or cartilage or fascia of other muscles

23. Attachments • indirect • much more common • durable & small • tendons are mostly tough collagenic fibers • cross bony projections • more tendons can pass over a joint • conserve space

24. Microscopic Anatomy of a Skeletal Muscle Fiber • muscle fiber • long, cylindrical cell • multiple oval nuclei • ~multinucleate • arranged just beneath sarcolemma (PM) • functions as a syncytium

25. Microscopic Anatomy of a Skeletal Muscle Fiber • myofibril • rodlike bundle of contractile filaments • ~hundreds to thousands in a single muscle fiber • make up 80% of cell volume

26. Microscopic Anatomy of a Skeletal Muscle Fiber • striations • repeating series of dark A bands and light I bands • nearly perfectly aligned • give cells its striped (striated) appearance • located along length of each myofibril

27. Microscopic Anatomy of a Skeletal Muscle Fiber • sarcomere • smallest contractile unit of a muscle fiber • consists of thick (myosin) filaments & thin (actin) filaments • arranged in a regular array • region of myofibril between two successive Z discs (lines)

28. Microscopic Anatomy of a Skeletal Muscle Fiber • Z disc ~ Z line • darker area of midline interruption in the I band • coin-shaped sheet • composed of proteins • connectins • anchors the thin filaments • connects each myofibril to the next throughout the width of muscle cell

29. Microscopic Anatomy of a Skeletal Muscle Fiber • thick filaments • composed primarily of protein myosin • each molecule has a rodlike tail terminates in two globular heads • head • AKA crossbridges • serve as actin binding sites • contain ATP binding sites & ATPase enzymes that split ATP to generate energy for contraction • link the thick & thin myofilaments together during contraction

30. Microscopic Anatomy of a Skeletal Muscle Fiber • thick filaments • myosin molecules are bundled together • tails form central part of the filament • heads face outward & in opposite directions at each end

31. Microscopic Anatomy of a Skeletal Muscle Fiber • thin filaments • composed chiefly of actin • polypeptide subunits of actin • AKA globular actin or G actin • contain active sites to which myosin crossbridges attach during contraction

32. Microscopic Anatomy of a Skeletal Muscle Fiber • thin filaments • also contains regulatory proteins • help to control myosin-actin interactions involved in contraction • two strands of tropomyosin • troponin

33. Tropomyosin • rod-shaped protein • spirals about actin core • helps stiffen it • successive molecules arranged end to end along actin filaments • relaxed muscle • block actin’s active sites • so myosin heads cannot bind to thin filaments

34. Troponin • three polypeptide complex • one inhibitory subunit • binds to actin • one positioning subunit • binds to tropomyosin • helps position it onto actin • one subunit binds to Ca

38. Myosin Molecule (fig 9.3a)

39. Actin Molecule (fig 9.4c)

40. Actin & Myosin Filaments (fig 9.3d)

41. Regulation of Contraction • assisted by two sets of intracellular tubules • located in skeletal muscle fibers • sarcoplasmic reticulum • SR • transverse tubules • T tubules

42. Sarcoplasmic Reticulum (SR) • specialized ER of muscle cells • interconnecting tubules • surround each myofibril • most run longitudinally along the myofibril • others form larger, perpendicular cross channels at A-I junctions • terminal cisternae (end sacs)

43. Sarcoplasmic Reticulum (SR) • function • regulate IC levels of ionic Ca • store Ca and release it on demand • when muscle fiber is stimulated to contract • Ca provides the final “go” signal for contraction

44. Transverse Tubules (T tubules) • elongated tube • invaginations of sarcolemma • run between terminal cisternae of SR formation of triads • successive groupings of three membranous structures • terminal cisternae • T tubule • terminal cisternae

45. Transverse Tubules (T tubules) • function • conduct impulses to the deepest regions of the muscle cell & to every sarcomere • signal for release of Ca from adjacent terminal cisternae • act as a “rapid telegraphy system” • ensures that every myofibril in muscle fiber contracts at the same time

46. SR & T tubules (fig 9.4)

47. Contraction • physiological definition • activation of myosin’s crossbridges • force generating sites

48. Contraction • shortening occurs • force or tension generated by cross bridges on the thin filaments > forces opposing shortening • shortening ends • cross bridges become inactive & tension generated declines • relaxation of muscle fiber

49. Tropomyosin • rod-shaped protein • spirals about actin core • helps stiffen it • successive molecules arranged end to end along actin filaments • relaxed muscle • block actin’s active sites • so myosin heads cannot bind to thin filaments

50. Troponin • three polypeptide complex • one inhibitory subunit • binds to actin • one positioning subunit • binds to tropomyosin • helps position it onto actin • one subunit binds to Ca