The Muscular System

1. 6 The Muscular System

2. The Muscular System Muscles are responsible for all types of body movement Three basic muscle types are found in the body Skeletal muscle Cardiac muscle Smooth muscle

3. Characteristics of Muscles Skeletal and smooth muscle cells are elongated (muscle cell = muscle fiber) Contraction and shortening of muscles is due to the movement of microfilaments All muscles share some terminology Prefixes myo and mys refer to “muscle” Prefix sarco refers to “flesh”

4. Table 6.1

5. Comparison of Skeletal, Cardiac, and Smooth Muscles

6. Comparison of Skeletal, Cardiac, and Smooth Muscles

7. Skeletal Muscle Characteristics Most are attached by tendons to bones Cells are multinucleate Striated—have visible banding Voluntary—subject to conscious control

8. Connective Tissue Wrappings of Skeletal Muscle Cells are surrounded and bundled by connective tissue Endomysium—encloses a single muscle fiber Perimysium—wraps around a fascicle (bundle) of muscle fibers Epimysium—covers the entire skeletal muscle Fascia—on the outside of the epimysium

9. Musclefiber(cell) Blood vessel Perimysium Epimysium (wraps entire muscle) Fascicle(wrapped byperimysium) Endomysium(betweenfibers) Tendon Bone Figure 6.1

10. Skeletal Muscle Attachments Epimysium blends into a connective tissue attachment Tendons—cord-like structures Mostly collagen fibers Often cross a joint due to toughness and small size Aponeuroses—sheet-like structures Attach muscles indirectly to bones, cartilages, or connective tissue coverings

11. Skeletal Muscle Attachments Sites of muscle attachment Bones Cartilages Connective tissue coverings

12. Smooth Muscle Characteristics Lacks striations Spindle-shaped cells Single nucleus Involuntary—no conscious control Found mainly in the walls of hollow organs

13. Circular layer of smooth muscle (longitudinal view of cells) Mucosa Longitudinal layerof smooth muscle(cross-sectionalview of cells) Submucosa (a) Figure 6.2a

14. Cardiac Muscle Characteristics Striations Usually has a single nucleus Branching cells Joined to another muscle cell at an intercalated disc Involuntary Found only in the walls of the heart

15. Cardiacmusclebundles (b) Figure 6.2b

16. Skeletal Muscle Functions Produce movement Maintain posture Stabilize joints Generate heat

17. Microscopic Anatomy of Skeletal Muscle Sarcolemma—specialized plasma membrane Myofibrils—long organelles inside muscle cell Sarcoplasmic reticulum—specialized smooth endoplasmic reticulum

18. Sarcolemma Myofibril Nucleus Light(I) band Dark(A) band (a) Segment of a muscle fiber (cell) Figure 6.3a

19. Microscopic Anatomy of Skeletal Muscle Myofibrils are aligned to give distinct bands I band = light band Contains only thin filaments A band = dark band Contains the entire length of the thick filaments

20. Z disc H zone Z disc Thin (actin) filament Thick (myosin) filament (b) Myofibril or fibril (complex organelle composed of bundles of myofilaments) I band M line I band A band Figure 6.3b

21. Microscopic Anatomy of Skeletal Muscle Sarcomere—contractile unit of a muscle fiber Organization of the sarcomere Myofilaments Thick filaments = myosin filaments Thin filaments = actin filaments

22. Microscopic Anatomy of Skeletal Muscle Thick filaments = myosin filaments Composed of the protein myosin Has ATPase enzymes Myosin filaments have heads (extensions, or cross bridges) Myosin and actin overlap somewhat Thin filaments = actin filaments Composed of the protein actin Anchored to the Z disc

23. Sarcomere M line Z disc Z disc Thin (actin) filament Thick (myosin) filament (c) Sarcomere (segment of a myofibril) Figure 6.3c

24. Microscopic Anatomy of Skeletal Muscle At rest, within the A band there is a zone that lacks actin filaments Called either the H zone or bare zone Sarcoplasmic reticulum (SR) Stores and releases calcium Surrounds the myofibril

25. Thick filament Bare zone Thin filament (d) Myofilament structure (within one sarcomere) Figure 6.3d

26. Stimulation and Contraction of Single Skeletal Muscle Cells Excitability (also called responsiveness or irritability)—ability to receive and respond to a stimulus Contractility—ability to shorten when an adequate stimulus is received Extensibility—ability of muscle cells to be stretched Elasticity—ability to recoil and resume resting length after stretching

27. The Nerve Stimulus and Action Potential Skeletal muscles must be stimulated by a motor neuron (nerve cell) to contract Motor unit—one motor neuron and all the skeletal muscle cells stimulated by that neuron

28. Axon terminals at neuromuscular junctions Spinal cord Motor unit 2 Motor unit 1 Nerve Axon ofmotorneuron Motor neuroncell bodies Muscle Muscle fibers (a) Figure 6.4a

29. Axon terminals at neuromuscular junctions Muscle fibers Branching axonto motor unit (b) Figure 6.4b

30. Neuromuscular junction Association site of axon terminal of the motor neuron and muscle The Nerve Stimulus and Action Potential PLAY A&P Flix™: Events at the Neuromuscular Junction

31. Figure 6.5

32. The Nerve Stimulus and Action Potential Synaptic cleft Gap between nerve and muscle Nerve and muscle do not make contact Area between nerve and muscle is filled with interstitial fluid Action potential reaches the axon terminal of the motor neuron Calcium channels open and calcium ions enter the axon terminal

33. Transmission of Nerve Impulse to Muscle Calcium ion entry causes some synaptic vesicles to release their contents (acetylcholine, a neurotransmitter) by exocytosis Neurotransmitter—chemical released by nerve upon arrival of nerve impulse in the axon terminal The neurotransmitter for skeletal muscle is acetylcholine (ACh)

34. Transmission of Nerve Impulse to Muscle Acetylcholine attaches to receptors on the sarcolemma of the muscle cell In response to the binding of ACh to a receptor, the sarcolemma becomes permeable to sodium (Na+) Sodium rushes into the cell generating an action potential and potassium leaves the cell Once started, muscle contraction cannot be stopped

35. Synaptic vesicle containing ACh Action potential reaches axonterminal of motor neuron. 1 Axon terminal of motor neuron Mitochondrion Ca2+ Ca2+ Synapticcleft Sarcolemma Fusing synapticvesicle Sarcoplasmof muscle fiber ACh Folds ofsarcolemma AChreceptor Figure 6.5, step 1

36. Synaptic vesicle containing ACh Action potential reaches axonterminal of motor neuron. 1 Axon terminal of motor neuron Mitochondrion Ca2+ 2 Calcium (Ca2+) channelsopen and Ca2+ enters the axon terminal. Ca2+ Synapticcleft Sarcolemma Fusing synapticvesicle Sarcoplasmof muscle fiber ACh Folds ofsarcolemma AChreceptor Figure 6.5, step 2

37. Synaptic vesicle containing ACh Action potential reaches axonterminal of motor neuron. 1 Axon terminal of motor neuron Mitochondrion Ca2+ 2 Calcium (Ca2+) channelsopen and Ca2+ enters the axon terminal. Ca2+ Synapticcleft Sarcolemma Fusing synapticvesicle Sarcoplasmof muscle fiber 3 Ca2+ entry causes somesynaptic vesicles to release theircontents (acetylcholine, a neurotransmitter) by exocytosis. ACh Folds ofsarcolemma AChreceptor Figure 6.5, step 3

38. Synaptic vesicle containing ACh Action potential reaches axonterminal of motor neuron. 1 Axon terminal of motor neuron Mitochondrion Ca2+ 2 Calcium (Ca2+) channelsopen and Ca2+ enters the axon terminal. Ca2+ Synapticcleft Sarcolemma Fusing synapticvesicle Sarcoplasmof muscle fiber 3 Ca2+ entry causes somesynaptic vesicles to release theircontents (acetylcholine, a neurotransmitter) by exocytosis. ACh Folds ofsarcolemma AChreceptor 4 Acetylcholine diffuses acrossthe synaptic cleft and binds toreceptors in the sarcolemma. Figure 6.5, step 4

39. Ion channel insarcolemma opens;ions pass. K+ Na+ 5 ACh binds and channels open that allow simultaneous passage of Na+ into the muscle fiber and K+ out of the muscle fiber. More Na+ ions enter than K+ ions leave and this produces a local change in the electrical conditions of the membrane (depolarization), which eventually leads to an action potential. Figure 6.5, step 5

40. ACh Degraded ACh Ion channel closed;ions cannot pass. Na+ ACh effects are ended by itsbreakdown in the synaptic cleft bythe enzyme acetylcholinesterase. 6 Acetylcholinesterase K+ Figure 6.5, step 6

41. Neuromuscular junction Muscle cellor fiber Nervefiber Small twig Striations Matchflame 1 Na+ diffusesinto the cell. 1 2 2 Flame ignites the twig. Action potential spreadsrapidly along the sarcolemma. Flame spreads rapidly along the twig. (a) (b) Figure 6.6a-b

42. The Sliding Filament Theory of Muscle Contraction Activation by nerve causes myosin heads (cross bridges) to attach to binding sites on the thin filament Myosin heads then bind to the next site of the thin filament and pull them toward the center of the sarcomere This continued action causes a sliding of the myosin along the actin The result is that the muscle is shortened (contracted)

43. Actin Myosin Z Z H A I I (a) Z Z I A I (b) Figure 6.7a–b

44. Protein complex In a relaxed muscle cell, the regulatory proteins formingpart of the actin myofilaments prevent myosin binding(see a). When an action potential (AP) sweeps along itssarcolemma and a muscle cell is excited, calcium ions(Ca2+) are released from intracellular storage areas (thesacs of the sarcoplasmic reticulum). Myosinmyofilament Actinmyofilament (a) Figure 6.8a

45. Myosin-binding site The flood of calcium acts as the final trigger forcontraction, because as calcium binds to the regulatoryproteins on the actin filaments, the proteins undergo a change in both their shape and their position on the thinfilaments. This action exposes myosin-binding sites onthe actin, to which the myosin heads can attach (see b),and the myosin heads immediately begin seeking out binding sites. Ca2+ Upper part of thick filament only (b) Figure 6.8b

46. PLAY A&P Flix™: The Cross Bridge Cycle Figure 6.8c

47. Contraction of Skeletal Muscle Muscle fiber contraction is “all or none” Within a skeletal muscle, not all fibers may be stimulated during the same interval Different combinations of muscle fiber contractions may give differing responses Graded responses—different degrees of skeletal muscle shortening

48. Contraction of Skeletal Muscle Graded responses can be produced by changing: The frequency of muscle stimulation The number of muscle cells being stimulated at one time

49. Types of Graded Responses Twitch Single, brief contraction Not a normal muscle function

50. Figure 6.9a