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The Muscular System

6. The Muscular System. The Muscular System. Muscles are responsible for all types of body movement Three basic muscle types are found in the body Skeletal, Cardiac, Smooth Skeletal and smooth muscle cells are elongated (muscle cell = muscle fiber)

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The Muscular System

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  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, Cardiac, Smooth Skeletal and smooth muscle cells are elongated (muscle cell = muscle fiber) Contraction and shortening of muscles is due to the movement of microfilaments

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

  4. Microscopic Anatomy of Skeletal Muscle Sarcolemma—specialized plasma membrane Myofibrils—long organelles inside muscle cell Sarcoplasmic reticulum—specialized smooth endoplasmic reticulum Sarcolemma Myofibril Nucleus Light(I) band Dark(A) band (a) Segment of a muscle fiber (cell) Figure 6.3a

  5. 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 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

  6. 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) Thin filaments = actin filaments Actin Troponin-tropomyosin complex

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

  8. Microscopic Anatomy of Skeletal Muscle Sarcoplasmic reticulum (SR) -stores and releases calcium T Tubules –transfer action potential to SR

  9. 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

  10. 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

  11. 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

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

  13. Figure 6.5

  14. 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

  15. 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)

  16. 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

  17. 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

  18. 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)

  19. 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

  20. 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

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

  22. Muscle Response to Strong Stimuli Muscle force depends upon the number of fibers stimulated More fibers contracting results in greater muscle tension Muscles can continue to contract unless they run out of energy

  23. Energy for Muscle Contraction Initially, muscles use stored ATP for energy ATP bonds are broken to release energy Only 4–6 seconds worth of ATP is stored by muscles After this initial time, other pathways must be utilized to produce ATP

  24. Figure 6.10a

  25. Energy for Muscle Contraction Aerobic respiration Glucose is broken down to carbon dioxide and water, releasing energy (about 32 ATP) A series of metabolic pathways occur in the mitochondria This is a slower reaction that requires continuous oxygen Carbon dioxide and water are produced

  26. Figure 6.10c

  27. Figure 6.10b

  28. Muscle Fatigue and Oxygen Deficit When a muscle is fatigued, it is unable to contract even with a stimulus Common cause for muscle fatigue is oxygen debt Increasing acidity (from lactic acid) and lack of ATP causes the muscle to contract less

  29. Types of Muscle Contractions Isotonic contractions Myofilaments are able to slide past each other during contractions The muscle shortens and movement occurs Example: bending the knee; rotating the arm Isometric contractions Tension in the muscles increases The muscle is unable to shorten or produce movement Example: push against a wall with bent elbows

  30. Effect of Exercise on Muscles Exercise increases muscle size, strength, and endurance Aerobic (endurance) exercise (biking, jogging) results in stronger, more flexible muscles with greater resistance to fatigue Makes body metabolism more efficient Improves digestion, coordination Resistance (isometric) exercise (weight lifting) increases muscle size and strength

  31. Figure 6.11a-b

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