Ch. 7: Muscular System

1. Ch. 7: Muscular System

2. Introduction • Most abundant tissue in the body • Always working • Movement • Locomotion • Breathing • Sitting/standing upright • Beating heart

3. Muscular System Functions • Body movement • Via muscle contraction; create overall body movements • Posture maintenance • Constant skeletal muscle tone • Respiration • Thoracic muscles • Body heat production • Heat released as byproduct of skeletal muscles • Communication • Oral, written, body language • Organ & vessel contraction • Move/mix food & water in GI tract, excrete secretions, regulate blood flow • Heart beat • Cardiac muscle contraction; propels blood to body

4. Types of Muscle • Skeletal muscle • Cardiac muscle • Smooth muscle

5. Comparison of Muscle Types

6. Characteristics of Skeletal Muscles • ~40% BW • Attached to skeleton • Contractility • Muscles can shorten with force causing movement of structures to which they’re attached; lengthen passively (gravity or force of opposing muscle) • Excitability • Muscles respond to stimuli (usually nerves cause contraction of skeletal muscle) • Extensibility • Skeletal muscle can be stretched to resting length and a little beyond • Elasticity • Ability of muscles to recoil to original resting length after stretching

7. Structure of Skeletal Muscle • Hierarchical organization: • Each muscle (organ) surrounded by epimysium (upon + muscle) /fascia (fillet) – connective tissue • Fasciculus (bundle) – bundles of muscle cells/fibers; surrounded by perimysium (around + muscle) – loose connective tissue • Several muscle fibers per fasciculus; surrounded by endomyosium (within + muscle) – loose connective tissue

8. Structure of Skeletal Muscle Cells

9. Structure of Skeletal Muscle Cells • Microscopic structure • Each fiber composed of myofibrils • Myofibrils composed of myofilaments (protein filaments) arranged in sarcomeres (flesh + part) • Actin myofilaments (thin) • Myosin myofilaments (thick) • Each muscle fiber has: • Sarcolemma (cell membrane) • Sarcoplasm (cytoplasm) • Sarcroplasmic reticulum (ER) - ↑[Ca2+] – muscle contractions • Transverse tubules (T tubules) – network of tubes connecting sarcolemma to sarcoplasmic reticulum

10. Structure of Skeletal Muscle Cells • Banding Pattern (striations) (See 7.3 a) • I-band (I-LIGHT): thin myofilaments only (actin) • A-band (A-DARK): thick & thin myofilaments (myosin & actin) • Z-disk: dark line in middle of I-band; site of actin attachment • H-zone: lighter, central region in middle of A-band; thick myofilaments only (myosin) • M-line: dark line in middle of H-zone; site of myosin attachment • Sarcomere: functional contractile unit of muscle cell; runs from A-disk to Z-disk

11. Membrane Potentials • Outside of most cell membranes is positively charged relative to inside • A resting membrane potential is created = charge difference across the membrane of a resting cell • Occurs because: • Higher [K+] inside than outside • Cell membrane more permeable to K+ than other ions (K+ channels open) • Action potential = change in membrane potential in an excitable tissue that is propagated as an electrical signal • Made via Sodium-Potassium pump.

12. Resting membrane potential. Na+ channels & some K+ channels closed. A few K+ diffuse down c.g. through open K+ channels, making outside positive. • Depolarization. Na+ channels open. A few Na+ diffuse down c.g. through open Na+ channels, making inside positive. • Repolarization. Na+ channels closed; Na+ movement into cell stops. K+ channels open. K+ movement outside cell increases, making outside positive.

13. Neuromuscular Communication • Motor neurons innervate all muscle tissue • The “motor unit”: 1 motor neuron + all muscle fibers it stimulates • From 2-3 fibers/unit (fine control) to 2000 fibers/unit (large power movements) • Neuromuscular junction (NMJ) structures: • Presynaptic terminals – bulbous ends of motor neuron (ends of axons) • NMJ or synapse – region of chemical communication between cells • Synaptic vesicles contain neurotransmitter (acetylcholine) • Synaptic cleft – space between presynaptic terminals and postsynaptic membrane (sarcolemma)

14. Neuromuscular Communication, continued.. • Basic events of muscle contraction: • Action potential (nerve impulse) arrives at presynaptic terminals • Synaptic vesicles release acetylcholine (ACh) into synaptic cleft via exocytosis • ACh binds to receptors sites on Na+ channels in postsynaptic membrane, causing them to open • Na+ moves into cell causing an action potential which travels along sarcolemma & T-tubules • ACh triggers Ca2+ release from sarcoplasmic reticulum • Ca2+ inside muscle triggers actin & myosin “sliding filament” action • ATP is spent • Acetylcholinesterase degrades remaining ACh to limit contraction stimulus

15. Neuromuscular Communication, continued.. • Sliding Filament Model • Actin & myosin myofilaments slide past one another causing sarcomeres to shorten • H-zones and I-bands shorten; A-bands maintain constant width (Fig 7.7)

16. Neuromuscular Communication, continued.. • Sliding Filament Model, continued… • Ca2+ binds to troponin • Tropomysin molecules slide into groove • Myosin attaches to exposed sites on actin myofilament • Energy from ATP stored in myosin heads • Myosin heads bind to actin forming cross-bridges • Stored energy used to myosin heads causing actin myofilament to slide • ATP binds to myosin causing head to release & return to resting position • Cycle repeats if Ca2+ still bound to troponin & ATP still available

17. Muscle Twitch, Summation, Tetanus, & Recruitment • Muscle twitch - contraction of a muscle fiber in response to a stimulus • Lag phase – time between stimulus application and beginning of contraction • Contraction phase – time of contraction • Relaxation phase – time of relaxation

18. Muscle Twitch, Summation, Tetanus, & Recruitment, continued… • Strength of Muscle Contraction (↑’d by:) • Summation – increasing force of contraction of muscle fibers in muscle • Recruitment – increasing number of muscle fibers contracting within muscle • Tetanus (convulsive tension) – sustained muscular contraction caused by a series of nerve stimuli repeated so rapidly that there is no relaxation, instead a sustained contraction results

19. Energy Requirements for Muscle Contraction • Mitochondria produce ATP • Creatine phosphate is energy storage • Used to generate ATP • Anaerobic respiration (w/o O2) • Generates 2 ATP; short-lived • Lactic acid as waste product (irritant) • Aerobic respiration (w/ O2) • Generates up to 38 ATP; long-term • Uses other nutrient molecules (F.A., a.a.) • Oxygen debt – must be repaid after labor

20. Fatigue • Psychological fatigue – CNS causes the perception that continued muscle contraction is impossible • Muscle fatigue – force of muscle contraction becomes increasingly weak when ATP is used faster than can be produced and lactic acid builds up faster than it can be removed • Physiological contracture – muscles cannot contract or relax (not enough ATP to do either)

21. Types of Muscle Contractions • Isometric contractions (equal distance) – tension increases, muscle same length • Isotonic contractions (equal tension) – tension is constant, muscle shortens • Concentric – tension increases as muscle shortens • Eccentric – tension constant as muscle lengthens

22. Slow & Fast Fibers

23. Smooth & Cardiac Muscle