Chapter 8 Notes. Muscles

Transcription

1 Chapter 8 Notes Muscles

2 8.1 Intro Three muscle types Skeletal Smooth cardiac

3 8.2 Structure of Skeletal Muscle Composition Skeletal muscle tissue Nervous tissue Blood Connective tissue

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5 Connective tissue coverings Fascia connective tissue (CT) covers muscle Separates different muscles If projecting beyond muscle end to form tendon Intertwines with periosteum of bone connecting to bone

6 Aponeuroses Broad fibrous sheet which may attach to adjacent muscles Epimysium CT closely surrounds skeletal muscle (SM) Perimysium CT that extends inward from epimysium Separates muscle into smaller compartments (bundles) Endomysium Covers muscle bundles

7 Skeletal muscle fibers (SMF) SMF is a single cell Contracts responding to stimuli / relaxes when stimuli ends Thin elongated cylinder w/ round ends May extend length of muscle Beneath sarcolemma (cell membrane) is the sarcoplasm (cytoplasm) that contains mitochondria and nuclei

8 Myofibrils also in sarcoplasm Sarcomere is the functional unit of a myofibril Play key role in contraction Contain Actin and Myosin A & M form light and dark striations in SMF

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10 Striataions (bands) I bands (light bands) Actin filaments attached to Z lines A bands (dark bands) Myosin filaments (thick) overlapping with actin filaments (thin) H zone (central region) of only thick myosin filaments M line Between two Z lines is a sarcomere

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12 Sarcoplasmic reticulum (SR) Surround myofibrils Transverse tubules (T-tubules) Extend from outside SMF through fiber Runs between two enlarged portions of SR called cristernae in area of actin /myosin overlap SR and t-tubules activate muscle contraction when stimulated

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14 Neuromuscular junction Each muscle fiber connects to nerve fiber motor neuron Contraction takes place when motor neuron stimulates muscle fiber Connection point of motor neuron and muscle fiber is the neuromuscular junction Connection point is the motor end plate w/ lots of mitochondria and nuclei and cell membrane with many folds (SMF)

15 Motor neuron highly branched and can stimulate many SMF s

16 Distal end motor neuron fiber Many mitochondria Synaptic vesicles (SV) storing neurotransmitters Nerve impulse from brain causes SV to release neurotransmitters into synaptic cleft (gap between motor end plate and SMF

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18 8.3 Skeletal Muscle Contraction Actin / myosin role Myosin 2 twisted protein strands with projecting cross-bridges Actin a globular structure with binding sites for myosin, sometimes a double helix filament Troponin Tropomyosin Proteins intertwined around actin

19 Sliding filament theory

20 Sliding filament theory cont. 1. Depolarization (nerve impulse reaches synaptic bulb. 2. Nerve impulse induces Ca + ions to enter the bulb 3. In response to Ca + ions, vesicles migrate toward the cell membrane 4. Vesicles fuse with the cell membrane 5. Vesicles release Acetylcholine (Ach); a neurotransmitter, into synaptic cleft.

21 Sliding filament theory cont. 6. Ach diffuses across the synaptic cleft 7. Ach binds to receptor proteins on the motor end plate (on sarcolemmamuscle cell membrane) 8. Motor end plate on the sarcolemma depolarizes 9. Depolarization generates an action potential (a change in electrical potential voltage on the surface of a muscle cell)

22 Sliding filament theory cont. 10. The action potential moves along the sacrolemma (cell membrane) 11. Moves along transverse tubules down into the fiber (cell). 12. Moves into the sarcoplasmic reticulum (SR) 13. The action potential causes the SR to release Ca + ions 14. Ca + ions expose binding sites on actin.

23 Sliding filament theory cont. 15. ATP binds to myosin heads (low energy configuration) 16. myosin heads hydrolyze ATP to ADP + P (high energy shape). As a result, myosin changes shape and is able to bind with actin. 17. Myosin heads bind to actin forming a cross bridge.

24 Sliding filament theory cont. 18. Myosin heads lose the ADP + P; myosin relaxes to its low energy state this action slides the thin filament past myosin 19. Myosin heads pull actin toward the center of the sarcomere causing the sarcomere to shorten. (thin filament actin slides toward the center of the sarcomere).

25 Sliding filament theory cont. 20. Binding on a new molecule of ATP releases the myosin head; cross bridges between myosin heads and actin break. 21. Sarcomere returns to its original shape and the muscle relaxes.

26 Relaxaion The Ach that started the contraction is broken down by acetylcholinesterase Rapid Ach breakdown prevents a single nerve impulse continuously stimulating a nerve fiber. Ca + ions are actively transported back into SR

27 Energy for contraction ATP provides the energy Only small stores of ATP in SMF so ATP must be regenerated if longer contraction Creatine phosphate (contains high energy phosphate bonds) is more abundant in SMK than ATP Acts as extra energy storage in SKM Transfers energy to ADP converting it to ATP

28 Energy sources continued After creatine phosphate used up then cellular respiration kicks in converting glucose to ATP

29 Oxygen supply and Cellular respiration RBC s carries oxygen loosely bound to hemoglobin to SMF s Myoglobin synthesized in the muscle can temporarily store the oxygen that will be used in cellular respiration making it not essential to have the continuous blood supply during a muscular contraction

30 Oxygen debt When muscles worked strenuously oxygen supplies can t meet cellular respiration demands Glycolysis breakdown of glucose occurs ATP is produced in lesser amounts than if glucose had gone through CR Lactic acid is a byproduct Oxygen debt develops w/ LA buildup

31 Oxygen debt cont. The oxygen debt is the amount of oxygen needed to convert accumulated lactic acid into glucose plus amount to restore ATP and creatine phosphate levels back to normal Process could take several hours

32 Muscle fatigue. Loss of ability for muscle to contract Possible causes Interruption of blood supply Lack of Ach Accumulation of lactic acid Lactic Acid lowers ph making SMF no longer responsive to stimulation

33 Muscle fatigue cont. Cramps Sustained muscle contraction Changes in extracellular fluids around SMF and motor units somehow trigger uncontrolled stimulation of a muscle

34 8.4 Muscular Responses Threshold stimulus There is a minimal threshold of stimulation before a contraction can occur An impulse in a motor neuron usually releases enough Ach to bring SMF to threshold

35 Muscle contraction Twitch If threshold met muscle will go through a contraction / relaxation the process called a muscle twitch

36 All or none response Threshold strength stimulus makes muscle respond to its fullest extent If it contracts at all it is to its fullest extent

37 Summation When a muscle fiber is exposed to a series of stimuli of increasing frequency reaches a point where it can not fully relax before the next stimuli

38 Recruitment of motor units Single motor neuron controls a motor units Motor units stimulates all its SMF at the same time but a muscle can have several motor units contained in it Different motor units have different stimulation thresholds If low stimulation only low sensitive SMF contract. If high stimulation then all would contract in same muscle

39 Recruitment motor units cont. Increase in motor units stimulated is called recruitment. More recruitments greater muscle contraction force

40 Sustained contractions Summation and recruitment can produce sustained contraction of increasing strength Most muscle contractions are sustained Muscle tone is a response to nerve impulses that originate repeatedly from spinal cord maintains our posture. If interrupted such as in fainting you collapse

41 Use and disuse of SM Vs. Why the difference?

42 Use Disuse cont. Muscular hypertrophy Forcefully exercised muscles enlarge Muscular atrophy Unused muscles decrease in size and strength Exercise response Muscles contracting weakly as in swimming or running activates specialized group of SMF called slow fibers

43 Use Disuse cont. Slow fibers are fatigue resistant Develop more mitochondria and extensive capillary network Size and strengths remain the same Fast fibers Develop from forceful exercise such as weight lifting Are fatigable Produce new filaments of actin and myosin increasing diameter of muscle fiber

44 Use Disuse cont. Fast fibers cont. Entire muscle enlarges No new muscle fibers are produce Strength of contraction increased Disuse Capillary networks shrink Mitochondria numbers decrease Actin myosin filaments decrease Muscle atrophies