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Chapter 8 Muscular System

Chapter 8 Muscular System. 8.3 and 8.4. 8.3. Skeletal Muscle Contraction. Role of Myosin and Actin. Myosin Structure. Actin Structure. Myosin molecule = 2 twisted proteins with globular ends ( cross-bridges ) Many myosin molecules form a thick filament.

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Chapter 8 Muscular System

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  1. Chapter 8Muscular System 8.3 and 8.4

  2. 8.3 Skeletal Muscle Contraction

  3. Role of Myosin and Actin Myosin Structure Actin Structure • Myosin molecule = 2 twisted proteins with globular ends (cross-bridges) • Many myosin molecules form a thick filament • Actin molecule = globular structure with binding site • Many actin molecules twist into a helix and for a thin filament • Proteins troponin and tropomyosin are also part of the actin filament

  4. Role of Myosin and Actin • Sliding filament model for muscle contraction • Thick and then filaments do not change length, but slide past one another • Sarcomeres shortened by force of myosin cross-bridges pulling on actin filaments at binding sites

  5. Role of Myosin and Actin • Role of ATP • Globular portions of myosin filaments contain ATPase (catalyzes breakdown of ATP to ADP and phosphate, releasing energy) • Energy puts cross-bridge in “cocked” position • Cocked cross-bridge binds to actin and pulls it • Another ATP binds to the cross-bridge and allows it to be released

  6. Role of Myosin and Actin

  7. Stimulus for Contraction Skeletal muscle fibers do not contract until a neurotransmitter (acetylcholine) stimulates it • Synthesized in the cytoplasm of the motor neuron and stored in vesicles • Released into the synaptic cleft when stimulated by a nerve impulse • Acetylcholine binds to protein molecules (receptors) on the cell membrane

  8. Stimulus for Contraction • Cell membrane increases permeability to sodium ions • Stimulates a muscle impulse (travels along sarcolemma and down into the T tubules) • Membranes of cisterne become more permeable to calcium ions (which are stored in the sarcoplasmic reticulum) as a result

  9. Stimulus for Contraction • Calcium binds to troponin and tropomyosin and causes them to move, exposing the binding sites on actin • Linkages between the actin and myosin filaments occur • The muscle contracts as long as acetylcholine is released and there is sufficient ATP http://www.youtube.com/watch?v=gJ309LfHQ3M

  10. Stimulus for Contraction • 2 events lead to muscle relaxation • Acetylcholinesterase(present in the neuromuscular junction) decomposes acetylcholine very rapidly • Calcium ions are actively transported back into the sarcoplasmic reticulum, causing the actin and myosin filaments to break and the muscle fiber to relax

  11. Energy Sources for Contraction • ATP molecules supply the energy for contraction, but there is only enough for a very short time • Cells must regenerate the ATP to continue contracting • This is made possible by creatine phosphate • Contains high energy phosphate bonds (like ATP) • 4 to 6 times more abundant in muscles than ATP • Stores excess energy from mitochondria • Does not directly supply energy for a cell

  12. Energy Sources for Contration • The energy from creatine phosphate can be transferred to ADP molecules, converting them back into ATP • Active muscles rapidly exhaust creatine phosphate supplies • What’s next?

  13. Oxygen Supply and Cellular Respiration • Glycolysis • Can take place without oxygen • Produces a net of 2 ATP • Cellular respiration • Requires oxygen • Produces a net of 36 ATP

  14. Oxygen Supply and Cellular Respiration • Blood carries oxygen from the lungs to the muscles • Oxygen is bound to hemoglobin • Pigment that causes blood to look red • Myoglobin is synthesized in muscle cells • Pigment that causes muscles to be reddish-brown • Can combine loosely with oxygen • Reduces muscle’s requirement for continuous blood supply

  15. Oxygen Debt • Skeletal muscles are used strenuously (even for just a minute or two) • There is not enough oxygen, so glycolysis occurs, producing pyruvic acid • Pyruvic acid reacts to form lactic acid (which causes a burning sensation in the muscles) • Blood carries lactic acid to the liver where it is converted back to glucose using ATP

  16. Oxygen Debt • During strenuous exercise, ATP is used to allow the muscle fiber to contract, rather than to synthesize glucose • As lactic acid accumulates, a person develops an oxygen debt that must be repaid • Oxygen debt = the amount of oxygen liver cells require to restore ATP and creatine phosphate to their original concentrations • It may take several hours to repay an oxygen debt

  17. Muscle Fatigue • Fatigue = when a muscle loses its ability to contract • May be caused by • Strenuous exercise over a long period • Interruption of the muscle’s blood supply • Lack of acetylcholine in the motor neuron axon • Most commonly, accumulation of lactic acid • Lowers pH • Muscle fibers no longer respond to stimulation • Cramp = sustained involuntary contraction

  18. Heat Production • Less than half of the energy released in cellular respiration is actually used • The rest becomes heat • All active cells generate heat • Muscle tissue is a major heat source because of its large proportion of total body mass • Blood transports heat to other tissues • Helps maintain body temperature

  19. 8.4 Muscular Responses

  20. Muscular Responses • Muscle contraction can be measured in several ways • One way is to remove a single muscle fiber and record its length as a result of different strength and frequency of stimuli

  21. Threshold Stimulus • Threshold stimulus = stimulation level that must be exceeded to elicit a muscle contraction • A single nerve impulse normally is enough (releases enough ACh) • A muscle will not contract if threshold is not reached • Twitch = contractile response of a single muscle fiber to a muscle impulse

  22. Recording a Muscle Contraction • Myogram = graphical recording of muscle activity • Has 3 periods • Latent period • Period of contraction • Period of relaxation

  23. Recording of a Muscle Contraction • The force generated by our muscles is controlled by the… • Frequency of muscle stimulation • Number of fibers that take part in the contraction of the whole muscle • The force that a muscle fiber can generate is not limited to the maximum force of a single twitch

  24. Summation • Summation = increased force of contraction by a skeletal muscle fiber when a twitch occurs before the previous twitch relaxes • Tetanic contraction = continuous, forceful muscular contraction without relaxation

  25. Recruitment of Motor Units • A single muscle is made of many different motor units • Recruitment = increase in the number of motor units that are activated as stimulation intensity increases • Motor neurons with low thresholds respond first, and those with higher thresholds respond as stimulation increases

  26. Sustained Contractions • Summation and recruitment together can produce a sustained contraction • A result of a rapid series of impulses from the brain and spinal cord • Muscle tone = a response to nerve impulses that originate repeatedly from the spinal cord and stimulate a few muscle fibers • Muscles may appear at rest, but are undergoing a sustained contraction • Important in maintaining posture

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