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Muscles The Sliding Filament Hypothesis suggests muscular contraction occurs in the sarcomeres of the muscle fibres. Explain how actin and myosin filaments in the sarcomere bind together causing muscular contraction. (4 marks) A. Filaments unable to bind due to tropomyosin B. Receipt of nerve impulse/action potential/electrical impulse/wave of depolarisation C. Sarcoplasmic reticulum (releases) D. Calcium (ions released) E. (Calcium) Attach to troponin (on actin filaments) F. Causes change of shape of troponin/moves tropomyosin G. Exposes myosin binding site (on actin filament)/ ATP H. Cross bridge formation I. Powerstroke occurs/Ratchet Mechanism/Reduce H zone/z lines closer together How can a performer vary the strength of muscular contractions to ensure that a skill is completed correctly? (4 marks) A. (Greater the force needed) larger motor units recruited B. More units recruited C. Need fast twitch fibres rather than slow twitch fibres D. Multiple unit summation/spatial summation E. All or none law/All or nothing law/or explanation F. Wave summation/frequency of impulse/innervations G. Motor unit unable to relax/increase the force H. Tetanus/titanic for powerful contraction I. Muscle spindles detect changes in muscle length/speed of contraction J. Send information to brain/CNS K. Compares information to long term memory to ensure correct force applied/past experiences L. Spatial summation – rotating the frequency of the impulse to motor units to delay fatigue During the race a swimmer has to dive off the starting blocks as quickly as possible. Identify the ‘muscle fibre type’ used to complete this action and justify your answer. (3 marks) A. Fast twitch fibres/type 2 B. Type 2b/fast twitch glycolytic/FTG C. Fast speed of contraction D. High force of contraction/powerful contraction/ strong contraction Identify the position of myosin and actin in the above diagram of a sarcomere. (2) A-Band = Myosin I-Band = Actin What are the functions of troponin and tropomyosin during muscle contraction? (5) • Troponin and tropomyosin are connected to actin. • During muscle contraction troponin binds with calcium ions. • This causes troponin to change shape. • This moves the tropomyosin molecules and allows myosin to bind to actin in order for muscle contraction to begin. • When calcium ions are no longer present troponin causes muscle to relax by preventing myosin from binding to actin, so myosin and actin return to their original relaxed positions. Explain how the movement of Calcium ions controls the contraction of skeletal muscle. (4) • Ca ions are stored in the sarcoplasmic reticulum (the liquid that surrounds each myofibril). • Electrical impulses sent into the myofibril via the transverse tubules cause the release of Ca ions. • Ca ions bind to troponin on the actin filaments, causing tropomyosin to move and exposing the myosin binding sites on actin. • This allows myosin to bind to actin and therefore muscle contraction to take place. • When the electrical impulse stops Ca ions are sent back to the sarcoplasmic reticulum, blocking the myosin binding sites and therefore stopping muscle contraction. What is the function of myosin ATPase during muscle contraction? (3) • Myosin ATPase is activated in the myosin head when it binds to actin. • Myosin ATPase causes ATP to breakdown and energy to be released. • This energy causes the myosin head to change shape and pull the actin filament along (resulting in muscle contraction). • Without myosin ATPase present to breakdown ATP muscle contractions could not occur. During the race a swimmer has to dive off the starting blocks as quickly as possible. Identify the ‘muscle fibre type’ used to complete this action and justify your answer. (3 marks) A. Fast twitch fibres/type 2 B. Type 2b/fast twitch glycolytic/FTG C. Fast speed of contraction D. High force of contraction/powerful contraction / strong contraction What are the main characteristics of the main type of motor unit used during sprinting? (4 marks) • • • • • • • • • Fast twitch fibres / type 2 fibres; Large number of muscle fibres per motor neurone; Fast myosin ATPase; High sarcoplasmic reticulum development; Low aerobic capacity; Very high anaerobic capacity; Fast contractile speed; Low fatigue resistance; High motor unit strength. What do you understand by the term ‘motor units’? (3 marks) • Motor unit is a motor neurone and its muscle fibres; • All muscle fibres within one motor unit are the same type (e.g. slow twitch); • Motor units follow the ‘All or nothing law’ – all muscle fibres either contract or don’t contract (there is no partial contraction). • Motor units vary in size, large motor units are used for gross movements and small motor units are used for fine control. How are motor units involved in spatial summation? (4 marks) • A motor unit is a motor neurone and its muscle fibres; • Motor units follow the ‘All or nothing law’ – all muscle fibres within the motor unit either contract or don’t contract. • When a more powerful muscle contraction is required more motor units are recruited – this is spatial summation. • Less motor units are activated for movements requiring fine control. The table below shows the percentage of slow twitch fibres in elite sprinters. Discuss whether the sampling of muscle is a good indicator of sprinting performance. (3 marks) Male Sprinters Range of percentage of slow twitch fibres Average percentage of slow twitch fibres 20-55 35 • Table shows that a high percentage of fast twitch fibres is common in male sprinters. • Large range of fast twitch muscle fibre percentages in sprinters shows that other factors must also be important. • Factors such as physique, motivation, commitment etc are also important. • However a high percentage of fast twitch fibres gives a natural advantage towards becoming a sprinter.