LCRS
Alexandra Burke-Smith
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Troponin binds to Ca2+ Tropomyosin displaces from myosin binding site Myosin head attaches to actin (along with ADP and Pi)
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Cross-bridge formation and Pi release Pi released from myosin ADP remains attached Myosin forms cross-bridge with actin; increase binding strength from weak to strong
3. Filament sliding and ADP release - ADP release causes change in orientation of myosin head - Myosin filament pulled towards M-line 4. ATP binding - New ATP molecule binds to myosin head 5. ATP hydrolysis and detachment - ATP molecule hydrolysed to ADP and Pi (which both remain attached) - Energy released used to return myosin head to original orientation, and detach from the actin filament Note: Isometric contraction - In isometric contraction, the myosin head changes orientation, but there is NO FILAMENT SLIDING - Force is directed towards the M line Key Features
ATP is used in each cycle to provide the energy o RIGOR MORTIS occurs if ATP concentration = 0 Direction of filament sliding is one way o Thin filament moves towards the centre of the sarcomere o Causes sarcomere shortening o However the attached bridge does resist a stretch applied to the ends of the muscle Step size is small o Sliding produced by one cycle is only 1% of sarcomere length o Many cycles in succession needed to cause large movements e.g. running
Variation in Force (in vivo) Recruitment of the Motor Units
Motor Unit: a motor neuron and all the muscle fibres is innervates (has synapses with) o Functional unit of normal skeletal muscle o Action potential in the motor neuron triggers an action potential in all the muscle fibres in the motor unit Vary the number of motor units that are active and thus vary the force as more muscle fibres are producing force Functional anatomy of motor units: o Motor unit size varies (muscle fibres per motor neurone) o There is a scattered distribution of motor units within the muscle 8