Myosin heads cannot bind to actin filaments

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Presentation transcript:

Myosin heads cannot bind to actin filaments troponin tropomyosin actin filament ADP myosin complex Pi myosin filament At rest, the actin-myosin binding site is blocked by tropomyosin, held in place by troponin Myosin heads cannot bind to actin filaments

Ca2+ binds to troponin, changing its shape Tropomyosin is pulled out of the binding site Myosin head can bind – bond is an actin-myosin cross bridge

Ca2+ activates ATPase, breaking down ATP to ADP + Pi Energy provided moves myosin head, pulling the actin filament along in a ratchet motion

Free ATP binds to head, changing shape Actin-myosin cross bridge breaks ATP is hydrolysed, and head returns to original shape

With continued stimulation the cycle is repeated

If stimulation ceases, Ca2+ is pumped back into sarcoplasmic reticulum Troponin and tropomyosin return to original positions Muscle fibre is relaxed

Supplying energy for muscle contraction Energy is required for the movement of the myosin heads that make the actin filaments slide and for the return of calcium ions into the reticulum of the sarcoplasm. The energy is provided by the breakdown of ATP to ADP and inorganic phosphate. Resting muscles only store enough ATP to maintain contraction for a short time. Muscle fibres contain large numbers of mitochondria that can generate ATP by respiration of glucose but this process is relatively slow. (even anaerobic respiration takes time to supply ATP) Muscle fibres also store phosphocreatine. This can be used to produce ATP very rapidly.

Using phosphocreatine to generate ATP Phosphocreatine is able to transfer a phosphate ion to ADP and is therefore able to replace the the ATP that has been broken down. ADP + Phosphocreatine ------ ATP + Creatine The amount of phospho creatine available is limited but there is enough to keep the muscles going until there is more ATP available from the mitochondria. Nevertheless this means that really intense muscle activity can only be maintained for a short time eg 10 secs for a 100m sprint.