Muscle/MuscleContraction/ThePowerstroke

The Powerstroke

With the muscle at rest the binding site on the thin filament is unavailable for the myosin head. The myosin head is primed for contraction but cannot act if the binding site is unavailable.

The “switch” for initiating muscle contraction is an elevation in intracellular calcium concentration. Calcium binds to troponin C, this moves the troponin-tropomyosin complex away from the binding sites on the thin filament. This allows the primed myosin head to attach the binding site. When the myosin head binds to actin it is known as the crossbridge.

Once attached to the thin filament the myosin head pivots on its hinge, this is known as the powerstroke. The powerstroke moves the thin filament towards the center of the sarcomere (M line). This movement drags the Z line with the thin filament, shortening the sarcomere.

Challenge Quiz

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<GIFT> //LEVEL:2 //RAND The binding of the myosin head to actin is prevented by {=the troponin-tropomyosin complex~calcium~the myosin head not being ready}

//LEVEL:2 //RAND An increase in {=calcium~sodium) in the (=intracellular~extracellular) fluid initiates muscle contraction.

//LEVEL:2 //RAND The calcium binds to {=troponin~tropomyosin~myosin}. This opens up the binding site on {=actin~myosin}.

//LEVEL:2 //RAND When the myosin head is bound to the thin filament it is called a crossbridge {=True~False}

//LEVEL:2 //RAND The powerstroke is when the myosin head pivots on its crossbridge {True~=False}

//LEVEL:2 //RAND The powerstroke moves the thin filament {=towards~away from} the center of the sarcomere.

//LEVEL:2 //RAND The overall effect of the powerstroke is to shorten the sarcomere {=True~False}