Human Physiology, 14th edition (2016)

Muscle 369

Figure 12.10 Activation of the myosin head. (1) The myosin head has an actin-binding site and an ATP-binding site, which
serves as an ATPase to hydrolyze ATP. (2) When ATP is hydrolyzed into ADP and P i , the myosin head becomes activated and changes
its orientation. It is now ready to bind to the actin subunits; at this point, ADP and P i are still attached to the myosin head.

ADP

Myosin tail

Troponin Actin Myosin binding site

Actin-binding

sites

AT P

ATP-binding

site

Pi

Thin

filament

Thick

filament

Myosin head

1 2

Tropomyosin

Figure 12.11 The cross-bridge power stroke. (1) The myosin head has been activated by the splitting of ATP into ADP
and P i , which remain bound. At this point, the myosin head has bonded to the actin, forming a cross bridge between the thick and
thin filaments. (2) After the P i group leaves the cross bridge, the myosin head changes its orientation, producing the power stroke that
moves the actin filament.

ADP

Power

stroke

Pi

Pi

Actin

1 2

action of the members is asynchronous. Some cross bridges
are engaged in power strokes at all times during the contrac-
tion. The force produced by each power stroke is constant,
but when the muscle’s load is greater, the number of cross
bridges engaged in power strokes is increased to generate
more force.

Regulation of Contraction

When the cross bridges attach to actin, they undergo power

strokes and cause muscle contraction. In order for a muscle to

relax, therefore, the attachment of myosin cross bridges to actin

must be prevented. The regulation of cross-bridge attachment