• Muscle Contraction is dependent on the interaction between the myosin ( thick ) and actin ( thin ) protein filaments in the sarcomere , the basic contractile unit of muscle tissue.

• To generate muscle contraction , the sarcomere functions as follows :

  1. When the muscle fiber is at rest , the myosin head is in its high-energy conformation ( upright and bound to $ADP$$ADP$ and $P_i$$P_i$ ) , and the actin filaments are bound by the regulatory proteins tropomyosin and troponin.

     • Tropomyosin is an elongated protein that wraps around the actin filament to block myosin-binding sites on this thin filament.

     • Troponin is a small protein complex associated with tropomyosin.

  2. Following a depolarizing stimulus by a motor neuron , $C{a}^{2+}$$Ca^{2+}$ ions are released from the sarcoplasmic reticulum into the cytosol.

     • The abundant cytosolic $C{a}^{2+}$$Ca^{2+}$ ions bind troponin , causing a conformational change that ultimately pulls on tropomyosin and exposes the myosin binding sites on the actin filaments.

  3. When the active sites are exposed , the myosin head is able to bind strongly to the actin filament , forming a cross-bridge and promoting the release of $ADP$$ADP$ and $P_i$$P_i$

  4. The dissociation of $ADP$$ADP$ and $P_i$$P_i$ causes the power stroke , which is the actual pivot of the actin-bound myosin head that drags the actin filament toward the center of the sarcomere.

     • This directly results in shortening of the sarcomere.

     • The myosin head is now in its low-energy conformation.

  5. A new ATP molecule bind the myosin head and the cross-bridge disassembles.

  6. Hydrolysis of the ATP molecule allows the myosin head to shift back into its upright , high-energy conformation in preparation for a new cycle of contraction.

  7. The cycle of cross-bridge formation and disassembly continues until motor neuron signaling ceases , and $C{a}^{2+}$$Ca^{2+}$ is sequestered back into the sarcoplasmic reticulum.

• When ATP bound to a myosin head hydrolyses , the myosin head shifts back and attaches to the actin filament.

• Dissociation of $ADP$$ADP$ and $P_i$$P_i$ leads to a power stroke , which shortens the sacromere.

• A new $ATP$$ATP$ molecule binds to the myosin head , causing it to dissociate from the actin filament.

• The action of many myosin heads cycling through this process leads to muscle contraction.