how muscle cell contracts

Demonstration of regulation of skeletal muscle contraction: http://www.sci.sdsu.edu/movies/actin_myosin.htmlHow does muscle cell contract?

The basic level of contraction is that myosin is needed to bind with pure actin and hydrolize ATP. However, thin actin filament has two proteins called tropomyosin and troponin that prevent myosin binding in the absence of Ca++.  Multiple tropomyosin molecules bind head to tail to form a continuous chain wrapping around the thin filament. It blocks the movement of myosin along the thin filament. When Ca++ is released from the sarcoplasmic reticulum after a signaling from motor nerve tells the muscle to contract, it binds to troponin, which moves tropomyosin out of the way, allowing myosin to move along the thin filament employing the cross-bridge cycle. Such a freedom of movement causes a contraction.

It seems like too many steps to take, but I understand the necessity to prevent unnecessary contraction.

jargon

Cdk activating kinase (CAK) and Cdc25 phosphatase antagonize the activity of Wee1.