Most people know that it is colder on top of a mountain compared to the ground, but few actually know why. And even less know that the atmosphere actually warms up again! The true temperature profile of the atmosphere is shown in the image.
The air at the surface up to around 10 kilometers is called the troposphere. The reason it is warmer at the surface is simple. The air is warmed by heat given off by the Earth! The farther away from the surface the air moves, the less heat there is to absorb.
From 10 to 20 kilometers the atmosphere is stable. This region is called the tropopause. From 20 to about 50 kilometers is the stratosphere. In this region the air actually warms with height! Ozone is concentrated in this part of the atmosphere and it absorbs ultraviolet light from the Sun. More light is absorbed at higher altitudes compared to the lower stratosphere, so the temperature increases.
But at 50 kilometers, the temperature levels out again in a region called the stratopause. At about 55 km, the mesosphere begins. In the mesosphere, the temperature decreases with height again, because there is very little ozone to warm up the air.
Finally, the mesopause divides the mesosphere from the thermosphere, which is the section of the atmosphere higher than 90 km. In this region, the temperature increases again! This time, it is molecular oxygen (O2) that causes the temperature increase. The oxygen absorbs light from the Sun, and since there is very little air in the thermosphere, just a little absorption can go a long way!
What exactly is your question
The answer is; Ca2+ from the sarcoplasmic reticulum binds to tropomyosin
When the ca2+ ions bind to the tropomyosins on the actin filament, the tropomyosins are displaced and this exposed the myosin binding sites on the actin filament. The ADP + Pi (attached to the myosins heads), from the previous cycle, is released allowing the cocking of the myosin heads hence pulling on the actin filament. ATP then binds on the ATP-site on the myosin heads casing a release of the myosin heads from their site in the actin. This also hydrolyzes the ATP. This is one cycle of the cross-bridge cycle.