Question: When you move a limb for example my bulging bicep, what happens? What happens in your brain, and then how do all my tiny little cells collectively move it afterwards?

Kylie Belchamber answered on 15 Jun 2016:

Good question Nerd123!
So, your arm is full of nerves which run from your brain, through your spinal cord and into your arm. When your brain wants to move an arm, it will send a signal down the nerve and into your muscle. Here, the nerve releases signals onto the muscle, telling it to move. When you move your bulging bicep up into a curl, the muscle contracts, meaning the muscle fibres get shorter. And when you relax the muscle, they get longer.

At a cellular level, the muscle cells contain stripes. One stripe is a protein called actin, and the other stripe is a protein called mysoin. These normally start relaxed so the two stripes sit next to each other (put your hands out in front of you, with your palms flat with fingers pointing to each other, as if you were pushing an invisible ball down). When the muscle contracts, the myosin slides over the actin by grabbing it and pulling at it (slide your hands so one goes over the other). This makes the muscle shorter, and because this is attached to the bone via tendons, it will move your arm up.
The signal from the nerves will activate lots of muscle cells, so they all move together.

I hope that made sense!!