So these heads are shown on this diagram, and these heads are responsible for interacting with the black region that thin filament. And that's exactly what causes that contraction. These myosin heads pull on our actin and that basically causes that contraction as we'll see in the next lecture. So now that we know what the composition of the sarcomere is, let's discuss these different regions. And let's begin with our H zone. So the H zone is basically this entire region that is found on the single sarcomere.