So we have two types of filaments. We have a thin filament, which are these black horizontal lines. We have 1234-5678 for this particular sarcomere, and we have the thick filament which is shown in purple. We have one, two, three of these thick filaments in this particular sarcommer. Now, let's begin by discussing what the composition of our thin filament is. The thin filament is composed of a type of globular spherical protein known as actin.