based on the molecule's atomic coordinates that were determined by mackinnon's group in 2001. some types of potassium channels are activated by muscarinic receptors and these are called muscarinic potassium channels (ikach). these channels are a heterotetramer composed of two girk1 and two girk4 subunits. examples are potassium channels in the heart, which, when activated by parasympathetic signals through m2 muscarinic receptors, cause an outward current of potassium, which slows down the heart rate. in fine art roderick mackinnon commissioned birth of an idea, a 5-foot (1.5 m) tall sculpture based on the kcsa potassium channel. the artwork contains a wire object representing the channel's interior with a blown glass object representing the main cavity of the channel structure.