The heart speeds up and slows down for various reasons, including its own internal pacemaker, stimulation of the vagal nerve or of the accelerator nerve (by the medulla oblongata). The article doesn't mention whether these organoids contain internal pacemakers or whether they're driven by an external pacemaker or whatever.<p>If these contained their own pacemakers, it would be really useful to test arrhythmia drugs. Might even be useful to test the causes of some cardiogenetic issues (genetically determined short circuits in the wiring, for instance). So many possibilities!