What is unusual about the cardiac muscles?

They are shorter then skeletal muscles cells. They only contain one nucleus. They are attached together by intercalated discs which stop the heart from tearing due to its constant contractions. It is completely involuntary. The action potential is different but I'm not sure you know what that is, if you do look it up really interesting comparing it to skeletal muscle action potentials. That should cover the basics :) have a nice day and enjoy the heart. It's the best organ.