How does heart muscle contract rhythmically in unison?

The heart is composed of a specialied muscle type called cardiac muscle. This has unique properties such that each muscle cell is connected together by gap junctions in their cell membranes (electrical synapse) to create a 'syncitium'.

There are specialised ion channels in the right atrium (sinoatrial node) that help to generate the basal rhythym of contraction (briefly mention base rhythmycity of all myoctes). This region conducts electrical signals to the surrounding musculature in the left and right atrium causing contraction. This conduction is slowed at the atrioventricular septum which is electrically insulating. Post delay, this signal then activates a second important node (special bundle of cells) called the atrioventricular node (AVN) which sends impulses down special conducting fibres called the bundle of His. (mention WPW syndrome) At the apex of the left ventricle, the purkinje fibres that compose the bundle of His spread out and the electrical signal that travels along these fibres post delay is what causes the subsequent ventricular contraction creating the characterstic 'lub-dub' sequence during auscaltation.

*Note that the mechanism and physiology of muscle contraction i.e. sarcomeres, calcium signalling and ion conductance is out of the scope fo this answer.