To answer your question, the heart is able to contract because it contains special cells called “pacemaker cells”. These cells have ion channels that are different from those found in muscle cells. They are able to generate spontaneous action potentials because the ion channels in their membranes are “leaky”. This means that K+ (potassium) and Na+ (sodium) will be constantly leaking into the cell, leading it towards depolarization. At a certain membrane potential (the membrane potential is the charge of the membrane, determined by the relative concentration of ions inside and outside of the cell) Ca₂₊ (calcium) channels open, causing Ca₂₊ to rush into the cell, leading to complete depolarization. Once the cell is completely depolarized, K+ channels will fully open and the Ca₂₊ channels will close, causing the cell to repolarize. Once repolarized, the K+ and Na+ ions will begin to leak in again, leading to another depolarization. These depolarizations/repolarizations are called action potentials. Action potentials in the pacemaker cells initiate action potentials in neighboring cells of the heart, which then spread very quickly to all the heart cells through channels called gap junctions. These are openings that connect the cytoplasm of neighboring cells which allow ions to flow in between. Once an action potential reaches a contractile cell in the heart, it causes the muscle fibers to contract just the same way as any other muscle in the body. Hope that helped you!

This is an interesting question. The reason why the heart can contract in a rhythmical way is because the heart is the organ in our body that has a natural pacemaker that sends the electrical signals to its muscles. This natural pacemaker is named Sinoatrial Node (also called the SA node or sinus node).

The sinus node is nestled in the upper are of the right atrium, and it sends the electrical impulse that triggers each heartbeat. The impulse is strong enough to spread through the atria, prompting the cardiac muscle tissue to contract in a coordinated wave-like manner.

The impulse that originates from the sinoatrial node strikes the Atrioventricular node (or AV node) which is situated in the lower portion of the right atrium. The atrioventricular node in turn sends an impulse through the nerve network to the ventricles, initiating the same wave-like contraction of the ventricles.

You can read and see pictures of the heart in the following link:

You can watch an interesting and fun video too in the link below:
how the heart works