 Dear students, in this topic we shall discuss the pacemakers and their types. The pacemaker is the excited tree region that generates rhythmical impulses which control the rhythmicity of the cardiac chambers. There are two basic types of pacemakers in animals that have a pumping heart. First, the neurogenic pacemakers and second, the myogenic pacemakers. First we shall discuss the neurogenic pacemakers which consist of neurons. They are found in many invertebrate hearts. Such hearts are known as the neurogenic hearts. Dear students, the myogenic pacemakers consist of specialized self-excitatory muscle cells. These types of pacemakers are found in some invertebrates and all vertebrate hearts. Such hearts are known as the myogenic hearts. Dear students, the pacemakers in the myogenic heart are sinus venoussus in the fish and amphibians. Sinus venousus is a heart chamber in the fish and amphibians. Whereas the amniots, that is birds, reptiles or mammals, are found in these pacemakers in the superior posterior lateral wall of the right atrium. Sinus venoussus does not exist in the amniots as a chamber but it has a vestigial remnant which is located in the superior posterior lateral wall of the right atrium. This pacemaker is known as the sinus node or sinoatrial SA node. Dear students, the sinoatrial node consists of cells which are small in size, they are flattened, they are weakly contractile and are specialized muscle cells which are capable of self-excitation. The sinoatrial node in humans is approximately 3mm wide, 15mm long and 1mm thick. Dear students, these SA nodes directly connect with the atrial muscle cells. This is because the action potential generated in the sinus node immediately spreads into the atrial muscle wall.