 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 myogenic pacemakers are located in the fissures and amphibians of Sinus venousus. Sinus venousus is a heart chamber located in the fissures and amphibians of Sinus venousus. These pacemakers are located in superior posterior lateral wall of the right atrium because Sinus venousus does not exist 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 Sino-atrial SA node. Dear students, the Sino-atrial 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 Sino-atrial node is about 3 mm wide, 15 mm long and 1 mm thick. Dear students, the Sino-atrial node is directly connected with the atrial muscle cells. The Sino-atrial node is directly connected with the muscle cells of atria. The reason is that the action potential generated in the Sinus node spreads immediately into the atrial muscle wall.