Cardiac Muscle : These muscles are found only on the walls of the heart in mammals. Cardiac muscles are small, spindle-shaped, and branched. Due to their branching nature, they form an interlocking network. These muscles have a central nucleus in their sarcoplasm. Intercalated discs are found in them, which cause them to remain divided into small sections. Similar to skeletal muscle fibres, each section of cardiac muscle fibres has deep and light stripes. These muscles are not under voluntary control; hence, they are involuntary muscles.
These heart muscles contract rhythmically and continuously without pausing, and this rhythmic contraction is termed as the heartbeat.