Cardiac muscle contracts as a unit because the electrical activity in the heart spreads over the entire muscle due to gap junctions in intercalated discs.
Heart is formed by a specialized tissue called myocardium. The myocardium is responsible for pumping blood through the heart. It contracts and expands involuntarily in response to electrical impulses from the nervous system. The heart has another tissue that contains pacemaker cells. These cells are responsible for generating electrical impulses. They determine the heart rate by controlling the frequency of pumping up of blood.
The myocardium, as a whole of cardiac muscle, is held together by desmosomes. Desmosomes are another specialized structure found inside the intercalated discs.
Two cardiac muscles are connected by intercalated discs. Intercalated discs have a gap junction inside them which transfers the electrical impulse from one cardiac muscle cell to another.
Therefore, cardiac muscle contracts due to the action of gap junctions inside the intercalated discs.
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