They can even stop contractions in the intestines if these feelings become severe. For this reason, people under emotional stress often find it difficult to digest food. Cardiac muscle makes up the walls of the heart. When cardiac muscle cells contract, they push blood out of the heart and into the arteries. The blood then circulates throughout the body, bringing nourishment to all body cells.
Cardiac muscle has characteristics of both skeletal and smooth muscles.
Like skeletal muscle fibers, cardiac muscle cells have striations. Like smooth muscle fibers, each cardiac muscle cell has only one nucleus and contracts automatically. The heart also contains a group of specialized cells called the sinoatrial node, or S-A node. The S-A node starts up each contraction of the cardiac muscle by giving off rhythmic signals to neighboring muscle cells. As these cells contract, they cause others to contract as well.