Final answer:
The middle muscle layer of the heart is known as the myocardium, consisting of cardiomyocytes that contract to pump blood. Its complex structure and the coordination of muscle cells and pacemaker cells facilitate cardiac function.
Step-by-step explanation:
The middle muscle layer of the heart is called the myocardium. It is the thickest layer and is comprised largely of specialized cells called cardiac muscle cells, or cardiomyocytes. These muscle fibers are essential for the heart's function, contracting to create the pumping action necessary to circulate blood throughout the body. The myocardium's structure is supported by a framework of collagenous fibers, which are intertwined with blood vessels that supply the heart tissue and nerve fibers that help regulate contractions. In an intricate arrangement, the muscle cells form swirling patterns that enhance the heart's efficiency in pumping blood. Cardiomyocytes and pacemaker cells work together within the myocardium to control heart rhythm and ensure effective blood flow.