HEART MUSCLE

A type of muscle found only in the walls of the heart; it is under control of the autonomic nervous system.

Cardiac muscle is like skeletal muscle in that it is striated and multinucleate, and like smooth muscle in that the nuclei are centrally located and many cells are required to span the length of the muscle. It differs from both skeletal muscle and smooth muscle in that its cells branch and are joined to one another via intercalated disks. Intercalated disks allow communication between the cells such that there is a sequential contraction of the cells from the bottom of the ventricle to the top. This makes possible the maximal ejection of blood from the ventricle during contraction and occurs without nervous innervation to each cell or group of cells.

Cardiac muscle also differs from the other two muscle types in that contraction can occur even without an initial nervous input. The cells that produce the stimulation for contraction without nervous input are called the pacemaker cells.

How cardiac muscle contracts?

Action potentials initiate the contraction of our hearts. The contraction of cardiac muscle is triggered in a different fashion then contraction of the skeletal muscle, which is caused by the electrical impulses from motor nerve terminals located at individual muscle fibers. These motor nerve terminals are networked and coordinated by the central nervous system.

Contrary to the skeletal muscle cells, the heart cells have no motor nerve terminals attached to them. If each of the millions of cardiac muscle cells were to have a separate nerve, the heart would be a huge organ. The cells of the heart remedy this lack of motor nerve terminals by using their own type of stimulus. They communicate with each other directly by passing electrical impulse to their neighbors through special channels called gap-junctions. This enables the heart to be stimulated at only one location, with the electrical impulse passing on to the rest of the heart muscle, causing it to contract.

What exactly happens to the muscle cells when they are stimulated?

Inside each cardiomyocyte are hundreds of myofibrils which are thin, elongated structures. Each myofibril, in turn, consists of thin filaments and thick filaments. Each of the thin filaments is composed of a protein called actin. Each of the thick filaments is composed of a protein called myosin. Each myosin filament is composed of about 200 myosin molecules. Each myosin molecule contains what is called a myosin head. Inside each cardiomyocyte there are compartments filled with calcium. The action potential causes these compartments to release the calcium into the cell. This calcium allows myosin heads to bind to actin filaments and pull them by a process called a power stroke. That is how action potential causes the individual muscle cells to contract.