Cardiac muscle tissue is an extremely specialized form of muscle tissue that has evolved to pump blood throughout the body. In fact, cardiac muscle is only found in the heart and makes up the bulk of the heart’s mass. The heart beats powerfully and continuously throughout an entire lifetime without any rest, so cardiac muscle has evolved to have incredibly high contractile strength and endurance. And because the heart maintains its own rhythm, cardiac muscle has developed the ability to quickly spread electrochemical signals so that all of the cells in the heart can contract together as a team.
Cardiac muscle tissue is made up of many interlocking cardiac muscle cells, or fibers, that give the tissue its properties. Each cardiac muscle fiber contains a single nucleus and is striated, or striped, because it appears to have light and dark bands when seen through a microscope. The dark bands represent areas of thick protein filaments made of myosin proteins that block light passing through the cell and appear dark. Between the dark bands are thin filaments made of actin protein that allow light to pass through and appear light. When the muscle fibers contract, myosin pulls the actin filaments together like an accordion to shrink the muscle cell and make it contract. While each cell is not very strong by itself, millions of cardiac muscle cells working together are easily able to pump all of the blood in the body through the heart in less than a minute.
Cardiac muscle cells have a branched shape so that each cell is in contact with three of four other cardiac muscle cells. Together all of the cardiac muscle cells in the heart form a giant network connected end to end. At the ends of each cell is a region of overlapping, finger-like extensions of the cell membrane known as intercalated disks. The intercalated disks form tight junctions between the cells so that they cannot separate under the strain of pumping blood and so that electrochemical signals can be passed quickly from cells to cell. The passage of signals from cell to cell allows cardiac muscle tissue to contract very quickly in a wave-like pattern to effectively pump blood throughout the body.
Another feature that is unique to cardiac muscle tissue is autorhythmicity. Cardiac muscle tissue is able to set its own contraction rhythm due to the presence of pacemaker cells that stimulate the other cardiac muscle cells. The pacemaker cells normally receive inputs from the nervous system to increase or decrease the heart rate depending on the body’s needs. However, in the absence of nervous system stimulation, the pacemaker cells can produce a regular heart rhythm.