Cardiac muscle under microscope | Image + structure

Types of muscles

There are three types of muscle tissues in the body. They are:

• Skeletal muscle
  • Striated, under voluntary control
  • Found in muscles attached to the skeleton
• Smooth muscle
  • Non-striated, under involuntary control
  • Located in the walls of hollow visceral organs such as liver, pancreas and intestine
• Cardiac muscle
  • Striated but involuntary control
  • Located in the walls of heart

Cardiac muscle is a special type of involuntary muscle present in the wall of the heart. These cells are also known as myocardium in vertebrates. These muscles cannot move or contract at will. But structurally they are striated and branched like voluntary muscles.

Cardiac muscle structure

Cardiac muscle is found only in the heart. The contractions and expansions occur continuously in a certain rhythm from birth to death of the animal. Cardiac cells are cylindrical, branched and relatively short. They are 0.8 mm in length and 12-18 mm in diameter.

The wall of the heart consists of three layers. The outer most layer of heart wall is called epicardium. Epicardium forms part of the pericardial sac that surrounds, protects as well as lubricates the heart. The inner layer is called endocardium which covers the cardiac valves and forms a connection between blood vessels and the heart. The middle layer is known as myocardium which is sandwiched between the inner endocardium and outer epicardium. Myocardium is a thick layer.

Intercalated discs are present at the junctions of one cell with another along the length. Intercalated discs are unique to cardiac cells and can only be found in cardiac cells. They are a complex structure that connect adjacent cardiac muscle cells. Intercalated discs allow the rapid transmission of electrical impulses through the network. Under light microscope, intercalated discs can be observed as thin, dark staining lines that divide adjacent muscle cells. They run perpendicular to the direction of muscle fibers. But They are more complex when seen under electron microscope.

Each cell in cardiac muscle contains only one nucleus. They contain all the cells genetic material. Cardiac muscle also contains a special type of cells called Purkinje fibers for rapidly conducting electrical signals.

Cardiac muscle contains Gap Junctions which are also the part of intercalated discs. When a cardiac cell is stimulated to contract, gap junction functions by transferring the stimulation to the next cardiac cell which allows the muscle to contract in a coordinated way.

Like gap junctions, there are also Desmosomes that are found within the intercalated discs too. They hold the cardiac muscle fibers together when a contraction happens.

The contraction capacity of these muscles is relatively fast and never fatigues.

Cardiac muscle under microscope

Here are some pictures of cardiac muscle under microscope.

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