Myocardium

The Heart Wall

This picture often confuses students. We start from the superficialNear the surface of the body. layer and move to the deepAway from the surface of the body., open cavity of the heart. The first membrane surrounding the heart is the fibrous pericardiumThe membrane surrounding the heart.. It secures the heart in place in the mediastinum.  The parietal pericardium and the visceral pericardium, also called the epicardium, are a serous membrane. There is a fluid-filled cavity between them called the pericardial cavity. 

This fluid filled cavity is meant to reduce friction. As with all serous membranesThin tissues that line body cavities and secrete fluid., it helps as the heart expands and contracts. If inflamed, this space limits the heart’s ability to expand.  You can also accumulate fluid in this space, a condition called cardiac tamponade.

The Myocardium

If we were to take a transverse section A cut or slice of the body or an organ for study. of the heart through the ventricles, we would see this picture here.  All this pink is the myocardium.  This is the contractile layer of the heart that contains the cardiac muscle fibers called cardiomyocytes.  The myocardium of the left ventricle is perfectly circular with a very thick myocardium.  The right ventricle myocardium is less thick and only semi-circle in shape. 

The left ventricle needs to generate enough force to get blood to your big toe.  Therefore, the left ventricle, with the thicker myocardium generates more force.  This make sense because the right ventricle only has to force blood into your lungs, which are right there.  Remember, the same volume is ejected from the heart, but at different pressures.

Cardiac Muscle

Recall from learning about tissues that there were three types of muscle tissue: smooth, cardiac, and skeletal.  Cardiac and smooth muscle were both involuntarily controlled compared to skeletal muscle that has voluntary control.  Cardiac and skeletal muscle were striated muscle. This striation occurred due to the arrangement of actin and myosin in sarcomeres compared to smooth muscle.   And, finally, cardiac muscle fibers had this branched cylinder appearance. This was compared to the cylinders of skeletal muscle. The spindles of smooth muscle also differed.  Although difficult to recognize, you can see the branching of the cardiomyocytes in this histological picture.  The sarcolemma or cell membranes are running up and down and branching at a few points.  Running side to side on these pictures are the striations that are barely able to be seen with student microscopes.

Functional Synctium

Functional syncytium is a concept that describes how the cardiomyocytes contract in a coordinated fashion.  They don’t all contract together.  If they did that, blood would flow no where.  They contract so that the atria squeeze blood into the ventricles. Then, the ventricles squeeze blood into the arteriesBlood vessels that carry oxygenated blood away from the heart (except pulmonary arteries, which carr. These two main events can’t happen together. They have to happen one after the other. After the one, after the other, after the one, and…you get it.  Let’s say that you and I are cardiomyocytes that touch each other.  I’m in the atria and you’re in the ventricle. I can’t contract until you are finished with your action potentialA rapid, temporary electrical charge that travels along neurons, allowing signal transmission.. You can’t contract until I am finished with my action potential.  We both have to wait for the other to coordinate our contractions. We are attached by corrugated edges of our cellsThe basic structural and functional units of life. that are kinda like these egg cartons. This allows for a greater strength of attachment. It also allows you and me to share more surface area at our borders. 

Along the border of cardiomyocytes are structures called intercalated discs Structures in cardiac muscle that allow electrical connectivity.. They perform two functions. First, they allow us to bind tightly to each other. Second, they allow us to share cytoplasmThe gel-like substance within a cell that contains organelles and cytosol..

Intercalated Discs

This is a drawing of a short, fat, branched muscle fiber or muscle cell. It is connected to another branching cardiac muscle cell. That connection is not just handholding. It’s similar to the episode of Family Guy where Stewie and Brian had their hands pasted together. An intercalated disk is genius. It has these things called desmosomes. Think of desmosomes as latches. What they do is they secure the cells to each other. The desmosomes are all these fibers and latches connecting the cells together, preventing separation.  This green structure between the desmosomes is a gap junction.  This allows sharing of the cytoplasm of these cells.  When one of these cells depolarizes, it allows an influx of sodium(Na⁺): Major ECF cation; important for fluid balance, nerve function. cations. These ionsCharged atoms or molecules. will flow through the gap junctions. They will depolarize the next cell, followed by the next, and so on.  This coordination of contraction creates functional synctium.