Hello everyone! It's Megan from Kenhub here, and in today's tutorial, we'll be discussing cardiac muscle.
Cardiac muscle is a type of tissue that develops from the mesoderm during embryonic development. It develops from the cells of the primitive heart tube, aligns into chain like arrays, and has the same arrangement of its contractile filaments as seen in skeletal muscle.
As the name suggests, cardiac muscle is only found in the heart. The highly synchronized contraction of the cardiac muscle cells transforms the heart into a pump where it sends blood throughout the body. To understand cardiac muscle better, let's first look at what muscle tissue is and the different types of muscle tissue that can be found in the human body.
First of all, it's important to know that there are four basic types of tissue found in the human body. One of them is muscle tissue and the other types of tissue are epithelial tissue, connective tissue and nervous tissue. Muscle tissue is a type of tissue who cells regularly and optimize contractility. In other words, the ability to contract.
Breaking it down further, there are three types of muscle tissue – smooth muscle also known as involuntary muscle which basically means that it contracts without conscious control, skeletal muscle which is known as voluntary muscle and is usually attached to bone, and cardiac muscle which is the subject of this tutorial and is found in the heart.
All three types of muscle tissue possess actin and myosin myofilaments that generate the forces that are needed for muscle contraction. The sliding interaction of the actin and myosin filaments over one another is what results in the contraction of the muscle tissue.
As we already discussed, cardiac muscle is found in the heart and the cells that make up the cardiac muscle are known as cardiomyocytes. These cells are found in the chambers of the heart. The right and left atria and the right and left ventricles are the chambers of the heart and they contract and relax to allow blood to enter and exit the heart. Here we see a micrograph taken from cardiac tissue highlighting the cardiomyocytes. These cardiac muscle cells are normally fifteen micrometers in diameter and eighty to a hundred micrometers in length. These cells usually possess one but sometimes two centrally located nuclei as you can see on your screen.
For the most part, cardiomyocytes are branched and here we see a micrograph highlighting exactly that. Such branched cardiac muscle cells often bind to the cells in adjacent muscle fibers. It should be noted that these branched cardiomyocytes are more often seen in samples taken from patients with hypertrophic cardiomyopathy or enlarged heart muscle. Here we see a heart again. If we extract the muscle fiber out of it, we'll see that these are composed of myofibrils and these, when observed even closer, are made up of sarcomeres – sarcomeres which are the functional contractile units of myofibrils found within the cardiac muscle cells.
The sarcomeres contain actin and myosin myofilaments that are contractile proteins present in all muscle. The repeating units of sarcomeres along myofibrils are what give the cardiac muscle its striated appearance – a visual feature of altering light and dark striations observed under a light microscope.
Here you can see the striations highlighted in green. As the contractile proteins in skeletal muscle are also arranged into sarcomeres, skeletal muscle also has a striated appearance. On the other hand, in smooth muscle, the contractile proteins are not arranged into such units, therefore, giving it a smooth or non-striated appearance.
Next, we can see the cardiomyocyte nuclei highlighted in green. As you can see and as we have already mentioned, the nuclei are centrally located in the cardiomyocyte cells. This is one feature that distinguishes cardiac muscle cells from skeletal muscle cells which are typically multinucleated.
Cardiac muscle fibers possess transverse lines that stain dark during H&E staining which are known as intercalated discs and are visible under a light microscope. These discs represent the highly specialized attachments between adjacent cardiac muscle cells. Intercalated discs contain structures called gap junctions which form channels between the adjacent cardiomyocytes. These junctions serve as communicating channels between the cells that allow for ions to pass between them ensuring ionic continuity. This ionic continuity allows cardiac muscle cells to act as a single functional organ or as a syncytium.
Intercalated discs also contain desmosomes which are intercellular junctions that form strong adhesive bonds between the cells. They function to bind or adhere the cardiac muscle cells to one another preventing them from pulling apart as a result of repeated contractions. So it's worth remembering that intercalated discs and their desmosomes have an important role of providing mechanical strength and stability to the cardiac muscle as to maintain its integrity when it's subjected to mechanical stress. It's also worth mentioning that cardiac muscle consists of bundles of cardiomyocytes such as the ones seen here in this micrograph. These bundles are surrounded by a tissue called perimysium.
So far, we've seen that cardiac muscle is made up of cells called cardiomyocytes. We've also seen that this type of muscle has a striated appearance owing to the presence of contractile units known as sarcomeres. It's important to remember that cardiac muscle is an involuntary muscle that is found in the walls of the heart. It's an involuntary muscle because it's regulated by the autonomic nervous system and is not under conscious control.
As we have just mentioned, cardiac muscle makes up the walls of the heart, however, for a better understanding of cardiac muscle, it should be noted that the heart wall is actually made up of three layers – the outer epicardium, the middle myocardium, and the inner endocardium.
The epicardium is the outermost layer of the three layers that make up the heart wall. The epicardium seen here highlighted in green surrounds the heart muscle acting as a protective layer. On the outer surface of the epicardium, there is a layer of epithelial tissue made up of simple squamous epithelial cells, also known as mesothelial cells. In addition to mesothelial cells, the epicardium is also composed of a connective tissue layer.
On its outer surface, the epicardium is in contact with the pericardium. The pericardium has two layers – the fibrous pericardium and the serous pericardium. The serous pericardium is further divided into two layers – the parietal layer and the visceral layer. The parietal layer lines the fibrous pericardium and the visceral layer actually forms part of the epicardium. Both of these layers secrete fluid lubricating the heart to prevent friction during heart activity.
Between the pericardium and the epicardium is a layer of fat or adipose tissue known as epicardial fat seen here in this micrograph. On its inner surface, however, the epicardium is fused to the myocardium. The myocardium is the second layer of the heart wall. It's the thickest layer of the three layers of the heart and is made up of cardiac muscle which we should now be getting familiar with.
As we saw earlier, the fibers of the cardiac muscle are arranged into fascicles or bundles that are bound by connective tissue. The myocardium provides scaffolding for the chambers of the heart assisting in contraction and relaxation of the cardiac walls and resulting in the pumping of blood in and out of the chambers of the heart. It should be noted that the myocardium layer varies in thickness. The ventricles have a thicker layer of myocardium in their walls than the atrium. On top of this, the left ventricle is two to three times thicker than the right one as it works hard to pump blood throughout the entire body whereas the right ventricle only has to pump blood to the lungs.
The third and innermost layer of the heart wall is the endocardium. In contrast to the myocardium, the endocardium layer is the thinnest within the walls of the ventricles and thickest within the walls of the atria. This layer of the heart is directly connected to the cardiac appendages of the heart. It's lined with endothelium which is a layer of simple squamous epithelial cells on the inner surface of the heart cavities. This layer is continuous with the endothelium that lines the blood vessels of the heart.
The deep part of the endocardium is composed of loose connective tissue seen here in this micrograph. This deep layer of the endocardium fuses with the myocardium. So, basically, the subendocardial connective tissue joins the endocardium with the myocardium. It contains large cardiac muscle cells called Purkinje fibers that you can see here on your screen.
The Purkinje fibers are larger than cardiomyocytes and they contain more mitochondria. These cells are specialized for conduction and keeping in mind that the subendocardial connective tissue is closely associated to the myocardium, the Purkinje fibers are able to transfer an action potential quickly and efficiently from the endocardium to the myocardium. It should also be noted that the Purkinje fibers spread from the atrioventricular bundle of His and go on to innervate the ventricles.
Here in this micrograph we see the trabeculae carneae which are muscular columns that project from the inner surface or walls of the left and right ventricles. In this illustration, we can see the trabeculae carneae from a ventral view of the right ventricle. It's believed that the trabeculae carneae plays a similar role to that of the papillary muscles preventing the tricuspid and bicuspid valves from inverting by pulling on the chordae tendinae. By doing so, this stop blood from flowing back into the atria.
When looking at a histological preparation taken from cardiac muscle, we should be able to identify other structures that are present in this tissue type. For example, here we see a magnified picture highlighting intramuscular capillaries that can be found in the myocardium. In addition to intramuscular capillaries, intramuscular arterioles such as those seen here in this micrograph can also be found in cardiac muscle. Simply put and as the name suggests, intramuscular arterioles are just arterioles that are situated within the cardiac muscle tissue and supply with oxygenated blood. These small blood vessels branch from arteries and connect to capillaries.
Moving now to larger vessels related to cardiac muscle, we see the coronary artery. As you can see here, this is a longitudinal section of the vessel. The coronary arteries are the arteries of the heart that supply the myocardium with oxygenated blood. The coronary system is not only comprised of arteries, arterioles and capillaries which carry oxygenated blood, it also comprises of veins and venules that carry deoxygenated blood.
Here we see a micrograph showing a cardiac vein in cross-section. The cardiac veins are responsible for the return of deoxygenated blood from the myocardium to the right atrium. It then travels into the right ventricle and from there, it's pumped to the lungs for gas exchange.
When cardiac muscle doesn’t receive sufficient oxygen as a result of insufficient blood flow, this can result in damage to the cardiac muscle in the area that's not receiving enough oxygen and nutrition. The most common reason for this reduction in blood flow is a critical stenosis in one or more of the arteries that supply the cardiac muscle with blood. This in turn leads to a myocardial infarction or to put in layman's terms, a heart attack. Symptoms of heart attack include chest pain usually radiating to the left arm, nausea, shortness of breath, fatigue or a cold sweat. In some cases, patient simply feel a discomfort that feels like heartburn or a pain in the chest that travels to the back, the shoulder, the arm, the neck or even the jaw.
After a heart attack, it's imperative that the patient is admitted to hospital to receive beta blockers, nitroglycerin, aspirin, and oxygen in order to improve their prognosis. In some cases, bypass surgery may also be recommended if multiple coronary arteries are blocked.
After a myocardial infarction, the patient will have to commit to a lifestyle change in order to reduce the risks of having another cardiac event. In addition, elimination and control of risk factors such as hyperlipidemia, diabetes, smoking and obesity and treatment with statins, aspirin, anti-diabetic drugs, exercise, and weight loss are also recommended.
And that brings us to the end of our tutorial.
So far, we've discussed that cardiac muscle is one of three types of muscle tissue. We've seen that it has a striated appearance due to the presence of contractile units called sarcomeres in the cardiac muscle cells, and we've also established that the cardiac muscle cells are called cardiomyocytes and that their nuclei are located at the center of the cell. We have seen that in between adjacent cardiac muscle cells are specialized attachments called intercalated discs which have gap junctions and desmosomes for the adhesion of neighboring muscle cells.
We also discussed the three layers of the heart wall – the epicardium, the myocardium, and the endocardium. And finally, we looked at some examples of other structures that can be found in cardiac muscle such as capillaries, arterioles, and veins.
I hope you enjoyed this tutorial and now have a better understanding of the structure, composition and function of cardiac muscle. Thanks for watching and see you next time!