Cardiac Muscle Tissue: Anatomy, Function, & Skeletal Muscles

Cardiac muscle (or myocardium) forms the thick middle layer of the heart. The three types of muscles found in the body are cardiac muscles, skeletal, and smooth muscles. The myocardium is surrounded by a thin outer layer called the epicardium and an inner endocardium.

Cardiac muscle contracts rhythmically and is not controlled voluntarily; thus, making it different from the functioning of skeletal muscles. One of the three main muscle types that are only present in the heart is cardiac muscle, also known as myocardium in vertebrates. 

Key Terms: Cardiac Muscles, Muscle Cells, Heart, Fibrillation, Cardiac, Fibers, Muscle Organization

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Cardiac Muscle Structure

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Here, let's take a closer look at the structure and function of the cardiac muscles.

Detailed Anatomy

Human heart is majorly made of cardiac muscle, commonly known as the myocardium. The outside epicardium also called the visceral pericardium, and the inner endocardium is joined to create the heart wall by a substantial layer of the myocardium. 

The endothelium that lines the blood veins that attach to the heart lines the cardiac chambers, which cover the cardiac joints and valves. While the epicardium, a component of the pericardium—the sac that envelops, protects and lubricates the heart—can be found on the myocardium's periphery.

Cardiac Muscle Cells

Cardiomyocytes, also known as cardiac muscle cells, are the pumping cells that enable the heart to beat. To effectively pump blood from the heart, each cardiomyocyte must contract in concert with its neighboring cells, creating what is known as a functional syncytium. The heart may not pump at all if this coordination fails, even when individual cells are contracting, as can happen with irregular heart rhythms like ventricular fibrillation.

Cardiac Muscle Anatomy

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T-Tubules

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The tiny tubes known as T-tubules extend from the cell's surface to its inside. These share a phospholipid bilayer structure and are one continuous membrane with the cell membrane. They are open at the cell's exterior, or extracellular fluid surface. 

T-tubules in cardiac muscle are fewer in number but wider and larger than those in skeletal muscle. They form a transverse-axial network in the center of the cell by running into and beside it. They are located close to the sarcoplasmic reticulum, the cell's internal calcium storage. In a dyad combination, a solitary tubule is paired with a terminal cisterna from the sarcoplasmic reticulum.

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What Distinguishes Skeletal Muscle from Cardiac Muscle?

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The body is moved by the contraction of skeletal muscles, which are affixed to bones. Heart muscles called cardiac muscles contract to pump blood.

Between 10 and 100 micrometers broad, skeletal muscle fibers are long and thin. With a width of 20 to 200 micrometers, cardiac muscle fibers are thicker. Unlike the heart muscle, skeletal muscle is also structured in bundles called fascicles.

When the heart needs to fill with blood, the branching architecture of the cardiac muscles permits them to do so (unlike skeletal muscles).

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Cardiac Muscle Organization

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Because it has a considerably more intricate structure than skeletal muscle, cardiac muscle is significantly distinct from the latter. In contrast to skeletal muscles, which can only contract, cardiac muscle may both contract and relax. 

As a result, heart tissue will be far more atypically shaped than skeletal muscles. The sarcoplasmic reticulum, which stores the calcium ions required for cell contraction, is connected to T tubules found in cardiac muscle cells.

The heart and the tissues connected to it are surrounded and safeguarded by the myocardium, a thin yet sturdy membrane. The myocardium, endocardium, and epicardium make up the heart wall.

All four of the heart's chambers are lined on the inside by the endocardium, which also covers the heart valves. The outside is covered by a thin coating of epicardium.

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Cardiac Muscle Disease Clinical Implications

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A collection of conditions known as cardiac myopathic illnesses have an impact on how well the heart can function. There are some of these illnesses that, if caught early enough, can be cured, but there are also some fatal diagnoses.

Cardiovascular disease refers to any chronic condition affecting the heart or blood vessels, such as those affecting the heart muscle, valves, or arteries. Congestive heart failure, hypertension, and other conditions are included in this phrase.

Diseases related to defects in the structure of the cardiopulmonary system or congenital abnormalities are often chronic. They range from common conditions like coronary artery disease to rare genetic diseases that affect many organs. Some examples include atrial septal defect (ASD) and patent ductus arteriosus (PDA).

Cardiac Muscle Disease Clinical Implications

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Things to Remember

• Cardiac muscle regulates the contractility of the heart and the pumping action. The cardiac muscle must contract with enough force and enough blood to supply the metabolic demands of the entire body. 
• Cardiac muscle tissue cells characteristics: involuntary and intrinsically regulated, branched, striated, and single nucleated.
• Cardiac muscle cells form a branched cellular network in the heart. 
• Cardiac muscles are connected by intercalated disks and are organized into layers of myocardial tissue wrapped around the chambers of the heart.
• Two types of cardiac muscle fibers: myocardial contractile cells and myocardial conducting cells (pacemaker cells).

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