Physiology of a Cardiac Myocyte

Understanding the cellular process.

The heart is a curious machine. Even though it contains the same kinds of cells that are found in other muscles, the cardiac myocyte sets it apart from other body tissues and structures. Cardiac myocytes are a special kind of cell that, through various means, keeps the body's most important pump supplied with electricity. An examination of the physiological processes within the cardiac myocyte reveals both its purpose and sophistication.

Basement Membrane

A basement membrane wraps the cardiac myocyte. It provides a means of uniting the extracellular space with the intercellular cytoskeleton by adhering to the sarcolemmic membrane. Through a molecular process that forms on its surface, the basement membrane provides a means of adhesion to the sarcolemmic membrane while keeping the internal cell structures bound.

Fibrillar Collagen

Collagen, by structure, is much like a rope. It is a form of connective tissue. Fibrillar collagen is attached to the basement membrane in a regularly occurring pattern. Physiologically, the fibrillar collagen is self-perpetuating by assembling itself in a string of proteins and amino acids. Fibrillar collagen connects in the extravascular space to provide cell stability.

Sarcolemma

Sarcolemma is a cellular membrane, much like the basement membrane to which it is attached. However, the difference between the two is that the sarcolemmic membrane acts as a conductor for electrical stimuli passing through muscle cells. This is especially important for cardiac muscle. The sarcolemmic membrane also acts as an attachment point for collagen fibrills.

Mitochondria

Mitochondria are specialized organelles directly responsible for several activities within cells--all dealing with how nutrients cross the cellular membrane and are then synthesized and transformed into energy. These organelles provide energy to cells through cellular respiration, a means of converting glucose into a substance called adenosine triphosphate (ATP). ATP is the chemical basis for the metabolic processes needed to sustain life.

Contractile Apparatus

The contractile apparatus of the myocyte contains a component known as a sarcomere. Sarcomeres are abundant in skeletal muscle tissue as well as cardiac tissue. The emblematic striation bands associated with skeletal and cardiac muscles are caused by the appearance of the contractile apparatus. The contractile apparatus also plays a key role in the transference of calcium between the extravascular space and the cytoskeletal intercellular space, primarily to the sarcoplasmic reticulum.

Sarcoplasmic Reticulum

The sarcoplasmic reticulum is a specialized form of endoplasmic reticuli. Like the contractile apparatus, the sarcoplasmic reticulum within cells plays a part in utilizing calcium as a means of readying the myocytes for contraction. Also like the contractile apparatus, chemical chains of molecular transfer are a vital component in cardiac contractility, specifically from the sarcoplasmic reticuli to the transverse tubules.

Transverse Tubules

Commonly referred to as T-tubules, the transverse tubules aid in contraction and in transferring calcium from the sarcoplasmic reticulum. Myocyte excitation, primarily though calcium-sensing mechanisms, is directly related to the ability of the T-tubules to aid in transferring energy once the excitability threshold is reached and the sinoatrial (SA) node releases the pace-making impulse responsible for myocardial contraction. This information is important to understanding the relationship between cardiac myocyte physiology and how it interacts with the electrophysiological impulse centers of the heart.

References

• Muscular Contraction Biomechanics: General Anatomy
• Georgia State University: Mitochondria
• Nature: A Calcium Sensor in the Sodium Channel Modulates Cardiac Excitability
• European Bioinformatics Institute: Sarcolemma
• The Journal of Thoracic and Cardiovascular Surgey: The Structure and Function of the Cardiac Myocyte

Resources

• European Bioinformatic Institute