Organization and structure of cardiomyocytes
The structural unit of the tissue is the typical cardiomyocyte, a cylindrical cell. Connecting at their ends, cardiomyocytes align into long strands — functional fibers.
To ensure the heart functions as a single whole, there are lateral branches (anastomoses) connecting parallel fibers to form an extensive three-dimensional network.
Unlike a skeletal muscle fiber (mysymplast), the cardiomyocyte nucleus (often polyploid) is located strictly in the center of the cell. This is because myofibrils occupy only about 40% of the cytoplasm volume and do not displace the nucleus to the periphery. Externally, each fiber is covered by a basement membrane that envelops only the lateral surfaces of the cells.
Intercalated discs as a key feature
The sites of end-to-end junctions between adjacent cardiomyocytes are called intercalated discs. Under light microscopy, they appear as thick transverse dark lines. Under an electron microscope, the disc is a complex set of junctions:
- Interdigitations — finger-like membrane folds that increase the cell contact area.
- Desmosomes — provide mechanical strength, securely coupling cells to prevent them from tearing apart during systole.
- Gap junctions (nexuses) — contain ion channels. Through these, excitation is instantly transmitted from cell to cell, causing them to contract synchronously.
Cardiac muscle cell populations
Although the myocardium is primarily composed of typical (contractile) cells, the tissue is heterogeneous. Three types of cardiomyocytes are distinguished:
- Typical (contractile) — perform the main pumping function, constituting the vast majority.
- Atypical — form the conduction system of the heart (generate and conduct impulses).
- Secretory — perform endocrine functions.
Energetics and metabolism
The heart operates continuously, which is why cardiomyocytes contain a huge number of mitochondria, lipid droplets, and myoglobin.
Metabolism here is exclusively aerobic. At rest, the heart is fueled by fatty acids and ketone bodies (supplied by the liver). However, during intense physical exertion, the myocardium switches to consuming lactate (produced by working skeletal muscles) and glucose.
With age, lipofuscin accumulates in the cells — an aging pigment formed in telolysosomes from incompletely digested structures.
Regeneration aspects
For a long time, it was believed that cardiomyocytes do not divide after birth. The presence of cardiac stem cells has now been proven (predominantly in the atria and at the apex), which function during the period of active body growth up to adolescence.
Nevertheless, in the adult (postnatal) state, their role is negligible. The main mechanism of adaptation to loads (e.g., in athletes) or compensation for damage (after a myocardial infarction) is intracellular hypertrophy. It increases the volume of existing cells, but hyperplasia (an increase in their number) does not occur.
Briefly: answers to topical questions
What functions do secretory cardiomyocytes perform, and in which part of the heart are they predominantly located?
Secretory cardiomyocytes perform an endocrine function and are located predominantly in the atria (prevailing in the right atrium).
They synthesize and store protein hormones in secretory granules:
- Glycoprotein — possesses anticoagulant activity.
- Natriuretic factor (ANF, ANP, natriuretic peptide) — enhances the excretion of sodium ions and water by the kidneys, leading to a hypotensive effect ( penurunan blood pressure).
Simultaneously with secretion, these cells retain the ability to contract, although their contractile apparatus is less developed than that of working cardiomyocytes.
Which structures (nodes and bundles) make up the cardiac conduction system formed by atypical cardiomyocytes?
The cardiac conduction system consists of two nodes and bundles extending from them:
- Sinus (sinoatrial) node (Keith-Flack node) — located in the upper wall of the right atrium. Three bundles extend from it into the atrial walls (sometimes called the Keith-Flack bundle).
- Atrioventricular node (Aschoff-Tawara node) — located in the lower wall of the right atrium near the septum.
- Bundle of His — extends from the atrioventricular node, enters the interventricular septum, and divides into right and left branches.
- Purkinje fibers — branches of the Bundle of His legs.
What are the structural features of the sarcotubular system (T-tubules and sarcoplasmic reticulum) in cardiomyocytes compared to skeletal muscle?
The following elements of the sarcotubular system are described in cardiomyocytes:
- T-tubules — deep invaginations of the plasmalemma surrounding the myofibrils.
- L-system / sarcoplasmic reticulum — a derivative of the smooth ER; includes L-tubules and terminal cisternae.
- The sarcoplasmic network in muscle tissues is involved in the storage of Ca²⁺ ions: upon excitation, the signal travels along the plasmalemma and T-tubules, then Ca²⁺ channels of the terminal cisternae open, and the release of Ca²⁺ into the sarcoplasm triggers myofibril contraction.
The provided sources do not indicate special differences between the sarcotubular system of typical cardiomyocytes and skeletal muscle.
Class and exam questions
Where is the nucleus located in a cardiomyocyte?
Unlike skeletal muscle tissue, in a typical cardiomyocyte the polyploid nucleus is located strictly in the center of the cell.
Why are gap junctions necessary in intercalated discs?
Gap junctions (nexuses) provide electrical coupling. Ions pass through their hydrophilic channels, allowing excitation to be rapidly transmitted and cells to contract synchronously.
Can cardiac muscle recover after a myocardial infarction?
Full regeneration with the formation of new fibers does not occur in adults. Replacement occurs through scarring, and the remaining cardiomyocytes adapt via hypertrophy (increase in volume).
What else to study on this topic
- Ultrastructure of the cardiomyocyte sarcomere (Z-lines, I- and A-bands)
- Membrane systems: T-tubules and L-system (sarcoplasmic reticulum)
- Differences between atypical and secretory cardiomyocytes
- Comparative table: cardiac and skeletal muscle tissues
- Mechanism of formation and significance of the lipofuscin pigment
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