Comparison of Skeletal and Cardiac Muscles: Histological Structure and Clinical Implications

Skeletal muscle is voluntary striated muscles, elastic and extensible. Its function concerned with posture maintenance and ability of voluntary movement (under the nervous system).

While cardiac muscle is involuntary striated muscle, their fibers are branched cells, cylindrical in shape, contain one or two central nuclei. Its function is to keep the heart pumping through involuntary movements.

Many diseases can affect skeletal muscles as muscular dystrophy, actin aggregate myopathy and myotubularmyopathies.

On the other hand, there are also several diseases that can influence cardiac muscles, such as myocardial infarction, cardiac hypertrophy and myocarditis.

The aim of this research is to shade more light on the histological structure of both skeletal and cardiac muscles, as well as the commonest diseases affecting them.

Histological structure of skeletal muscles

Skeletal muscle fibers under light microscope: are long, cylindrical and multinucleated as a result of fusion of myoblasts during early development. Hence, they form a structural syncytium. Nuclei are oval-shaped and located at the periphery of the cell.

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They are accompanied by satellite cells between the external lamina and sarcolemma. Thick sarcolemma, acidophilicsarcoplasm and transverse striations (in LS) are also appeared.(3,4) Skeletal muscle fibers under electron microscope: (2,4,5,6)

1. Euchromatic nucleus.
2. Sarcolemma - the membrane that surrounds the cells.
3. Terminal cisterna - extension of sarcolemma that stores calcium.
4. Sarcoplasm - the cytoplasm.
5. Golgi systems- enhance the “maturation” and trafficking of intrinsic membrane proteins.
6. Mitochondria- many rows between myofibrils, for energy production.
7. T-tubules - invaginations of sarcolemma that transmit action potentials to the inside of the muscle cell.
8. Sarcoplasmicreticulum –for the storage and release of calcium ions.
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9. Sarcomere – It is considered as the functional unit of skeletal muscle fiber, as it consists of thin and thick myofilaments; actin and myosin, respectively, in addition to other regulatory proteins. The sequential order of these myofilaments gives the striation appearance.
10. Accessory proteins- For the maintenance of the alignment of actin and myosin filaments such as titin, tropomodulin, alpha-actinin, and dystrophin. (2,4,5,6)Structure of skeletal muscle fiber

Myofibrils surrounded by three types of connective tissue sheaths, named according to their site.

1. Endomysium: reticular fibers, surround single muscle fibers. It is considered as a site of metabolic exchange.
2. Perimysium: less dense connective tissue surrounds a group of fibers to form a fascicle or bundle, which acts as the functional units of skeletal muscle tissue.
3. Epimysium: dense connective tissue, surrounds the entire collection of fascicles to form an individual muscle.(1,2,3,7)Types of skeletal muscle fibers

•  Type I - use aerobic metabolism to function; they appear red because of the high amount of myoglobin; they are slow-twitch and resistant to fatigue
•  Type IIa - get energy from oxidative glycolysis; have high amount of glycogen, they are fast-twitch and resistant to fatigue
•  Type IIb - get energy from anaerobic glycolysis, appear pink, fast-twitch and prone to fatigue.

Special internal membrane systems called triads control muscle contraction by regulating calcium release.(1,2,8)

Role of triad system in muscle contraction

Depolarization of sarcolemma by nerve impulse, then spread into muscle through T tubule, leading to calcium releases from SER which resulting in sliding of actinmyofilament over myosin, leading to contraction.(4)

Neuromuscular junction: It is a site where a motor neuron’s terminal meets the muscle fiber, to deliver contraction command; Neurotransmitter is acetylcholine.(1,2,9)

Histological structure of cardiac muscle

It is also called myocardium, involuntary muscle, of branched cylindrical cells, with one or two centrally located nuclei. Additionally, a collagenous tissue separates these fibers and strengthens the capillary network of cardiac tissue. Cross-striations are the more prominent feature of cardiac fibers, as a result of the arrangement pattern of myofilaments, which are crossed by intercalated discs.(10,11,12).Components of Cardiac muscle

Cardiomyocytes

Involve one central elongated nucleus. Within the sarcoplasm, which is eosinophilic, the cell organelles are located involving; mitochondria, Golgi apparatus, lipofuscin filled granules, and glycogen.

•  Mitochondria: for high energy requirements, they located between the myofibrils, involve multiple cristae.
•  Golgi apparatus: supports the synthesis of protein.
•  Lipofuscin: or wear and-tear-pigment which is a red-brown pigment that accumulates by aging.
•  Glycogen granules: For energy storage, they are also located between the myofibrils
•  Collagenous tissue fibers and capillaries: located between the muscle fibers for support and a blood supply.(10,11,12,13)

Intercalated discs

For joining of cardiac myocytes, coincide with Z lines. On examination by a light microscope: they appear as perpendicular lines which transverse the muscle fiber. While finger-like interdigitations appear in its ultrastructure to increase surface area of contact.(7,11,12)They involve three kinds of cell junctions:

• Fascia adherens: are a major part of the transverse component and help the visibility of hematoxylin and eosin (H&E) stained intercalated discs. They enhance the transmission of contraction force, as they considered as anchoring sites for actin filaments, and connect to the terminal sarcomere.(7,14)
• Maculae adherents or Desmosomes: are part of transverse and lateral components. They are responsible for Binding individual myocytes to one another, preventing their separation during contraction by binding intermediate filaments. (7,14)
• Gap junctions: are part of the lateral component. They allow action potentials to spread between cardiac cells which can act as a syncytium.As they permit the flow of ions between adjacent cells, leading to depolarization of the heart muscle.(7,14)

Myofibrils and sarcomeres

Myofibrils that are bundles of actin and myosin filaments. These filaments are arranged into aggregates of repeating units called sarcomeres, lies between two Z lines and are responsible for the cardiac tissue striation.(15,16)

The cytoplasmic regions in between the sarcomere branches, have many mitochondria and sarcoplasmicreticulum, which surrounds each myofibril. (10,11)

The membranous network of sarcoplasmicreticulum is transversed by T tubules, which are extensions of the sarcolemma. They form the T tubule system and their lumens are conducted with the extracellular space. A single tubule pairs with part of the sarcoplasmicreticulum called a terminal cisterna in a combination known as a diad. (17)

Cardiac conducting cells

They form the conducting system of the heart. These cells form nodes, bundles, and conducting fibers. The heart contraction is involuntary. However, their autonomy, they are not isolated from the nervous system. The sympathetic branch improves the impulse frequency, while the parasympathetic branch reduces it. (18)

Differences between skeletal and cardiac muscles.

There are many differences between skeletal and cardiac muscles, which can be illustrated in the following table.

Clinical diseases affecting skeletal muscles

Muscular dystrophy

It is a group of disorders having mutations in DNA coding for dystrophin proteins which leads to weakness of skeletal muscle. This muscular weakness may be due to two causes;1- dysfunction of neural tissue(neuropathy) or 2- dysfunction of muscle tissue (myopathy or muscular dystrophy). Some examples include Duchenne and Becker muscular dystrophy, and facioscapulohumeral muscular dystrophy.(19,20)

Actin aggregate myopathy

Severe muscle weakness and decreased muscle tone due to accumulation of actin filaments. This disease manifested by poor posture and fine motor skills and difficulty walking.Patients often do not survive past infancy because the diaphragm dysfunction.(19,21)

Myotubular (centronuclear) myopathies

It is a genetic disorder caused by a mutation in the dynamin protein, defined by a) numerous centrally placed nuclei on muscle biopsy and b) clinical features of a congenital myopathy. Additional but inconsistent histopathological features comprise a surrounding central zone either devoid of oxidative enzyme activity or with oxidative enzyme accumulation. Patients often present with paralysis of extraocular muscles.(19,22)

Clinical diseases affecting cardiac muscles

Myocardial infarction

It is caused by decreasing the oxygen and blood supply to the cardiac tissue, as a result of formation of plaques in the interior walls of the arteries resulting in reduced blood flow to the heart.( 23,24).

Cardiac hypertrophy

In which the cardiomyocytes are increased in size, having enlarged nuclei and more production of proteins. Hypertrophy has specific types according to the underline cause. Pressure overload hypertrophy: hypertension is the main cause, characterized by increasing the thickness of ventricular walls.(24,25). Volume overload hypertrophy: Returning of the blood to the heart, leading to lengthening of the myocyte and ventricular dilatation. The workload in cardiac hypertrophy is increased as a result of insufficient oxygen and blood supply leading to ischemia and even cardiac failure may be developed.(24,25)

Myocarditis

It is an inflammation of the heart muscle as a result of viral infection or autoimmune disorders. (26,27).

Conclusion

•  Skeletal muscle is voluntary striated muscles, concerned with posture maintenance and ability of voluntary movement (under the nervous system).
•  While cardiac muscle is involuntary striated muscle. Its function is to keep the heart pumping through involuntary movements.
•  There are many differences between skeletal and cardiac muscle fibers based on their site, size, branching, thickness of sarcolemma, striations, tubular system, regeneration, number and location of nuclei, the muscle action and innervation.
•  Skeletal muscles can be affected by many diseases, such as muscular dystrophy, actin aggregate myopathy and myotubularmyopathies.
•  Additionally, there are also several diseases that can influence cardiac muscles, such as myocardial infarction, cardiac hypertrophy and myocarditis.