What is the Difference Between Myofibrils and Myofilaments

The main difference between myofibrils and myofilaments is that myofibrils are elongated contractile treads that occur in striated muscles, whereas myofilaments are small structures in myofibrils, occurring in thick and thin forms. 

Myofibrils and myofilaments are two structures that occur in striated muscles. They are important in muscle contraction and locomotion. 

Key Areas Covered

1. What are Myofibrils
– Definition, Structure, Function
2. What are Myofilaments
– Definition, Structure, Function
3. Similarities Between Myofibrils and Myofilaments
– Outline of Common Features
4. Difference Between Myofibrils and Myofilaments
– Comparison of Key Differences

Key Terms 

Myofibrils, Myofilaments

What are Myofibrils

Myofibrils are the main rod-like structures of the muscle cells in muscles. The diameter of a myofibril is 1-2 micrometers. Myofibrils are synthesized during embryonic development. Also, myogenesis is the process of forming myofibrils. Myofibrils are made up of two types of proteins. They are long proteins, including actin, myosin, titin, and associated proteins that hold long proteins.

Figure 1: Muscle Fiber Structure

Furthermore, sarcomere is a subregion that occurs on myofibrils. It has a very dark color band as boundaries called Z-lines. Also, it composes of I bands and A bands. I bands contain thin actin filaments, while A bands contain actin and myosin filaments. The H band occurs in the middle of the A band. Muscle fibers are in a relaxed state in the H band. 

Moreover, muscle contraction is the main function of the myofibrils. Before contraction, the myosin head contains ADP and phosphate attached to it. With the arrival of the nerve impulse, Calcium ions change the structure of troponin and move the troponin + tropomyosin complex away, opening the myosin binding sites. Now, myosin heads form cross-bridges within actin filaments, releasing ADP. Then, the myosin head disconnects from actin, grabbing ATP. It releases the phosphate from ATP storing the energy for the later contraction. 

What are Myofilaments

Microfilaments are the protein filaments that occur in the myofibrils of the muscle fiber. The main function of myofilaments is contraction. Actin, myosin, and titin are the three types of proteins that occur in myofilaments. Also, actin and myosin are contractile proteins, while titin is an elastic protein. Importantly, there are three types of myofilaments. They are thin filaments, thick filaments, and elastic filaments. Additionally, myofibrils occur in skeletal and heart muscles.

Figure 2: Myofilament Structure

• Thick Filaments – Thick filaments are made up of myosin. The diameter of the filament is 15 nm. Myosin molecule comprises a double-globular head and a tail of two interwind chains. M region occurs in the middle of the head. 
• Thin Filaments – Thin filaments are made up of actin. The diameter of the thin filament is 7 nm. They compose of F-actin filaments. The F-actin filament comprises a string of globular G-actin, whose active site binds to the head of the myosin. In the relaxation of the muscle, approximately 40-60 tropomyosin molecules block the active sites. Importantly, all the thin filaments are bound to Z-line. 
• Elastic Filaments – Elastin filaments are made up of titin. The diameter of the elastic filament is 1 nm. They run through the core of the thick filament, anchoring to the Z-line. Also, they stabilize the thick filament, preventing overstretching the thick filament. They allow the thick filament to recoil like a spring. 

Similarities Between Myofibrils and Myofilaments

• Myofibrils and myofilaments are two components of striated muscles.
• They are important for contraction and locomotion.

Difference Between Myofibrils and Myofilaments

Definition

Myofibrils refer to any of the elongated contractile threads found in striated muscle cells, while myofilaments refer to threadlike structures that comprise the myofibril inside the muscle cell (or muscle fiber). 

Anatomy

Myofibrils are made up of thick and thin myofilaments, giving the muscle a striped appearance, while myofilaments are threadlike structures inside myofibrils.

Function

Myofibrils produce muscle contraction and relaxation, while myofilaments are the key molecular regulators of contraction.

Conclusion

In brief, myofibrils and myofilaments are two structural components of muscles. Their main function is muscle contraction and locomotion. Myofibrils are elongated contractile threads occurring in striated muscles. They are made up of thick and thin myofilaments. Their function is to contract and relax muscles. In comparison, myofilaments are threadlike structures occurring in myofibrils. They are the key molecular regulators of muscle contraction. Therefore, the main difference between myofibrils and myofilaments is their structure and function.   

References:

1. 15: Muscle contraction and locomotion – skeletal muscle fibers. Biology LibreTexts.

Image Courtesy:

1. “Blausen 0801 SkeletalMuscle” By BruceBlaus – Own work (CC-BY 3.0) via Commons Wikimedia
2. “Myofilament” By Mikael Häggström. – Own Work (Public Domain) via Commons Wikimedia