Difference Between Myofibrils and Sarcomeres

The key difference between myofibrils and sarcomeres is that myofibrils are the contracting units of muscles while sarcomeres are the small repeating units of the myofibril.

Skeletal muscles consist of myofibrils. They form from myocytes. The repeating unit of the skeletal muscle is myofibril. Similarly, the repeating unit of myofibril is the sarcomere. Sarcomere functions during the contraction and relaxation of muscles. They are active during movements and less active at sedentary conditions.

CONTENTS

1. Overview and Key Difference
2. What are Myofibrils 
3. What are Sarcomeres
4. Similarities Between Myofibrils and Sarcomeres
5. Side by Side Comparison – Myofibrils vs Sarcomeres in Tabular Form
6. Summary

What are Myofibrils?

Myofibrils are the structural units of muscle cells. They are rod-shaped structures. Myocytes give rise to myofibrils. Myogenesis is the process of forming muscle tissue and myofibrils. It takes place during embryonic development. Different proteins, including actin, myosin and titin, form myofibrils. However, actin and myosin play the main structural role in myofibrils. Myofibrils also have accessory proteins that bind main proteins together.

Figure 01: Myofibrils

There are two types of myofilaments in myofibrils. They are thin and thick myofilaments. The thin filaments are actin filaments while the thick filaments are myosin filaments. They arrange into repeating units called the sarcomere, which functions during skeletal and cardiac muscle contraction. Thus, the key function of myofibrils is to facilitate contraction of muscles with the help of calcium, troponin and tropomyosin. It takes place via the transmission of a nerve impulse. ATP is vital for this process. Thus, the muscle contraction is an energy-consuming process.

What are Sarcomeres?

Sarcomeres are the repeating units of myofibrils. They structurally provide the striated appearance to the skeletal as well as cardiac muscles. Sarcomeres have two important protein filaments that slide past each other during relaxation and contraction. They are the actin filaments and the myosin filaments. In the sarcomere, the actin filaments form thin bands, and the myosin filaments form thick bands. They are only present in cardiac and skeletal muscles and are absent in smooth muscles.

Figure 02: Sarcomere

Each sarcomere in the myofibril is separated from each other at the Z line. The Z line is the anchorage point for actin filaments. Next to the Z line, the I band is present. Here, the I band only consists of thin filaments. It does not have any superimposed thick filaments. The A band lies next to the I band. The A band only consists of thick filaments as well as thin filaments. Following the A band, the H zone is the zone that consists only of thick filaments. The M line within the H zone forms cross-connecting elements of the cytoskeleton. Thus, the main function of the sarcomere is the facilitate muscle contraction and relaxation in skeletal and cardiac muscles.

What are the Similarities Between Myofibrils and Sarcomeres?

• Both consist of proteins, such as actin and myosin.
• They are important in muscle contraction.
• Both require Calcium and ATP for the process of contraction.
• Moreover, they are important in movement and locomotion.

What is the Difference Between Myofibrils and Sarcomeres?

Myofibrils are the structural units of the muscle cells, whereas sarcomeres are the structural units of the myofibrils. So, this is the key difference between myofibrils and sarcomeres. Also, although they perform the same function, there is a difference between myofibrils and sarcomeres in their appearance under the electron microscope.

Moreover, myofibrils are generalized to all three types of muscle cells, while sarcomeres are only found in cardiac and skeletal muscles. Further, the presence of sarcomeres provides a striated appearance in these muscles.

The below infographic presents more information regarding the difference between myofibrils and sarcomeres.

Summary – Myofibrils vs Sarcomeres

Myofibrils are the structural and functional units of the muscle fibre that form the muscle. In skeletal and cardiac muscles, the sarcomeres form cross striations, which are also the repeating units of the myofibril. Together, they perform the process of muscle contraction and relaxation with the help of calcium, ATP and other binding proteins. Their actions are neurogenic. Thus, this summarizes the difference between myofibrils and sarcomeres.

Reference:

1. “Muscle Cell Structure and Physiology .” BIOL 237 Class Notes – Muscle Cells & Muscle Physiology, Available here.

Image Courtesy:

1. “Blausen 0801 SkeletalMuscle” By BruceBlaus. When using this image in external sources it can be cited as:Blausen.com staff (2014). “Medical gallery of Blausen Medical 2014”. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. – Own work (CC BY 3.0) via Commons Wikimedia
2. “Sarcomere” By User:Sameerb – en:WP; Author User:Sameerb in English WP, Copyrighted free use) via Commons Wikimedia

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