Difference Between Fibrils and Fibers

The key difference between fibrils and fibers is that myofibrils are long cylindrical structures that lie within the muscle fiber while muscle fibers are muscle cells that are long, cylindrical multinucleated cells.

The three types of muscles are smooth muscle, cardiac muscle and skeletal muscle. Muscle cell or muscle fiber is the basic unit of skeletal muscle. Myofibril is the basic rod-like unit of a muscle fiber. A muscle fiber contains thousands of muscle fibrils running the entire length of a muscle fiber. Therefore, myofibrils lie within the muscle fiber.

CONTENTS

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

What are Fibrils?

Muscle fibrils or myofibrils are the basic repeating unit of the skeletal muscle. Therefore, they are the structural units of muscle cells. They are rod-shaped structures. The myocytes give rise to myofibrils. Myogenesis is the process of forming muscle tissue and the myofibrils during embryonic development. There are different proteins that form myofibrils. They are actin, myosin and titin. However, actin and myosin play the main structural role in myofibrils. The myofibrils also have accessory proteins that bind the main proteins together.

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 are arranged into repeating units called the sarcomere, which functions during skeletal and cardiac muscle contraction. Thus, the key function of the myofibrils is to facilitate the 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. Therefore, muscle contraction is an energy-consuming process.

What are Fibers?

A muscle has bundles of tubular muscle cells or muscle fibers. Bundles of muscle fibers are known as fasciculi. A bundle of muscle fibers is protected with a connective tissue known as perimysium. There are many muscle fibers inside the perimysium. Each fascicle contains 10 to 100 muscle fibers. Large strong muscles have a large number of muscle fibers in each bundle. Smaller muscles contain fewer numbers of muscle fibers in the fascicle. Each muscle fiber is covered with a fibrous connective tissue called endomysium. The diameter of muscle fibers can range from 10 to 80 micrometers, and they can be extended up to 30 cm in length.

The muscle fiber is composed of numerous rod-like units or cylindrical organelles called myofibrils. Each muscle fiber contains hundreds to thousands of myofibrils which are bundles of myosin and actin proteins that run through the length of the muscle fiber. These myofibrils are important in muscle contraction.

What are the Similarities Between Fibrils and Fibers?

• Myofibrils run the entire length of the muscle fiber.
• They are long cylindrical structures.
• Both myofibrils and muscle fiber are important in muscle contraction.
• They run parallel in a muscle.

What is the Difference Between Fibrils and Fibers?

Muscle fibrils are the rod-like basic structural units of a muscle fiber, while muscle fiber is a long cylindrical multinucleated muscle cell. So, this is the key difference between fibrils and fibers. Moreover, fibrils lie within the fibers. Therefore, fibrils are smaller than fibers. Besides, structurally, a myofibril is a cylindrical organelle, while a muscle fiber is a cell with a nucleus and other organelles, including mitochondria. Thus, this is also a significant difference between fibrils and fibers.

The below infographic summarizes the differences between fibrils and fibers in tabular form.

Summary – Fibrils vs Fibers

Myofibril or muscle fibril is a rod-like structure that lies within the muscle fiber. It is composed of two types of myofilaments called thin and thick filaments. Muscle fiber is a muscle cell composed of thousands of myofibrils. Muscle fiber is a multinucleated long cylindrical cell. Both myofibrils and muscle fiber are cylindrical in shape. They are responsible for muscle contraction. Thus, this is the summary of the difference between fibrils and fibers.

Reference:

1. “Types of Muscle Tissue and Fibers.” Lumen, Available here.
2. “Myofibril.” Wikipedia, Wikimedia Foundation, 2 Oct. 2020, Available here.

Image Courtesy:

1. “Blausen 0801 SkeletalMuscle” By 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. “1022 Muscle Fibers (small)” By OpenStax – (CC BY 4.0) via Commons Wikimedia