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Sliding filament theory is the currently accepted mechanism behind how muscles contract. It tells us that muscles shorten or lengthen when the two filaments, known as actin and myosin, slide past each other. The filaments slide instead of changing length, which is what was previously thought.

The topic of sliding filament theory is covered in all the main personal training certifications. If you’re preparing for your first fitness certification exam, be sure to check out this cheat sheet from Trainer Academy.

As a budding fitness professional, mastering the sliding filament theory is essential for acing your exercise certification. This comprehensive guide unpacks one of the most fundamental concepts in muscle physiology, ensuring you grasp the intricate dance of actin and myosin filaments that power every muscle contraction. By the end of this article, you’ll not only understand the core principles of the sliding filament theory but also be equipped to apply this knowledge in your fitness career. Let’s dive into the microscopic world of muscle mechanics.

The sliding filament theory, developed in the 1950s and changed the way we looked at and understood how muscle cells operate, provides a detailed explanation of the interaction between actin and myosin filaments in muscle fibers. 

As a fundamental concept in muscle biology, the sliding filament theory elucidates how actin and myosin filaments help generate muscular contractions. 

Structure of Muscle Cells

Inside each muscle fiber are between 50 and 2000 myofibrils, long rod-shaped cells.

Each myofibril has subunits called sarcomeres. 

A sarcomere is the term for a basic contractile unit of muscle. There are thousands of sarcomeres inside most muscle cells, but a sarcomere is the smallest unit that has the ability to contract.

Inside each sarcomere are many parallel filaments. The two types of protein filaments in muscle tissue are actin and myosin. Actin is thin and myosin is thick. The actin filament is composed of individual actin molecules that form a helical arrangement. The myosin filament, on the other hand, consists of long, tail-like structures with globular heads that extend towards the actin filament during muscle contraction. 

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Now letโ€™s look inside the sarcomere. On each side of it are two dark lines called z-lines (also called z-discs, z-bands, or intermediate discs). 

The z lines mark the boundaries of the sarcomere, the fundamental unit of muscle contraction. 

Between each z-line are two bands: the a-band and the i-band. In a relaxed state, the a-band is where the myosin molecule lives. The i-band is where the actin lives. Also, within the a-band is an area known as the h-zone. 

This illustration taken from the NASM personal training book should give you a visual framework to look at. 

Visual graphic showing the inside of a muscle from the NASM textbook

In this diagram, you can see the inside of the muscle as well as a further look into the sarcomere and the filaments that slide across one another.

In this diagram, you can see the inside of the muscle as well as a further look into the sarcomere and the filaments that slide across one another.

This other page, taken from the NSCA CPT textbook, illustrates the a-band and i-band elements more closely. 

NSCA muscle cell diagram

Sliding Filament Theory Mechanism

The sliding filament theory explains how muscle cells generate force and movement through the interaction of actin and myosin within the sarcomeres.   

When the muscle contracts, the i-band shortens but the a-band stays constant. This means the actin and myosin overlap and the h-zone region disappears as the two filaments slide over each other. 

This overlapping creates muscle tension as the sarcomere shortens. During the shortening process, the myosin heads attach to the actin filaments, forming cross-bridges, and utilize ATP energy to pull the actin filaments towards the center, resulting in the overall shortening of the sarcomere and muscle contraction. 

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Cross-Bridge Theory

The myosin filament heads extrude and form cross-bridges with the actin filaments when the distance between the two of them decreases. When the myosin head and the actin bind together, they use the chemical energy gained from the breakdown of ATP to create a pulling force against each other. Then they detach and get ready to re-bind. This process of binding, creating force, and unbinding is called the cross-bridge cycle.

The cross bridges, made up of the myosin heads, play a crucial role in the sliding filament theory by attaching to the actin filaments, generating force, and then detaching to allow for continuous muscle contraction. 

Role of Calcium in Muscle Contraction

ATP provides the energy for action, but calcium ions play an important secondary crucial role in sliding filament theory. Calcium helps two proteins, troponin and tropomyosin. These proteins block the binding of myosin to actin. In a muscle contraction, calcium helps tropomyosin rotate around actin filaments to expose the myosin-binding sites. 

Without the presence of calcium there would be no muscle contractions. Both calcium and ATP are required for muscle contraction. 

When a muscle is stimulated to contract, an action potential travels down the motor neuron and reaches the neuromuscular junction, where it triggers the release of acetylcholine. Acetylcholine then binds to receptors on the muscle fiber, causing depolarization and the propagation of an action potential along the sarcolemma and into the T-tubules. This action potential activates voltage-gated calcium channels located on the sarcoplasmic reticulum (SR), a specialized network of membranes within the muscle fiber. The opening of these channels allows calcium ions to flow from the SR into the cytoplasm of the muscle fiber.

This influx of calcium ions triggers a cascade of events, including the activation of the sliding filament theory, where the myosin heads undergo hydrolysis of ATP to ADP and phosphate, providing the necessary energy for contraction. 

Sliding Filament Theory Historyโ€‹โ€‹

Two groups of scientists (Hugh Huxley and Jean Hanson, in addition to Andrew Huxley and the German scientist Rolf Niedergerke) proposed the sliding filament model in 1954 in two separate papers, providing a pioneering step in scientific research that led the way for future discoveries regarding muscle tissue. 

Before the 1950s very little was known about muscle structure at a deep level save that striated muscles had complicated repeating patterns of bands and lines. Scientists knew that actin and myosin existed, but the theories behind why muscles contracted varied. 

Some thought that the a-bands attracted each other or that the a-bands swelled during muscle activation, filling up with fluid. There was no consensus. 

More powerful X-ray technology took a while to be developed, so you could view more detail under a microscope. In 1953, the double helix of DNA was discovered. 

A year later, in 1954, two groups of scientists both named Huxley (Andrew and Hugh) discovered that when stretching a relaxed muscle, the length of the A-band remained constant while the sarcomere length changed.

Both these scientists, along with Jean Hanson and Rolf Niedergerke, developed the theory of sliding filaments based on their x-ray observations of muscle. Building upon their x-ray observations of muscle, Jean Hanson, Rolf Niedergerke, Andrew Huxley, and Hugh Huxley utilized electron microscopy to delve deeper into the intricacies of the sliding filament theory. 

The groundbreaking x-ray diffraction technique enabled the scientists to analyze the structural changes that occur in muscle during contraction, providing crucial evidence to support their sliding filament theory. 

Hugh Huxley later published the theory of cross bridging in a 1969 article called โ€œThe Mechanism of Muscular Contraction.โ€ 

Although Huxley’s article provided a foundation for understanding muscle contraction, recent advancements in technology and the accumulation of new data have allowed scientists to further refine and expand upon the sliding filament theory. 

Summary

The sliding filament model, proposed in 1954 by Hugh Huxley and Jean Hanson, revolutionized our understanding of muscle contraction and laid the foundation for the sliding filament theory. Andrew Huxley and Rolf Niedergerke’s additional studies contributed to what we know now. 

Their research paved the way for the development of the sliding filament theory, which elucidates the mechanism of muscle contraction through the interaction of actin and myosin filaments, where myosin heads undergo a power stroke, leading to the sliding of filaments and subsequent muscle shortening.

The sliding filament theory provides a crucial explanation of muscle contraction at the molecular level, as it suggests that the interaction between myosin and actin filaments occurs within the muscle cell. 

The sliding filament theory suggests that during muscle contraction, the myosin heads attach to the actin filaments in specific regions, forming cross-bridges that undergo a series of coordinated movements. 

By unraveling the intricacies of the sliding filament theory, scientists have been able to gain valuable information about how muscle contractions occur and the role of proteins in this process. 

Sliding Filament Theory Frequently Asked Questions (FAQs)

What is the sliding filament theory?

The sliding filament theory describes how muscles contract by the actin (thin) and myosin (thick) filaments sliding past each other, shortening the sarcomere without changing the length of the filaments themselvesโ€‹.

Is the sliding filament theory universally accepted in muscle physiology?

Yes. The sliding filament theory, a cornerstone of muscle physiology, explains muscle contraction. Supported by numerous studies, including electron microscopy, it visualizes the sliding motion of filaments. While research continues to refine molecular processes, the theory remains fundamental in understanding muscle function.

Does the sliding filament theory apply to all types of muscles?

Absolutely! The sliding filament theory is a fundamental concept in muscle physiology that explains how muscles contract. This theory applies to all types of muscles, including skeletal, cardiac, and smooth muscles. While there may be some variations in the specific mechanisms and regulatory factors involved in each type of muscle, the underlying principle of filament sliding remains consistent across all muscle types.

What are the steps involved in the sliding filament theory?

First, when a muscle receives a signal from the nervous system, calcium ions are released into the muscle fibers. These calcium ions bind to troponin, causing it to change shape and expose binding sites on actin filaments. Next, myosin heads attach to these exposed binding sites on actin, forming cross-bridges. As ATP is hydrolyzed, the myosin heads undergo a conformational change, pulling the actin filaments towards the center of the sarcomere. This movement is known as the power stroke.

What diseases or conditions are associated with abnormalities in the sliding filament theory?

Abnormalities in the sliding filament theory, crucial for muscle contraction, are linked to diseases like muscular dystrophy and myasthenia gravis. Muscular dystrophy involves protein defects, such as dystrophin, causing muscle weakness. Myasthenia gravis, an autoimmune disorder, affects neuromuscular transmission and muscle function.

What term is used to describe the movement of actin and myosin in a muscle contraction?

The term used to describe the movement of actin and myosin in muscle contraction is “cross-bridge cycling,” where myosin heads attach to actin filaments and pull them toward the center of the sarcomereโ€‹.

Which are the thin filaments of the sarcomere?

The thin filaments of the sarcomere are actin filaments. Actin filaments are essential components in the contraction process and interact with myosin to produce muscle contractionโ€‹.

What binds to actin at its binding sites, allowing the formation of cross-bridges?

Myosin heads bind to actin at its binding sites, forming cross-bridges. This interaction is facilitated by the presence of calcium ions, which expose the binding sites on actinโ€‹.

What are actin and myosin?

Actin and myosin are the primary contractile proteins in muscle fibers. Actin forms the thin filaments, while myosin forms the thick filaments. Their interaction is crucial for muscle contractionโ€‹.

What is a power stroke during muscle contraction?

A power stroke occurs when the myosin heads pivot and pull the actin filaments toward the center of the sarcomere, using energy from ATP hydrolysis. This action shortens the muscleโ€‹.

What is a sarcomere?

A sarcomere is the basic functional unit of muscle fibers, spanning from one Z-line to the next. It contains the actin and myosin filaments whose interaction causes muscle contractionโ€‹.

What happens to the length of the muscle when it contracts?

When a muscle contracts, its length shortens. This is due to the sliding of actin and myosin filaments past each other, reducing the length of the sarcomereโ€‹.

The binding of calcium to which molecule causes the myosin binding sites to be exposed?

The binding of calcium to troponin causes a conformational change that moves tropomyosin away from the myosin binding sites on actin, allowing cross-bridge formationโ€‹.

References

  1. Krans, Jacob. โ€œThe Sliding Theory of Muscle Contraction.โ€ Nature News, Nature Publishing Group, 2010, www.nature.com/scitable/topicpage/the-sliding-filament-theory-of-muscle-contraction-14567666/.
  2.  Clark, Micheal A, and Scott C Lucett. โ€œBasic Exercise Science .โ€ NASM Essentials of Personal Fitness Training, 7th ed., Jones & Bartlett Learning, 2018, p. 41. 
  3. Coburn, Jared W, and Moh H Malek. โ€œStructure and Function of the Muscular, Nervous, and Skeletal Systems.โ€ NSCAโ€™s Essentials of Personal Training, 2nd ed., Human Kinetics, p. 6. 
  4. Squire, John M. โ€œMuscle Contraction: Sliding Filament History, Sarcomere Dynamics and the Two Huxleys.โ€ Global Cardiology Science & Practice, U.S. National Library of Medicine, 30 June 2016, www.ncbi.nlm.nih.gov/pmc/articles/PMC5642817/.
  5. Huxley, H E. โ€œThe Mechanism of Muscular Contraction .โ€ Science.Org, 20 June 1969, www.science.org/doi/10.1126/science.164.3886.1356.
  6. Huxley, Hugh E. โ€œFifty Years of Muscle and the Sliding Filament Hypothesis.โ€ FebsPress, European Journal of Biochemistry, 1 Apr. 2004, febs.onlinelibrary.wiley.com/doi/10.1111/j.1432-1033.2004.04044.x.
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