Sliding Filament Theory

The sliding filament theory explains how muscle fibres contract. The sliding filament theory can be best explained as how muscles contract by the interaction of actin and myosin filaments sliding past each other within muscle cells. The process requires ATP for energy.

The sliding filament theory was proposed in 1954 by Andrew Huxley and Rolf Niedergerke. In this article, we will study the sliding filament theory of muscle contraction notes in detail.

What is Sliding Filament Theory?

The sliding filament theory states that the two main types of muscle filaments slide past each other during contraction, causing the muscle to shorten. The actin filaments are thin and have a double helix structure, while the myosin filaments are thick and have a globular head. 

The myosin heads bind to the actin filaments and pull them towards the center of the muscle fiber. This causes the muscle fiber to shorten and the muscle to contract. The sliding filament theory is important because it helps us to understand how muscles work.

What is Sarcomere in Muscle?

A sarcomere is the fundamental unit of muscle contraction and consists of bundle of thick and thin filaments. It has the following key features:

• Sarcomeres are present in series to form a myofibril and span from Z-line to Z-line. It is only a few micrometers long. Z-lines mark the boundaries of a sarcomere and anchor the thin filaments.
• It consists of overlapping actin and myosin filaments.It is present in a repeating pattern.
• Actin filaments are thin and extend from the Z-line towards the center. Myosin filaments on the other hand, are thicker and are located in the center.
• H- Zone is the central region of a sarcomere where only myosin filaments are present. It shortens during muscle contraction.
• I-band is the region containing only actin filaments, extending from the Z-line towards the center, and shortening during muscle contraction.
• A-band is the central region of the sarcomere where both actin and myosin filaments overlap.
• M-line is present at the center of the A-band that anchors the myosin filaments.
• Muscle contraction occurs as actin and myosin filaments slide past each other, causing the sarcomere to shorten.
• Sarcomeres contract when stimulated by a nerve impulse, leading to the shortening of the muscle fiber and the generation of force.

Sarcomere Diagram

Sliding Filament Theory of Muscle Contraction

Sliding Filament theory describes the molecular changes that occur during muscle contraction at the sarcomere level, which is the basic functional unit of a muscle fiber.

• In the resting state, myosin heads are in a low-energy position, and the actin and myosin filaments do not overlap.
• When a motor neuron signals a muscle fiber to contract, an action potential is generated. It travels along the muscle cell membrane and into the muscle fiber through the transverse tubules.
• The action potential triggers the release of calcium ions from the sarcoplasmic reticulum.
• Calcium binds to troponin that results in change in shape of troponin. It allows tropomyosin to move away from the myosin-binding sites on actin.
• With the myosin-binding sites exposed, myosin heads can bind to actin, forming cross-bridges.
• The myosin heads undergo forceful contraction, pulling the thin actin filaments towards the center of the sarcomere.
• As myosin heads continue to cycle through binding, pulling, and releasing, the actin filaments slide past the myosin filaments, causing the sarcomere to shorten.
• When the action potential ceases and calcium ions are actively pumped back into the sarcoplasmic reticulum, the troponin-tropomyosin complex returns to its original position, blocking the myosin-binding sites on actin. This leads to muscle relaxation.

Sliding Filament Theory Diagram

The following is a well-labeled diagram of sliding filament theory:

Summary of the Sliding Filament Theory Steps

The sliding filament theory of muscle contraction involves the steps:

• Resting State: Actin and myosin filaments overlap only slightly and muscle fibers are relaxed.
• Excitation of the nerve: A nerve impulse stimulates the muscle fiber. It causes the release of calcium ions from the sarcoplasmic reticulum into the sarcoplasm.
• Cross-Bridge Formation: Calcium ions bind to troponin, causing tropomyosin to move. It expose the myosin-binding sites on actin. Myosin heads then bind to these sites and forms the cross-bridges.
• Role of ATP: The ATP molecule is hydrolyzed and causes the myosin head to pivot. It pull actin filaments towards the center of the sarcomere.
• Repeat: The cycle continues as long as calcium ions are present and ATP is available, resulting in the shortening of sarcomeres and muscle contraction.

How Does Muscle Contraction Occur?

Muscle contraction is a physiological process where muscle fibers generate tension and exert a force, resulting in movement or the stabilization of body parts.

• Muscle contraction begins with a signal from the central nervous system through a motor neuron.
• The neuromuscular junction connects the motor neuron to the sarcolemma.
• Acetylcholine is released at the neuromuscular junction. It results in the action potential in the sarcolemma.
• An action potential triggers the release of calcium ions from the sarcoplasmic reticulum into the sarcoplasm.
• Calcium ions bind to troponin on actin filaments. It exposes the myosin-binding sites.
• Myosin binds to the exposed active sites on actin and forms the cross bridges.
• The hydrolysis of ATP at the myosin head causes sliding of thin filaments over thick filaments.
• As thin filaments slide, the Z lines are pulled closer together. It leads to muscle contraction.
• The cycle of cross-bridge formation, contraction, and sliding repeats until calcium ions are actively pumped back into the sarcoplasmic reticulum.
• With decreasing calcium levels, troponin covers the myosin-binding sites on actin, allowing for muscle relaxation.
• Recurrent muscle activation may lead to the accumulation of lactic acid, contributing to muscular fatigue.
• Myoglobin, a pigment in muscles, contributes to their red color. Muscles rich in myoglobin are adapted for sustained, aerobic activities.
• Red fibers with myoglobin-rich content also have a large number of mitochondria, supporting energy production during prolonged activities.
• Muscles lacking myoglobin appear white and are associated with anaerobic, short cycle of activity.
• As calcium is pumped back, the Z lines return to their initial positions, and the muscle returns to a relaxed state.

Importance of Sliding Filament Theory

The sliding filament theory is the most widely accepted theory for explaining how muscle fibers contract. It describes how the interaction between actin and myosin filaments produces contractile force.

• Explains the molecular mechanism behind muscle contraction.
• Forms the basis for understanding different body movements.
• It help in diagnosing and treating muscle-related disorders.
• Forms the basis for studying muscle physiology and related disorders.

Conclusion: Sliding Filament Theory

The sliding filament theory is the most accepted theory that explains how muscle fibers contract. It states that when a muscle contracts, the actin and myosin filaments slide past each other, causing the sarcomere to shorten. The filaments themselves do not change in length. The number of fibers that contract determines the strength of the muscular force. The theory help us to understand various muscle functions and different body movements.

FAQs on Sliding Filament Theory

What is the Sliding Filament Theory Explain?

The sliding filament theory is a theory that explains how muscles contract at the cellular level. 

 What are Cross Bridges?

A cross-bridge formation occur during the attachment of myosin to actin filament within a muscle cell.

What is the Role of ATP in the Sliding Filament Theory?

In the sliding filament theory, ATP, or energy currency of the cell, releases myosin from actin filaments. It causes muscle relaxation.

What is the Function of Troponin?

Troponin regulate muscle contraction in skeletal and cardiac muscles.

What can Stop a Muscle Contraction?

The depletion of ATP and the reuptake of calcium ions into the sarcoplasmic reticulum can stop muscle contraction . It prevents cross-bridge formation and energy-dependent cycling.

Why is Sliding Filament Theory Important?

The sliding filament theory is important as it explains the cross bridge formation and the process of muscle contraction.

Why are Calcium Ions Necessary for Skeletal Muscle Contraction?

Calcium ions are necessary for skeletal muscle contraction. They trigger muscle contraction by interacting with the troponin.