How Do Muscles Contract: The Sliding Filament Theory Explained

EndurElite Chief Endurance Officer Matt Mosman discusses what your muscles are made up and explains what causes a muscle to contract.

The Sliding Filament Theory and How Muscles Contract Fast Facts:

  • All muscles are made up of two contractile proteins called actin and myosin.
  • Groups of actin and myosin form myofibrils, which group together to form a muscle fiber, which group together to form a fascicle, which group together to form the whole muscle.
  • When a muscle contraction occurs a nerve impulse is sent to the neuromuscular junction that causes the release of acetylcholine.
  • This compound causes the release of calcium from the sarcoplasmic reticulum which then attaches to a binding protein called troponin.
  • This calcium also causes a shift in another binding protein called tropomyosin.
  • These two binding proteins cause actin and myosin to attach and a muscle contraction occurs.This is called the sliding filament theory.

All Exercise Is A Series Of Muscles Contracting Repeatedly

Whether you run, bike, lift weights or participate in other exercises the movements you make are all part of a complex process of muscular contractions. Before we explain how muscles contract (the sliding filament theory) it is necessary to understand the individual components of what makes up a muscle. This article will discuss the micro and macrostructure of the muscular system, what need to happen for a muscle to contract, and the physiological process known as the sliding filament theory

The Muscular System Explained Simply

Each skeletal muscle is an organ that contains muscle tissue, connective tissue, nerves, and blood vessels. Fibrous connective tissue, or epimysium, covers the body’s more than 430 skeletal muscles.

The Macro & Micro Structure Of Muscles

Muscle fibers, are long cells about the diameter of a human hair). When grouped together they form fascicles that may be made of up to 150 fibers, with the group surrounded by perimysium (connective tissue). Each fiber is surrounded by connective tissue called endomysium, which is encircled by the sarcolemma. All the connective tissue in the body—epimysium, perimysium, and endomysium—is continual with the tendon, so tension developed in a muscle is transfered to the tendon.

The intersection between a motor neuron and the muscle fibers it innervates is called the neuromuscular junction. Each muscle cell has only one neuromuscular junction. A motor neuron combined with the muscle fibers it innervates is called a motor unit. All the muscle fibers of a motor unit contract together when they are stimulated by the motor neuron. This is called the all-or-none principle.

The sarcoplasm contains components that contract along with mitochondria and the sarcoplasmic reticulum.

Hundreds of myofibrils are found in the sarcoplasm. Myofibrils contain the structure that contracts the muscle cell, which consists primarily of two types of contractile proteins: myosin and actin.

All Muscle Are Composed Of Actin And Myosin

The myosin filaments can contain up to 200 myosin molecules. Round heads called cross-bridges stick out from the myosin at regular intervals. The actin filaments are made up of two strands in a double helix formation. Myosin and actin filaments are the smallest contractile component of skeletal muscle, the sarcomere.

Adjacent myosin attaches to each other at the M-bridge in the center of the sarcomere. Actin is aligned at both ends of the sarcomere and is attached at the Z-line. Z-lines run through the entire myofibril. Six actin filaments surround each myosin filament, and each actin filament is surrounded by three myosin filaments.

It is the arrangement of the myosin and actin filaments and the Z-lines of the sarcomeres that give skeletal muscle striated appearance under a microscope. The dark A-band corresponds with the alignment of myosin, whereas the light I-band corresponds with the areas in two adjacent sarcomeres that contain only actin. The Z-line is in the middle of the I-band and is a thin, dark line running through the I-band. The H-zone is in the center of the sarcomere where only myosin is present. When muscles contract, the H-zone decreases as the actin slides over the myosin toward the center of the sarcomere. The I-band also decreases as the Z-lines are pulled toward the center of the sarcomere.

Surrounding each myofibril is something called the sarcoplasmic reticulum (SR), that ends as vesicles in the area of the Z-lines. Calcium is stored in the sarcoplasmic reticulum. The control of calcium dictates muscle contractions. T-tubules run perpendicular to the (SR) and end at the Z-line between two vesicles. This pattern of a T-tubule spaced between and perpendicular to two sarcoplasmic reticulum vesicles is called a triad. Because the T-tubules run between outlying myofibrils and are continuous with the sarcolemma at the surface of the cell, discharge of an action potential (an electrical nerve impulse) arrives nearly simultaneously from the surface to all depths of the muscle fiber. Calcium is thus released throughout the muscle, producing a coordinated contraction.

The Sliding-Filament Theory of Muscular Contraction

Simply put, the sliding-filament theory happens as follows. Actin filaments slide inward on myosin, drawing the Z-lines toward the center of the sarcomere and shortening the muscle fiber. As actin slides over myosin, the H-zone and I-band shrink. The flexion of myosin cross-bridges pulling on actin is responsible for the movement of the actin filament. Because only a very small displacement of the actin filament occurs with each flexion of the myosin cross-bridge, very fast, continual flexions must occur in multiple cross-bridges throughout the muscle for movement to happen.

The Resting Phase Of The Sliding Filament Theory

When at rest, most of the calcium required for muscular contractions to occur is stored in the sarcoplasmic reticulum, so very few myosin cross-bridges are attached to actin. In this state the muscle is at rest because no tension is developed in the muscle.

The Excitation-Contraction Coupling Phase Of The Sliding Filament Theory

Before myosin cross-bridges can flex, they must first attach to the actin filament. When the sarcoplasmic reticulum releases calcium ions (due to acetylcholine), the calcium attaches to troponin. This causes another molecule, called tropomyosin, which runs along the length of the actin filament. The myosin cross-bridge head now binds more rapidly to actin, allowing cross-bridge flexion to occur. The amount of force produced by a muscle is directly related to the number of myosin cross-bridge heads attached to actin filaments at that instant in time.

The Contraction Phase Of The Sliding Filament Theory

The energy for cross-bridge flexion comes from the breakdown of ATP to adenosine ADP and phosphate, a reaction catalyzed by ATPase. Another molecule of ATP must replace the ADP on the myosin cross-bridge head in order for the head to detach from the active actin site and recock. This allows the contraction process to be continued or relaxation to occur.

The Recharge Phase Of The Sliding Filament Theory

Muscle contractions only happen when this sequence of events—binding of calcium to troponin, coupling of the myosin cross-bridge with actin, cross-bridge flexion, dissociation of actin and myosin, and recocking of the myosin cross-bridge head—is repeated over and over again. This occurs as long as calcium is available, ATP is available to assist in uncoupling the myosin from the actin, and sufficient ATPase is available for the breakdown of ATP.

The Relaxation Phase Of The Sliding Filament Theory

Muscular relaxation happens when the stimulation of the motor nerve stops. Calcium returns back to the sarcoplasmic reticulum, which stops the binding process between actin and myosin.