The sarcomere is the fundamental unit of muscle contraction. These repeating units are responsible for generating the force that allows muscles to shorten and produce movement. Understanding how sarcomeres function provides insight into the mechanics of muscle contraction throughout the body.
Sarcomere Structure
A sarcomere is defined as the segment between two Z-discs, dense protein structures that anchor thin filaments. Within each sarcomere, two primary protein filaments are arranged in a specific pattern: thick filaments composed of myosin and thin filaments composed of actin. This arrangement creates the striated appearance characteristic of skeletal and cardiac muscle.
Several distinct regions are observed within a sarcomere due to the arrangement of these filaments. The A-band, appearing as a dark band, encompasses the entire length of the thick myosin filaments and includes regions where thin and thick filaments overlap. The I-band, a lighter band, contains only the thin actin filaments and is located between the ends of the thick filaments. In the center of the A-band lies the H-zone, which contains only thick myosin filaments and no thin filament overlap. The M-line, a protein structure, bisects the H-zone and connects the middle portions of the thick filaments.
The Sliding Filament Mechanism
Muscle contraction occurs through a process known as the “sliding filament theory,” where actin and myosin filaments slide past one another without changing their individual lengths. This theory proposes that active force is generated as the thin actin filaments are pulled inward by the thick myosin filaments. This sliding action causes the sarcomere to shorten, leading to muscle contraction.
The molecular basis for this sliding is the cross-bridge cycle, a repetitive series of events involving myosin heads. Each myosin head has binding sites for actin and adenosine triphosphate (ATP). When a myosin head binds to an actin filament, a cross-bridge forms.
Following cross-bridge formation, a “power stroke” occurs: the myosin head pivots and pulls the actin filament towards the M-line. This movement shortens the sarcomere. After the power stroke, the myosin head detaches from actin, re-cocks, and reattaches to a new binding site further along the actin filament, ready for another cycle. This continuous, asynchronous cycling of numerous myosin heads drives sustained filament sliding, causing sarcomere shortening.
How Sarcomere Shortening is Controlled
The initiation and control of sarcomere shortening are precisely regulated, beginning with a nerve impulse. When a motor neuron stimulates a muscle cell, it releases a neurotransmitter called acetylcholine at the neuromuscular junction. This triggers an action potential in the muscle cell membrane, which propagates along the sarcolemma and into T-tubules.
The action potential’s arrival at the T-tubules leads to the release of calcium ions (Ca²⁺) from the sarcoplasmic reticulum, a specialized internal membrane system within the muscle cell that stores calcium. These released calcium ions then bind to troponin, a protein complex located on the thin actin filaments. The binding of calcium to troponin causes a conformational change in troponin, which moves tropomyosin away from the myosin-binding sites on the actin filament.
With the myosin-binding sites on actin now exposed, the myosin heads can attach and begin the cross-bridge cycle. ATP serves as the energy source for this process. ATP binding to myosin causes detachment from actin, and its hydrolysis into ADP and inorganic phosphate (Pi) provides the energy to re-cock the myosin head, preparing it for the next binding and power stroke.
From Shortening to Muscle Contraction
The collective shortening of countless sarcomeres within muscle fibers translates into the overall contraction of a whole muscle. As each sarcomere shortens by a small amount, the cumulative effect across thousands of sarcomeres in series results in a noticeable change in muscle length. The force generated by this sarcomere shortening is known as muscle tension.
Muscle contraction can manifest in different ways depending on the load encountered. In isotonic contractions, the muscle shortens while generating tension sufficient to move a load. An example is lifting a weight, where the biceps muscle visibly shortens. Conversely, isometric contractions involve generating muscle tension without a significant change in muscle length. This occurs when the force produced by the sarcomeres is not enough to overcome the load’s resistance, such as pushing against an immovable object. In both types of contractions, the fundamental mechanism of sarcomere shortening remains the underlying force generator.