Microscopic Anatomy of Skeletal Muscle Fibers

Myofibrils and the Sarcomere

The interior of a muscle fiber is dominated by myofibrils, which are densely packed, rodlike contractile elements. These myofibrils account for approximately 80% of the total muscle cell volume.

Myofibrils are not continuous, uniform rods; rather, they are composed of repeating functional units called sarcomeres. You can visualize sarcomeres as the individual "boxcars" of a train, aligned end-to-end along the length of the myofibril. The boundaries of a sarcomere are defined by Z discs (or Z lines).

The arrangement of proteins within these sarcomeres creates the visible striations (stripes) seen under a microscope. These stripes are categorized into specific bands:

• A bands: The dark regions that span the entire length of the thick filaments.
• H zone: A lighter region located in the very center of the A band.
• M line: A protein line (composed of myomesin) that bisects the H zone vertically.
• I bands: The lighter regions that contain only thin filaments; these are bisected by the Z disc.

The Molecular Composition of Myofilaments

The actual "machinery" of contraction resides in the myofilaments—the protein filaments organized within the sarcomere. There are two main types: thick and thin filaments.

Thick Filaments (Myosin)

Thick filaments are composed of the protein myosin. Each myosin molecule features a long tail and a globular head. The heads are highly functional; they contain binding sites that allow them to form cross bridges with the thin filaments during contraction.

Thin Filaments (Actin and Regulatory Proteins)

Thin filaments are primarily composed of the protein actin. On a molecular level, actin is made of kidney-shaped G actin (globular) subunits that link together to form long, fibrous strands called F actin (filamentous).

Crucially, thin filaments are regulated by two other proteins:

1. Tropomyosin: A protein that wraps around the actin strands.
2. Troponin: A regulatory protein complex bound to tropomyosin.

Structural and Elastic Proteins

To maintain the integrity of the myofibril, other proteins are present:

• Titin: An elastic filament that holds thick filaments in place, helps the muscle recoil after being stretched, and resists excessive stretching.
• Dystrophin: A protein that links the thin filaments to the proteins of the sarcolemma, ensuring the force of contraction is transmitted to the cell membrane.

The Sliding Filament Model of Contraction

How does a muscle actually shorten? According to the sliding filament model, contraction is not caused by the filaments themselves shrinking, but by them sliding past one another.

When the nervous system stimulates a muscle fiber, calcium is released from the sarcoplasmic reticulum into the sarcoplasm. This calcium allows the myosin heads to bind to the active sites on the actin filaments, forming cross bridges. In a "ratcheting" action, the myosin heads pull the thin filaments toward the center of the sarcomere.

As the thin filaments slide toward the M line, the following structural changes occur:

1. The Z discs move closer together.
2. The I bands shorten.
3. The H zone shortens and may eventually disappear.
4. The A band remains the same length, as it represents the total length of the thick filaments.

Summary Table: Changes During Contraction