Sarcomere Structure and Sliding Filament Theory: Study Notes for Anatomy & Physiology

Sliding Filament Theory

Overview of Muscle Contraction

The sliding filament theory explains how muscles contract by describing the interaction between actin and myosin filaments within the sarcomere. Muscle shortening occurs as these filaments slide past each other, resulting in contraction.

• Myosin (thick filament): Anchored to the center of the sarcomere. Myosin acts like a protein that wants to pull on a rope.

• Actin (thin filament): Anchored to the ends of the sarcomere. Actin acts like a protein that is the rope being pulled.

• During contraction: The filaments themselves do not change length; instead, the overlap between actin and myosin increases, shortening the sarcomere.

Example: When muscles contract, the actin filaments slide toward the center of the sarcomere, increasing overlap with myosin.

Proteins of the Sarcomere

Contractile Proteins

Contractile proteins are responsible for generating force during muscle contraction.

• Myosin: The thick filament, often described as having many heads (like Medusa), which bind to actin and pull.

• Actin: The thin filament, forming the track along which myosin heads move.

Regulatory Proteins

Regulatory proteins control the interaction between actin and myosin, thus regulating muscle contraction.

• Tropomyosin: A thread-like protein that wraps around actin and blocks myosin binding sites on actin.

• Troponin: A globular protein that binds to calcium ions (Ca2+). When Ca2+ binds to troponin, it causes tropomyosin to move, exposing the myosin binding sites on actin and allowing contraction to occur.

Example: If troponin is absent or non-functional, myosin binding sites remain blocked, and contraction cannot occur.

Structural Proteins

• Titin: An elastic filament that helps the sarcomere retain its shape and provides passive elasticity to muscle.

Structure of the Sarcomere: Bands, Zones, Discs & Lines

Discs and Lines

The sarcomere is divided into distinct regions based on the arrangement of actin and myosin filaments, as seen under a transmission electron microscope (TEM).

• Z Disc: The boundary of the sarcomere; anchors actin filaments.

• M Line: The middle of the sarcomere; anchors myosin filaments.

Bands and Zones

• I Band: The light band containing only actin (thin filaments).

• A Band: The dark band containing both actin and myosin filaments; the length of the A band is equal to the length of the myosin filament.

• H Zone: The central region of the A band containing only myosin (thick filaments).

Example: During muscle contraction, the I band and H zone become shorter, while the A band remains the same length.

Component	Change During Contraction
A Band	no change
I Band	gets shorter and disappears
H Zone	gets shorter and disappears
Z Disc	no change
M Line	no change

Clinical Application: Myosin Storage Myopathy

Pathology and Sarcomere Changes

Myosin storage myopathy is a congenital condition where myosin folds incorrectly, forming clumps in skeletal muscle. This leads to muscle weakness and delayed motor development.

• Thick filament (myosin): Forms protein clumps in affected individuals.

• Thin filament (actin): Remains unaffected in this condition.

Example: Examination of the sarcomere in affected individuals would reveal abnormal aggregation of myosin filaments.

Practice Questions and Applications

• During muscle contraction, the myosin pulls on the actin, shortening the muscle.

• The box in the provided image surrounds the A Band structure.

• The region that is the same length as the actin filament is the I Band.

• The M Line is not found in the A band.

Key Terms and Definitions

• Sarcomere: The basic contractile unit of muscle fiber, defined by the region between two Z discs.

• Actin: Thin filament protein involved in muscle contraction.

• Myosin: Thick filament protein that interacts with actin to produce contraction.

• Tropomyosin: Regulatory protein that blocks myosin binding sites on actin.

• Troponin: Regulatory protein that binds calcium and moves tropomyosin to expose myosin binding sites.

• Titin: Elastic protein that stabilizes the sarcomere.

Equations and Formulas

• Force of Muscle Contraction:

$\text{Force} \propto \text{Number of cross-bridges formed between actin and myosin}$

• Sarcomere Length:

$\text{Sarcomere length} = \text{Distance from one Z disc to the next}$

Additional info: The notes provide a concise overview of sarcomere structure, the sliding filament theory, and related clinical conditions, suitable for college-level Anatomy & Physiology students.