BIO 2101 AM 11-06-2023

Overview of Muscle Anatomy

Muscle structures include two important outer coverings: fascia and epimysium.

• Fascia: This connective tissue is continuous through the tendon and merges with the periosteum of the bone, providing support and structure for the muscles.

• Epimysium: This layer encases the entire muscle and assists in the formation of the tendon, although it does not extend directly to the bone, allowing for flexibility during movement.

Fasciculus Structure

Inside the muscle, we find fasciculi, which are bundles made up of individual muscle cells known as muscle fibers.

• Perimysium: This connective tissue surrounds and separates fasciculi, holding muscle cells together and providing pathways for blood vessels and nerves.

• Peripheral nuclei: These nuclei are located around muscle cells, aiding in identification and playing a role in muscle repair and regeneration.

Cellular Structure of Muscle

Each muscle cell, also referred to as a muscle fiber, contains two main boundaries:

• Endomysium: This delicate connective tissue surrounds each individual muscle cell, providing support and a pathway for nutrients and waste products.

• Sarcolemma: The cell membrane of muscle fibers, it plays a critical role in potential electrical signals necessary for muscle contraction.

Muscle cells contain myofibrils, which are cylindrical structures that form the contractile units of muscles, enabling contraction and movement.

Myofibril Organization

Myofibrils consist of subunits known as actin and myosin, which are essential for muscle contraction.

• Actin: Referred to as thin filaments, it appears in light-staining areas in muscle tissue and plays a critical role during contraction.

• Myosin: Known as thick filaments, these are found in dark-staining areas and interdigitate with actin, forming cross-bridges during contraction.

Banding Patterns

Different staining patterns lead to specific names for areas within myofibrils:

• I Band: This area contains only actin (light-staining), which indicates that this region is only part of thin filaments.

• A Band: Extends from one end of myosin to the other, containing both actin and myosin filaments, revealing how filaments overlap during contraction.

• H Zone: Area within the A Band that contains only myosin, indicating regions where only thick filaments are present.

• M Line: The middle line of the H zone, providing structural support to myofibrils, and serves as an attachment point for myosin filaments.

The darkest area in stained tissue indicates overlap between actin and myosin, crucial for the process of contraction.

Sarcomere Functionality

The sarcomere is the functional unit of muscle contraction, defined as the space between two Z lines.

• The I Band can be defined as spanning two sarcomeres or belonging to one depending on the context of contraction.

• The zone of overlap within the A Band represents the contraction phase, indicating structural changes that occur during muscle contraction.

Changes Upon Contraction

As muscles contract, the following changes occur:

• Sarcomeres shorten, pulling Z lines closer together, which is essential for muscle shortening and movement.

• The A Band length remains unchanged, while the I Band and H Zone become narrower, illustrating the sliding filament theory of contraction.

• Contraction leads to the overlap of actin and myosin, absorbing more dye and appearing darker under microscopic examination.

Mechanism of Contraction

The mechanism of muscle contraction involves several steps:

• Binding: Myosin heads bind to actin at binding sites, which are previously blocked by tropomyosin. This action is initiated by calcium ion availability.

• Power Stroke: Upon binding, myosin heads swivel, pulling actin inward, which is a fundamental step in shortening the muscle fiber.

• Release: After contraction, myosin heads must detach from actin, which requires a new molecule of ATP. This is a critical step to avoid muscle fatigue.

• Resetting: ATP is hydrolyzed to reset the myosin head for the next cycle of contraction, illustrating the energetic demands of muscle function.

Role of Calcium

Calcium ions play a crucial role in muscle contraction by binding to troponin, leading to a conformational change that causes tropomyosin to shift and expose binding sites for myosin heads.

• Calcium release is triggered by action potentials that travel through the transverse tubules, resulting from the release of acetylcholine at the neuromuscular junction.

Energetics of Muscle Contraction

Muscle contraction continues as long as action potentials and calcium ions remain available.

• ATP is essential for muscle contractions, and its depletion leads to rigor mortis, illustrating the critical connection between ATP levels and muscle activity.

• Energy is stored in the chemical bonds of ATP and released upon hydrolysis, enabling the continuation of the contraction cycle, which is a key aspect of muscle metabolism and endurance.

Final Points

Effective muscle contractions require synchronized activity across numerous myosin heads. This synchronized cycling prevents simultaneous release which can halt contraction.

• Further study of muscle physiology, including animations and detailed physiological processes, is encouraged for a complete understanding of these complex mechanisms. Emphasis on the biochemical aspects of muscle metabolism can provide deeper insights into muscle health and performance.