Skeletal Muscle Anatomy

Introduction

Skeletal muscle is a specialized tissue responsible for voluntary movement in the human body. Its structure and function are essential topics in Anatomy & Physiology, focusing on the cellular organization, protein composition, and mechanisms of contraction.

Skeletal Muscle Fiber (Cell)

Structure and Characteristics

• Large, Cylindrical Cells: Skeletal muscle fibers are elongated, multinucleated cells that can span the entire length of a muscle.

• Myofibrils: Each fiber contains hundreds to thousands of myofibrils, which are the contractile elements of the cell.

• Multiple Nuclei: Muscle fibers are unique in having many nuclei, located at the periphery of the cell.

• (Myo)satellite Cells: These are stem cells involved in muscle repair and regeneration.

Development of Muscle Fibers

• Myoblasts: Embryonic cells that fuse to form a mature muscle fiber.

• Satellite Cells: Remain associated with muscle fibers and aid in growth and repair.

Microscopic Anatomy

• Striations: Alternating dark and light bands are visible under the microscope, reflecting the organized arrangement of contractile proteins.

• Nuclei: Located at the edges of the muscle fiber.

Muscle Fiber Components

Cellular Structures

• Sarcolemma: The plasma membrane of the muscle fiber.

• T-tubules: Invaginations of the sarcolemma that conduct action potentials into the cell's interior.

• Sarcoplasm: The cytoplasm of the muscle fiber, containing glycogen (energy storage), myoglobin (oxygen binding), mitochondria (energy production), and creatine phosphate (energy buffer).

• Sarcoplasmic Reticulum (SR): Specialized smooth endoplasmic reticulum that stores calcium ions necessary for contraction.

• Terminal Cisterns: Enlarged regions of the SR adjacent to T-tubules, forming a triad (2 terminal cisterns + 1 T-tubule).

Myofibrils and Myofilaments

Organization

• Myofibrils: Long protein bundles running the length of the muscle fiber; each fiber contains hundreds to thousands.

• Myofilaments: The contractile proteins within myofibrils, classified as thick (myosin) and thin (actin) filaments.

• Sarcomere: The basic contractile unit of muscle, defined as the region between two Z-lines.

Sarcomere Structure

• Z-line (disc): Anchors thin filaments.

• A band: Length of thick filaments; includes regions of overlap with thin filaments.

• I band: Contains only thin filaments.

• H zone: Central region of A band with only thick filaments.

• M line: Center of H zone; anchors thick filaments.

Proteins of Skeletal Muscle

Thick Filaments (Myosin)

• Myosin: Composed of hundreds of myosin protein molecules, each with a tail and a head region.

• Myosin Head Features:

  • ATP binding site (with ATPase activity)

  • Actin binding site (forms cross-bridges)

  • Flexible hinge region (allows movement during contraction)

Thin Filaments (Actin, Tropomyosin, Troponin)

• Actin: Globular (G-actin) molecules form a twisted double strand (F-actin); contains binding sites for myosin.

• Tropomyosin: Lies along actin strands, blocking myosin binding sites in resting muscle.

• Troponin: Globular protein with three subunits:

  • Troponin-T: binds tropomyosin

  • Troponin-I: binds actin

  • Troponin-C: binds calcium ions

Classification of Muscle Proteins

• Contractile Proteins: Generate force (Actin, Myosin)

• Regulatory Proteins: Control contraction (Troponin-Tropomyosin complex)

• Structural Proteins: Provide alignment, elasticity, and link myofibrils to the sarcolemma (Actinin, Myomesin, Titin, Nebulin, Dystrophin)

Structural Proteins and Their Functions

• Actinin: Cross-links thin filaments at Z lines.

• Myomesin: Cross-links thick filaments at the M line.

• Titin (Connectin): Large, elastic protein from Z disk to M line; stabilizes thick filaments and provides elasticity.

• Nebulin: Large protein along thin filaments; stabilizes their position.

• Dystrophin: Links thin filaments to the sarcolemma.

Sliding Filament Theory of Contraction

Mechanism of Muscle Contraction

• The length of thick and thin filaments does not change during contraction.

• The sarcomere shortens as Z discs move closer together.

• The I band and H zone decrease in width.

• There is an increase in overlap between thick and thin filaments.

• Actin and myosin slide past each other, with myosin heads pushing actin toward the M line.

Key Equations

• Force Generation:

Summary Table: Sarcomere Changes During Contraction

Review Questions

• What proteins are associated with thick filaments? Myosin, Myomesin, Titin

• What proteins are associated with thin filaments? Actin, Tropomyosin, Troponin, Nebulin, Dystrophin, Actinin

• What are the three features associated with the myosin head? ATP binding site, Actin binding site, Flexible hinge region

• For each part of a sarcomere, indicate if there is an increase, decrease, or no change during muscle contraction. See summary table above.

Additional info: Academic context and definitions have been expanded for clarity and completeness.