Skeletal Muscle Tissue Structure

Objectives

This section introduces the foundational concepts of skeletal muscle tissue, focusing on its structure, function, and microscopic anatomy. Students will learn to:

• Identify the general characteristics of skeletal muscle tissue structure and function.

• Describe the three connective tissue sheaths and two types of muscle attachments.

• Explain the microscopic anatomy of skeletal muscle fibers.

• Understand the sliding filament model of muscle contraction.

Universal Characteristics of Muscle

Functional Properties of Muscle Cells

All muscle cells share several universal characteristics essential for their function:

• Excitability (Responsiveness): Ability to respond to chemical signals, stretch, and electrical changes across the plasma membrane.

• Conductivity: Local electrical excitation sets off a wave of excitation that travels along the muscle fiber.

• Contractility: Muscle cells shorten when stimulated, producing movement.

• Extensibility: Capable of being stretched between contractions.

• Elasticity: Returns to its original resting length after being stretched.

Muscle tissue can be classified by control (voluntary or involuntary) and by appearance (striated or non-striated):

• Skeletal muscle: Voluntary and striated.

• Cardiac muscle: Involuntary and striated.

• Smooth muscle: Involuntary and non-striated.

All muscle cells possess contractility, excitability, extensibility, and elasticity. Muscles only pull, never push.

Skeletal Muscle

Structural and Functional Features

Skeletal muscle is a voluntary, striated muscle type usually attached to bones. Its striations are alternating light and dark transverse bands, resulting from the arrangement of internal contractile proteins.

• Voluntary control: Usually subject to conscious control.

• Muscle fibers (myofibers): Skeletal muscle cells are called muscle fibers or myofibers. They are large, cylindrical cells, typically 100 μm in diameter and up to 30 cm long.

• Body mass: Skeletal muscles make up 30–40% of total body mass.

• Tendons: Bands of connective tissue that attach skeletal muscle tissue to bones.

Skeletal Muscle Fibers

Microscopic Anatomy

Skeletal muscle fibers are multinucleated, elongated cells with visible striations under the microscope. The image below (not shown) highlights key features:

• Muscle fiber: The basic cellular unit of skeletal muscle.

• Nuclei: Multiple nuclei per fiber, located at the periphery.

• Endomysium: Thin connective tissue sheath surrounding each muscle fiber.

• Striations: Alternating bands due to the arrangement of myofilaments.

Additional info: Striations are caused by the regular arrangement of actin and myosin filaments within sarcomeres.

Connective Tissue Sheaths of Skeletal Muscle

Organization and Function

Three connective tissue sheaths organize and protect skeletal muscle:

• Endomysium: Surrounds each individual muscle fiber; provides support and electrical isolation.

• Perimysium: Bundles muscle fibers into fascicles; contains blood vessels and nerves.

• Epimysium: Encloses the entire muscle; contributes to muscle efficiency and power output.

These sheaths are composed mainly of collagen and elastic fibers, which resist excessive stretching and help return muscle to its resting length.

Muscle Attachments

Direct and Indirect Attachments

Muscles attach to bones via two main mechanisms:

• Direct attachment: Epimysium of the muscle is fused directly to the periosteum of bone.

• Indirect attachment: Connective tissue extends beyond the muscle as a tendon or aponeurosis, attaching to the bone's periosteum.

Tendons are cord-like structures, while aponeuroses are flat, sheet-like attachments.

Functions of Skeletal Muscle

Major Roles

• Produce movement: By contracting and pulling on bones.

• Maintain posture and position: Stabilize the body and joints.

• Stabilize joints: Reinforce and support joint structures.

• Generate heat: Muscle contractions produce heat, helping maintain body temperature.

Summary Table: Connective Tissue Sheaths

Summary Table: Muscle Attachments