Skeletal Muscle Tissue: Structure, Function, and Physiology

Chapter Overview: Skeletal Muscle Tissue

Introduction

Skeletal muscle tissue is a primary component of the human musculoskeletal system, responsible for voluntary movements, posture, and heat generation. This chapter explores the structure, function, and physiology of skeletal muscle, comparing it to cardiac and smooth muscle tissues.

Functional Properties of Muscle Tissue

Key Characteristics

• Contractility: Muscle cells shorten due to the interaction of myofilaments (actin and myosin proteins).

• Excitability: Muscle cells respond to nerve signals; impulses travel along the sarcolemma.

• Extensibility: Muscle can be stretched by contraction of opposing muscles.

• Elasticity: Muscle recoils after being stretched.

Terminology Specific to Muscle Tissue

Definitions

• Myo, mys: Refer to "muscle".

• Sarco: Means "flesh".

• Sarcolemma: Plasma membrane of a muscle cell.

• Sarcoplasm: Cytoplasm of a muscle cell.

Functions of Muscle Tissue

Major Roles

• Produce movement: Skeletal muscle moves bones; smooth muscle moves substances through organs.

• Open and close passageways: Sphincters control flow in hollow organs.

• Maintain posture and stabilize joints: Muscle tone supports posture and joint stability.

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

Types of Muscle Tissue

Classification and Features

• Skeletal muscle: Striated, voluntary, comprises about 40% of body weight.

• Cardiac muscle: Striated, involuntary, found only in the heart.

• Smooth muscle: Non-striated, involuntary, found in walls of hollow organs.

Skeletal Muscle Structure

Components of Skeletal Muscle

• Skeletal muscle tissue

• Connective tissue

• Blood vessels

• Nerves

Connective Tissue Coverings

Layers and Functions

• Epimysium: Dense regular connective tissue surrounding the entire muscle.

• Perimysium: Fibrous tissue surrounding each fascicle (bundle of fibers).

• Endomysium: Thin connective tissue surrounding each muscle fiber.

• All layers merge into tendons, transmitting force and providing elasticity.

Nerves and Blood Vessels

Supply to Muscles

• Each muscle is supplied by one nerve, one artery, and one or more veins.

• Branches serve each muscle fiber, ensuring efficient communication and nutrient delivery.

Muscle Attachments

Origins and Insertions

• Muscles attach to bones at origins (less movable) and insertions (more movable).

• Direct attachments: Connective tissue fibers are short.

• Indirect attachments: Via tendon or aponeurosis.

• Bone markings for muscle attachment include tubercles, trochanters, and crests.

Microscopic Anatomy of Skeletal Muscle

Muscle Fiber Structure

• Muscle fibers are long, cylindrical cells (10–100 μm diameter).

• Formed by fusion of embryonic cells; multinucleate.

• Nuclei are located at the periphery of the cell.

Myofibrils and Sarcomeres

Organization and Components

• Myofibrils: Long rods in cytoplasm, making up 80% of cell volume.

• Sarcomeres: Functional units; repeating segments within myofibrils.

• Z disc: Defines boundaries of each sarcomere.

• Actin (thin) filaments: Extend from Z disc.

• Myosin (thick) filaments: Located at the center of the sarcomere.

Bands of the Sarcomere

• A band: Length of thick filaments.

• H zone: Contains only thick filaments.

• M line: Holds thick filaments together.

• I band: Contains only thin filaments.

Titin

Role in Muscle Structure

• Titin: Springlike molecule extending from Z disc to M line.

• Holds thick filaments in place and unfolds during stretch, resisting overstretching.

Sarcoplasmic Reticulum and T Tubules

Functions and Structure

• Sarcoplasmic reticulum (SR): Specialized smooth endoplasmic reticulum around each myofibril; forms terminal cisterns.

• T tubules: Deep invaginations of the sarcolemma; form triads with cisterns.

• Functions: Release calcium for contraction and coordinate signal transmission.

Mechanism of Contraction

Sliding Filament Theory

• Myosin heads attach to actin, pulling thin filaments inward.

• Filaments slide past each other; do not shorten.

• Requires calcium ions and ATP.

Changes During Contraction

• Z discs move closer; sarcomere shortens.

• I bands narrow; H zone disappears.

• A band remains unchanged.

Innervation of Skeletal Muscle

Motor Units and Neuromuscular Junctions

• Motor unit: One motor neuron and all the muscle fibers it innervates.

• Neuromuscular junction: Synapse between neuron and muscle fiber.

• Terminal boutons: Axon endings containing neurotransmitters.

• Synaptic cleft: Space between axon and sarcolemma.

Muscle Fiber Types

Classification by Function

• Slow oxidative fibers: Red, fatigue-resistant, aerobic metabolism.

• Fast oxidative fibers: Intermediate endurance.

• Fast glycolytic fibers: White, anaerobic, suited for power bursts.

Diseases of Muscle Tissue

Muscular Dystrophy and Related Disorders

• Muscular dystrophy: Genetic muscle-degenerating disease.

• Duchenne muscular dystrophy: Most common form; X-linked inheritance.

• Myotonic dystrophy: Characterized by delayed relaxation after contraction.

Muscle Tissue Throughout Life

Development and Aging

• Body mass: Males ~42%; females ~36% (due to androgens).

• Aging: Connective tissue increases, muscle fibers decrease.

• Sarcopenia: Muscle wasting; strength drops by ~50% by age 80.

Comparison of Muscle Tissue Types

Structural and Functional Differences

Type	Striations	Control	Location	Key Features
Skeletal	Present	Voluntary	Attached to bones	Multinucleate, rapid contraction
Cardiac	Present	Involuntary	Heart	Intercalated discs, rhythmic contraction
Smooth	Absent	Involuntary	Walls of hollow organs	Spindle-shaped cells, slow contraction

Key Equations

ATP Requirement for Muscle Contraction

Muscle contraction requires ATP hydrolysis:

$ATP \rightarrow ADP + P_i + \text{energy}$

Force Transmission

Force generated by muscle fibers is transmitted through connective tissue layers:

$\text{Total Force} = \sum_{i=1}^{n} F_{\text{fiber}_i}$

Summary

Skeletal muscle tissue is essential for movement, posture, and metabolic functions. Its unique structure, innervation, and ability to adapt to physiological demands distinguish it from other muscle types. Understanding its anatomy and physiology is fundamental for students of Anatomy & Physiology.