Chapter 9: Skeletal Muscle Tissue and Muscle Organization

Introduction

  • There are three types of muscle tissue:

    • Skeletal Muscle: Voluntary control, includes all skeletal muscle tissue.

    • Cardiac Muscle: Involuntary, found in the heart.

    • Smooth Muscle: Involuntary, found in walls of hollow organs.

Functions of Skeletal Muscle

  • Skeletal muscles attach to bones and perform various essential functions:

    1. Produce Skeletal Movement: Enables motion of bones.

    2. Maintain Posture and Body Position: Stabilizes the body against the force of gravity.

    3. Support Soft Tissues: Provides support and protection for internal organs.

    4. Regulate Entry and Exit of Materials: Controls openings, such as sphincters.

    5. Maintain Body Temperature: Generates heat during muscle contraction.

Anatomy of Skeletal Muscles

Gross Anatomy

  • Each muscle fiber is encased in three layers of connective tissue:

    • Epimysium: Surrounds the entire muscle.

    • Perimysium: Surrounds bundles (fascicles) of muscle fibers.

    • Endomysium: Surrounds individual muscle fibers.

  • Muscles attach to other structures via tendons or aponeuroses.

  • Contraction communication occurs across the neuromuscular junction.

Microanatomy of Skeletal Muscle Fibers

  • A skeletal muscle cell comprises:

    • Plasma Membrane/Sarcolemma: Protects the cell.

    • Cytoplasm/Sarcoplasm: Contains organelles and myofibrils.

    • Sarcoplasmic Reticulum (SR): Functions similarly to endoplasmic reticulum, stores calcium ions.

  • Deep indentations called T Tubules facilitate the transmission of action potentials.

  • Myofibrils contain myofilaments, organized into sarcomeres (functional units of muscle contraction).

Muscle Contraction

The Sliding Filament Theory

  • Explains tension exertion and muscle shortening through the sliding of filaments.

  • Involves interaction between thin (actin) and thick (myosin) filaments:

    • Myosin bridges undergo a cycle of attachment, pivoting, detachment, and returning.

    • At resting state, protein complexes (tropomyosin and troponin) inhibit interactions.

Neural Control of Muscle Fiber Contraction

  • Neuronal control links chemical release to electrical activity initiating contraction.

  • Neuromuscular junction consists of:

    • Axon terminal

    • Synaptic vesicles

    • Synaptic cleft

  • Acetylcholine (ACh) release triggers contraction, and breakdown by acetylcholinesterase (AChE) limits stimulation duration.

Muscle Contraction Summary

  • Contraction steps:

    1. ACh release from synaptic vesicles.

    2. ACh binds to the motor end plate.

    3. Action potential generation in the sarcolemma.

    4. Propagation of the action potential through T tubules.

    5. Calcium release from SR.

    6. Exposure of active sites on thin filaments.

    7. Cross-bridge formation and contraction.

Motor Units and Muscle Control

  • Motor units determine the precision of muscle movement.

  • A muscle twitch is a single, momentary contraction due to a stimulus.

  • The all-or-none principle states muscle fibers either contract fully or not at all.

Muscle Tone

  • Even at rest, random motor unit stimulation maintains muscle tone, stabilizing joints and bones.

Muscle Hypertrophy and Atrophy

  • Hypertrophy: Muscle enlargement due to repetition of near-maximal tension.

  • Atrophy: Muscle shrinkage due to lack of stimulation.

Types of Skeletal Muscle Fibers

  • Three types of skeletal muscle fibers:

    1. Fast Fibers: Large, high glycogen, fewer mitochondria; short, powerful contractions.

    2. Slow Fibers: Smaller, slower to contract, designed for endurance.

    3. Intermediate Fibers: Similar to fast but more fatigue-resistant.

Distribution of Muscle Fibers

  • Fiber types vary among muscles; each muscle comprises a mix but each motor unit contains one type.

  • The fiber type composition is genetically determined and influenced by exercise.

Organization of Skeletal Muscle Fibers

  • Classification based on fiber arrangement:

    • Parallel Muscles: Fibers aligned with the muscle's long axis.

    • Convergent Muscles: Broad base, fibers converge at a single attachment.

    • Pennate Muscles: Oblique fibers relative to a tendon, generate more tension.

      • Can be unipennate, bipennate, or multipennate.

    • Circular Muscles (Sphincters): Fibers arranged around an opening.

Muscle Terminology

Origins and Insertions

  • Muscles are identified by their origin (stationary) and insertion (moves).

Actions

  • Muscles are classified as:

    • Agonist: Prime mover.

    • Antagonist: Opposes the action.

    • Synergist: Assists the agonist.

    • Fixator: Stabilizes the origin.

Names of Skeletal Muscles

  • Muscle names often reveal their location, orientation, or function.

Levers and Pulleys

  • Levers: Rigid structures pivoting at a fulcrum, altering direction, speed, or distance of movement.

  • Classified as first-class, second-class, or third-class; third-class is most common.

  • Anatomical pulleys: Bony structures changing muscle force direction.

Aging and the Muscular System

  • Aging leads to reduced muscle size, elasticity, and power.

  • Exercise tolerance and recovery from injuries decline with age.