Muscles & Muscle Tissue

Module 6 - Muscles & Muscle Tissue

Introduction

  • Course: BIOL 2457 - Anatomy & Physiology I

Learning Objectives

By the end of this lesson, students will be able to:

  • Identify the three different types of muscle tissue:

    • Skeletal muscle

    • Cardiac muscle

    • Smooth muscle

  • Describe unique characteristics that differentiate muscle types.

Structures of Skeletal Muscles

  • List the structures from epimysium to endomysium:

    1. Epimysium: outer layer

    2. Perimysium: surrounds fascicles (bundles of fibers)

    3. Endomysium: surrounds individual muscle fibers

Muscle Fiber Anatomy

  • Describe the parts of a single skeletal muscle fiber and their functions:

    • Sarcolemma: plasma membrane

    • Sarcoplasm: cytoplasm of muscle fibers, contains multiple nuclei

    • Myofibrils: contractile elements within the fibers

    • Sarcoplasmic reticulum: calcium storage

    • T tubules: transmit action potentials into the fiber

Anatomy of a Myofibril

  • Contractile proteins:

    • Actin (thin filaments)

    • Myosin (thick filaments)

  • Arrangement in sarcomeres includes:

    • Z discs: anchor actin

    • M line: anchors myosin

    • A band: area of overlap

    • I band: only actin

    • H zone: only myosin

Muscle Contraction Mechanism

  1. Excitation-Contraction Coupling

    • Neurons release acetylcholine at the neuromuscular junction.

    • ACh binds to receptors, opening sodium channels.

    • Sodium influx leads to action potential generation.

  2. Action Potential Generation

    • Action potential travels across the sarcolemma and into T-tubules.

    • Change in membrane charge opens voltage-gated channels.

  3. Calcium Release

    • Action potential activates calcium channels on the sarcoplasmic reticulum (SR).

    • Calcium diffuses into the sarcoplasm and binds to troponin.

  4. Cross Bridge Cycle

    • Myosin heads bind to exposed actin, forming cross-bridges:

    1. Cross-Bridge Formation

    2. Power Stroke: actin is pulled toward the M line.

    3. Cross-Bridge Detachment: ATP binds myosin, causing release from actin.

    4. Cocking of Myosin Head: ATP hydrolysis resets myosin.

    • Tension is generated as muscle fibers shorten on contraction.

Muscle Contraction Types

  • Isotonic Contractions: Muscle changes length, producing movement:

    • Concentric: muscle shortens

    • Eccentric: muscle lengthens under tension

  • Isometric Contractions: Muscle length remains unchanged

Muscle Twitch Phases

  1. Latent Period: initial phase, few seconds where excitation-contraction coupling occurs.

  2. Period of Contraction: cross-bridges are active, tension increases.

  3. Period of Relaxation: calcium pumped back into SR, tension declines.

Motor Units

  • Defined as the muscle fibers controlled by a single motor neuron.

  • Small motor units for fine control; large motor units for gross movements.

Types of Muscle Fibers

  • Fast Glycolytic (FG) fibers:

    • High glycogen, low myoglobin, rapid contraction rate.

    • Suitable for short bursts of power.

  • Fast Oxidative (FOG) fibers:

    • Intermediate properties, adaptable to aerobic and anaerobic conditions.

  • Slow Oxidative (SO) fibers:

    • High myoglobin content, suitable for endurance activities.

Effects of Age, Sex & Exercise on Muscle Fibers

  • Age: Sarcopenia, decline in muscle fibers, affecting strength and elasticity.

  • Sex: Males typically have greater muscle mass due to hormone differences.

  • Exercise: Changes muscle fiber composition, hypertrophy in response to strength training, and improved endurance through aerobic exercise.

Summary of Key Muscle Tissue Characteristics

  • Characteristics of Muscle Tissue:

    • Excitability: ability to respond to stimuli.

    • Contractility: ability to shorten forcefully.

    • Extensibility: ability to stretch without damage.

    • Elasticity: ability to return to resting length after contraction or stretching.

Functions of Muscle Tissue

  1. Producing body movements.

  2. Stabilizing body positions.

  3. Storing and utilizing substances.

  4. Thermogenesis (heat production).

Review and Conclusion

  • Muscle tissue is essential for various functions within the body, integrating with other systems for coordinated movement, stability, and maintenance of internal conditions. Effective understanding of their structure and function informs broader physiological implications and potential therapeutic interventions in muscle-related conditions.