Skeletal Muscles Structure and Function

Skeletal Muscles I. Structure and Function

  • Overview of the transcript, introducing the primary focus on skeletal muscles, their structure, and function.

    • Organization:

    • Structured by body system.

    • Covers standard scope and sequence requirements.

    • Uses clear text, strategically constructed art, and links to external learning tools.

    • Features:

    • Contains surgical videos.

    • Includes histology and interactive diagrams.

  • Senior Contributing Authors:

    • J. Gordon Betts, Tyler Junior College

    • Kelly A. Young, California State University, Long Beach

    • James A. Wise, Hampton University

    • Eddie Johnson, Central Oregon Community College

    • Brandon Poe, Springfield Technical Community College

    • Dean H. Kruse, Portland Community College

    • Oksana Korol, Aims Community College

Learning Objectives and Answers

  1. Understand the overall function of muscles and their properties.

    • Answer: Muscles are essential for survival, primarily facilitating movement by generating tension to exert force on the skeleton. Their key properties include: Electrical Excitability (ability to respond to stimuli), Contractility (capacity to shorten and generate force), Extensibility (ability to be stretched without damage), and Elasticity (ability to return to original shape after being stretched).

  2. Understand the overall levels of organization of the human body.

    • Answer: While the note primarily focuses on muscle-specific organization, it details: Organ Structure (e.g., biceps muscle, composed of muscle tissue, connective tissue, nervous tissue, and blood/vascular tissue, and part of neuromuscular and musculoskeletal systems) and Microscopic Anatomy (fascicle \$\rightarrow\$ muscle fibers (cells) \$\rightarrow\$ myofibrils).

  3. Recognize that muscles act on the skeletal structure, serving as a basis for movement.

    • Answer: Muscles generate tension to exert force on the skeleton, which directly facilitates body movement. Tendons, formed by the fusion of connective tissue layers (epimysium, perimysium, endomysium), connect muscles to bones and transmit this tension.

  4. Describe the cellular components and structure of muscle fibers.

    • Answer: Muscle fibers (cells) contain a plasma membrane (sarcolemma), intracellular fluid/cytoplasm, a cell nucleus, mitochondria (ATPATP production), and contractile organelles. These contractile organelles consist of thick and thin filaments arranged into repeating compartments called sarcomeres. They also contain myofibrils, which house these filaments.

  5. Describe the functional structure of sarcomeres.

    • Answer: Sarcomeres are the contractile units of skeletal muscle, giving muscle fibers their striated appearance. They are composed of several key structures: Z discs (boundaries), M line (center of H zone), H zone (center of A band, contains only thick filaments at rest), I band (contains only thin filaments), and A band (entire length of thick filaments, includes overlapping thin filaments). Thick filaments are made of Myosin proteins (with tails pointing inward and globular heads outward), while thin filaments are composed of Actin, Troponin, and Tropomyosin. Troponin acts as a 'lock' for tropomyosin, which covers actin's myosin-binding sites at rest.

  6. Describe the intrinsic processes underlying muscle contraction via the sliding filament mechanism.

    • Answer: Muscle contraction occurs via the Sliding Filament Mechanism, where thin filaments slide over thick filaments. The process involves: at rest, tropomyosin blocks actin's myosin-binding sites. Upon muscle activation, Calcium ions (Ca2+) are released from the sarcoplasmic reticulum. Ca2+ binds to troponin, causing a structural change that moves tropomyosin, exposing actin's binding sites. Cross-bridges then form as myosin heads bind to actin. This is followed by a power stroke, where the myosin heads swivel and pull the thin filaments towards the center of the sarcomere. This shortening occurs simultaneously across all sarcomeres, leading to whole muscle contraction and increased overlap between thin and thick filaments.

Function of Muscles

  • Essential for Survival:

    • Movement is fundamentally crucial for basic survival.

Types of Muscles

  • Skeletal Muscle

  • Cardiac Muscle

  • Smooth Muscle

  • Copyright \$\copyright\$

    John Wiley & Sons, Inc. All rights reserved.

Properties of Muscle

  1. Electrical Excitability:

    • The ability to respond to stimuli.

  2. Contractility:

    • The capacity to shorten and generate force.

  3. Extensibility:

    • The ability to be stretched without damage.

  4. Elasticity:

    • The ability to return to original shape after being stretched.

Organizational Structure of Muscle

  • Organ Structure:

    • Composed of skeletal muscle (e.g., biceps muscle).

    • Includes muscle tissue, connective tissue, nervous tissue, and blood/vascular tissue (endothelial).

    • Part of the neuromuscular and musculoskeletal systems.

Muscle Contraction

  • During contraction, muscles generate tension to exert force on the skeleton, facilitating body movement.

    • Adapted from source: OpenStax / CC BY (https://creativecommons.org/licenses/by/4.0)

Connective Tissue Components

  • Superficial Fascia Components:

    • Skin

    • Fatty layers

    • Biceps brachii muscle

    • Periosteum

    • Bone marrow

    • Deep fascia

    • Bone

    • Cephalic vein

    • Brachialis muscle

    • Radial nerve

    • Dorsal antibrachial cutaneous nerve

    • Radial collateral artery

    • Lateral intermuscular septum of humerus

    • Brachial artery and veins

    • Median nerve

    • Medial antibrachial cutaneous nerve

    • Basilic vein

    • Ulnar nerve

    • Superior ulnar collateral artery

    • Medial intermuscular septum of humerus

    • Humerus

    • Triceps brachii muscle

    • Cross-section through upper arm

Connective Tissue Components (2)

  • Muscle Connective Tissue Layers:

    • Epimysium: Surrounds the whole muscle.

    • Perimysium: Surrounds a fascicle (bundle of muscle fibers/cells).

    • Endomysium: Surrounds an individual muscle fiber/cell.

  • Other Components:

    • Blood vessels feed the muscle fibers.

    • Muscle fibers make up fascicles, which are encased in connective tissue layers.

Tendons and Their Function

  • Tendons: Fusion of epimysium, perimysium, and endomysium connects muscle to bone.

    • Function: Transmits tension from muscle contraction to exert force on the skeleton.

Aponeurosis

  • Definition: Flat connective tissue extensions for flat muscles.

    • Examples:

    1. Abdominal Aponeurosis and Linea Alba:

      • Forms a sheath around the abdominal muscles (fibrous band running down the middle).

    2. Epicranial Aponeurosis:

      • Connects the frontal belly with the occipital belly of the occipitofrontalis muscles.

Muscles Not Connecting to Bone

  • Some muscles do not connect directly to bone, such as muscles of facial expression.

Historical Context

  • Guillaume B. A. Duchenne (1862):

    • Explored mapping facial expressions.

    • Utilized galvanic current to 'shock' the facial muscles for study.

Microscopic Anatomy of Muscles

  • Organization:

    • Structure: Skeletal muscle comprises a hierarchy of components: fascicle \$\rightarrow\$ muscle fibers (cells) \$\rightarrow\$ myofibrils.

Cellular Components of Muscle Fibers

  • Cell Components Identified:

    • Plasma membrane:

    • Intracellular fluid/cytoplasm:

    • Contractile organelles:

    • Consist of thick and thin filaments arranged in compartments called sarcomeres.

Sarcomeres

  • Contractile Unit of Skeletal Muscle:

    • Gives striated appearance to muscle fibers.

    • Visual references provided from Wikimedia Commons.

Sarcomere Structure

  • Components Identified:

    • Z disc

    • M line

    • H zone

    • I band

    • A band

  • Magnification: Transmission Electron Microscopy (TEM) at 21,600x.

Thick Filaments

  • Composed of Myosin Proteins:

    • The tail points towards the center, whereas globular heads point outward.

    • Visual Elements: Representation of thick filament and myosin molecules within the sarcomere.

Thin Filaments and Protein Arrangement

  • Composed of Actin, Troponin, & Tropomyosin Proteins:

    • Actin molecules arranged in a helical structure.

    • Troponin Functionality:

    • Acts as a 'lock' securing tropomyosin’s position over actin when the muscle is at rest.

    • Tropomyosin blocks actin's binding site to myosin.

Troponin Binding Sites

  • Binding Sites on Troponin:

    • Actin

    • Tropomyosin

    • Calcium Ion (Ca2+)

    • At rest, troponin binds actin and tropomyosin. Upon activation:

    • Calcium ions (Ca2+) released into the sarcoplasm bind to troponin, causing a structural change that loosens its grip, allowing tropomyosin to move and exposing actin’s binding site to myosin.

Sliding Filament Mechanism

  • Definition: Filaments slide along each other during muscle contraction.

    • Thin filaments slide over thick filaments during contraction.

Filaments at Rest

  • Key Components Identified:

    • Myosin binding site on actin

    • Tropomyosin position

Filaments Activated

  • Involvement of Sarcoplasmic Reticulum:

    • Release of Ca2+ ions upon muscle activation.

Cross-Bridge Formation

  • Details of the activation process leading to cross-bridge formation between actin and myosin.

Sarcoplasmic Reticulum Overview

  • Function: Organelle in muscle cells where Ca2+ ions are stored and released during activation.

    • Includes:

    • Sarcolemma (cell membrane)

    • Mitochondria (produce energy, ATP)

    • Cell nucleus

    • Myofibrils containing the filaments.

Power Stroke Mechanism

  • Power Stroke Action:

    • Pulls thin filaments towards the center of the sarcomere during contraction.

Dynamics of Sarcomere Contraction

  • Mechanism of Shortening:

    • Contraction lengthens sarcomeres, increasing overlap between thin and thick filaments.

    • All sarcomeres contact simultaneously leading to whole muscle contraction.

Cross-Bridge Description and Mechanism

  • The Cross-Bridge Formation:

    • Occurs when myosin binds to actin, followed by a power stroke as the myosin head swivels, pulling thin filaments toward the center of the sarcomere.

    • Results in muscle contraction and shortening.

Summary of Muscle Structure and Function

  • Structure Overview:

    • Muscle cells (fibers): cell components and roles in contraction.

    • Connective tissue components transmit tension generated within a muscle to exert force on other structures (skeleton or skin).

    • Geometric arrangement of thin and thick filaments within sarcomeres influences contraction, defined by zones and bands.

    • Protein constituents of filaments:

    • Contractile Proteins: Myosin

    • Regulatory Proteins: Actin, Troponin, Tropomyosin

    • The sliding filament mechanism and actin-myosin cross-bridge formation are foundational for tension production in muscle.