Muscle Tissue

Chapter 10: Muscle Tissue

Overview of Muscle Tissue Types

  • Three Types of Muscle Tissue:

    • Skeletal

    • Cardiac

    • Smooth

Skeletal Muscle Tissue

  • Characteristics:

    • Striated

    • Voluntary

    • Moves skeleton

    • Comprised of long cylindrical cells, referred to as skeletal muscle fibers

    • Multinucleated, arising from the fusion of multiple myoblasts

Cardiac Muscle Tissue

  • Characteristics:

    • Striated

    • Involuntary

    • Responsible for heart's contraction, facilitating the pumping of blood and oxygen

    • Composed of short, branched cells

    • Contains intercalated discs, which connect adjacent cells

Smooth Muscle Tissue

  • Characteristics:

    • Non-striated

    • Involuntary, often referred to as visceral muscle

    • Composed of spindle-shaped cells

Functions of Skeletal Muscle

  1. Body Movement:

    • Enables movement of bones, facilitates facial expressions, speech, breathing, and swallowing

  2. Maintenance of Posture:

    • Stabilizes joints and maintains body position

  3. Protection and Support:

    • Encloses internal organs and holds them in position

  4. Regulation of Material Elimination:

    • Circular sphincters control passage at orifices

  5. Heat Production:

    • Aids in maintaining body temperature

Characteristics of Skeletal Muscle Tissue

  1. Excitability:

    • Ability to respond to stimuli by changing the electrical membrane potential

  2. Conductivity:

    • Involves conduction of electrical changes down the cell membrane

  3. Contractility:

    • Ability to shorten in response to signals from the brain; observed through filament sliding

  4. Elasticity:

    • Ability to return to original length post-stretching or shortening

  5. Extensibility:

    • Ability to be stretched

Gross Anatomy of Skeletal Muscle

  • Structure:

    • Muscle consists of fascicles

    • Fascicle is composed of muscle fibers (muscle cells)

    • Contains nerves, blood vessels, and connective tissue

Connective Tissue Components

  • Three Layers of Wrapping:

    • Epimysium:

    • Dense irregular connective tissue wrapping the entire muscle

    • Perimysium:

    • Dense irregular connective tissue encasing a fascicle

    • Endomysium:

    • Areolar connective tissue surrounding individual muscle fibers

  • Attachments:

    • Tendon: Cordlike dense regular connective tissue attachment

    • Aponeurosis: Thin, flattened sheet of dense irregular tissue

    • Deep Fascia: Separates and binds different muscles

    • Superficial Fascia (Hypodermis): Areolar and adipose tissue separating muscles from skin

Blood Vessels and Nerves in Skeletal Muscle

  • Extensive blood vessels to supply oxygen and nutrients while removing waste

  • Innervated by somatic neurons

  • Neurons branch and end at neuromuscular junctions

Microscopic Anatomy of Skeletal Muscle

  • Muscle Fiber (Cell) Structure:

    • Sarcoplasm:

    • Contains organelles and contractile proteins

    • Multinucleation:

    • Arises from fusion of multiple myoblasts, some become satellite cells for repair

    • Sarcolemma and T-tubules:

    • Sarcolemma has voltage-gated ion channels for electrical signals; T-tubules allow action potential propagation

Sarcoplasmic Reticulum

  • Smooth endoplasmic reticulum with calcium pumps and channels

  • Terminal Cisternae: Reservoirs for calcium ions, organized in triads with T-tubules

Myofibrils

  • Bundles of myofilaments (contractile proteins) found in muscle fibers

  • Comprises thick (myosin) and thin (actin) filaments

Thick Filaments

  • Comprised of many myosin protein molecules

  • Myosin heads contain binding sites for thin filaments and ATPase sites

Thin Filaments

  • Primarily composed of two helically twisted strands of actin

  • Regulated by troponin and tropomyosin, which bind calcium ions to expose myosin binding sites

Organization of a Sarcomere

  • Definition: Repeating units of myofilaments

  • Z Discs: Mark boundaries of sarcomeres and anchor thin filaments

  • I Bands: Light regions of thin filaments

  • A Bands: Dark regions with thick filaments and thin filament overlap

  • H Zone: Area in A Band with only thick filaments

  • M Line: Center of H Zone where thick filaments attach

Sliding Filament Model of Muscle Contraction

  1. Nerve impulse triggers calcium release

  2. Myosin binding site on actin exposed

  3. Cross-bridge forms between myosin and actin

  4. Power stroke: myosin pulls actin closer

  5. Myosin detaches after ATP binds

  6. Muscle relaxes when calcium is pumped back

Steps of Muscle Contraction

  1. Calcium Ion Release:

    • Action potential reaches neuromuscular junction, causing Ca2+ entry and ACh release

  2. Exposing Myosin Binding Sites:

    • Calcium binds to troponin, moving tropomyosin and exposing myosin-binding sites

  3. Cross-Bridge Formation:

    • Myosin heads attach to actin after ATP hydrolysis

  4. Power Stroke:

    • Myosin head pivots, pulling thin filament toward M-line

  5. Detachment:

    • New ATP binds to myosin, allowing it to detach from actin

  6. Relaxation:

    • Calcium ions are reabsorbed into the sarcoplasmic reticulum

Muscle Fiber Energy Supply

  • Muscle fibers use stored ATP only for a few seconds

  • Sources of ATP Generation:

    1. Immediate Supply via Phosphate Transfer:

    • Myokinase and creatine kinase assist in rapid ATP generation

    1. Short-Term Supply via Glycolysis:

    • Anaerobic process generating 2 ATP per glucose in cytosol

    1. Long-Term Supply via Aerobic Cellular Respiration:

    • Produces up to 38 ATP per glucose, requires oxygen

Muscle Fiber Types

  • Fiber Classification:

    • According to contraction type and ATP supply method

Slow Oxidative Fibers (Type I)

  • High endurance and red in color due to myoglobin

  • ATP produced aerobically

Fast Oxidative Fibers (Type IIa)

  • Fast and powerful contractions; light red in color

Fast Glycolytic Fibers (Type IIb)

  • Fast, powerful contractions; white in color, primarily anaerobic

Muscle Fiber Variability

  • Skeletal muscles contain a mixture of fiber types influenced by genetics and training

  • Specific muscle groups show variation in proportions of fiber types

Muscle Tension

  • Defined as the force generated by muscle contraction

Muscle Twitch

  • Defined as a brief contraction to a single stimulus, includes three phases:

    • Latent Period: No tension change occurs

    • Contraction Period: Tension increases

    • Relaxation Period: Tension decreases to baseline

Motor Unit Recruitment

  • Motor units allow varying degrees of force output based on the number of fibers activated

Types of Muscle Contraction

  • Isometric Contraction: Muscle tension increases without shortening

  • Isotonic Contraction: Muscle length changes while tension remains constant

    • Concentric: Muscle shortens

    • Eccentric: Muscle lengthens

Length-Tension Relationship

  • Tension produced based on muscle length during stimulation, optimal length produces maximum force

Muscle Fatigue

  • Reduced muscle tension primarily due to depletion of glycogen stores

  • Other causes include: insufficient calcium, altered ion concentrations, and build-up of by-products

Cardiac Muscle Tissue

  • Characterized by short, branching fibers, typically one or two nuclei, striated, and intercalated discs

    • Autorhythmic pacemaker cells control contractions, influenced by the autonomic nervous system

Smooth Muscle Tissue

  • Smooth muscle is found in various organ systems including blood vessels, bronchioles, and the digestive system

    • Characterized by fusiform shape, absence of striations, presence of caveolae, and contractile proteins oriented at angles

Smooth Muscle Contraction Mechanics

  • Calcium entry from the extracellular environment activates the myosin light-chain kinase, which phosphorylates myosin heads enabling contraction

Functional Categories of Smooth Muscle

  • Multiunit Smooth Muscle: Found in structures like iris and larger air passages

  • Single Unit Smooth Muscle (Visceral): Contractions triggered in unison due to the presence of gap junctions

Clinical Views

  1. Myasthenia Gravis: Autoimmune disorder affecting ACh receptors leading to muscle weakness

  2. Anabolic Steroids: Synthetic testosterone mimickers, can enhance performance but carry serious health drawbacks.