Muscle Histology and Physiology

Muscle Histology & Physiology

Overview of Muscle Tissue

• Muscle tissue constitutes nearly half of the body's mass.

• Muscle tissue transforms chemical energy (ATP) into directed mechanical energy, which enables it to exert force.

• There are three main types of muscle tissue:

  • Skeletal

  • Cardiac

  • Smooth

• Prefixes used for muscle tissue: Myo-, mys-, and sarco-.

Types of Muscle Tissue

1. Skeletal Muscle

• Location: Organs attached to bones and skin.

• Cell Structure: Contains elongated cells called muscle fibers.

• Appearance: Striated (striped).

• Control: Voluntary (conscious control).

• Contraction Speed: Contracts rapidly but tires easily, powerful, requires nervous system stimulation.

2. Cardiac Muscle

• Location: Only found in the heart; forms the bulk of heart walls.

• Appearance: Striated.

• Control: Involuntary (does not require nervous system stimulation).

• Contraction Ability: Can contract without nervous system stimulation.

3. Smooth Muscle

• Location: Found in the walls of hollow organs (e.g., stomach, urinary bladder, airways).

• Appearance: Not striated.

• Control: Involuntary (does not require nervous system stimulation).

• Contraction Ability: Can contract without nervous system stimulation.

Functions of Skeletal Muscle

• Body Movement: Moves bones, facilitates facial expressions, speaking, breathing, swallowing.

• Maintenance of Posture: Stabilizes joints, maintains body position.

• Protection and Support: Packages and holds internal organs in place.

• Regulating Elimination of Materials: Circular sphincters control the passage of materials at orifices.

• Heat Production: Maintains body temperature.

Characteristics of Skeletal Muscle Tissue

• Excitability: Ability to respond to a stimulus by changing its electrical membrane potential.

• Conductivity: Involves sending an electrical change down the length of the cell membrane.

• Contractility: Exhibited when muscle filaments slide past each other, enabling movement.

• Elasticity: Ability to return to its original length after being stretched or shortened.

• Extensibility: Ability to be stretched.

Gross Anatomy of Skeletal Muscle

• Hierarchy of Structures:

  • A whole muscle contains many fascicles.

  • A fascicle consists of many muscle fibers.

  • A muscle fiber is a muscle cell.

• Components of Skeletal Muscle: includes nerves, blood vessels, and connective tissue.

Connective Tissue Components

• Epimysium: Dense irregular connective tissue wrapping the whole muscle.

• Perimysium: Dense irregular connective tissue wrapping each fascicle; houses blood vessels and nerves.

• Endomysium: Areolar connective tissue wrapping each individual muscle fiber; delicate layer providing electrical insulation and capillary support.

• Attachments:

  • Tendon: Cordlike structure of dense regular connective tissue.

  • Aponeurosis: Thin, flattened sheet of dense irregular tissue.

• Deep Fascia: Sheet of dense irregular connective tissue located external to epimysium, separates different muscles, houses nerves, blood, and lymph vessels.

• Superficial Fascia: Areolar and adipose tissue located superficial to deep fascia, separating muscles from skin.

Blood Vessels and Nerves

• Skeletal muscle has extensive blood vessel networks that deliver oxygen and nutrients while removing waste products.

• Innervated by somatic neurons; axons branch and terminate at neuromuscular junctions for voluntary contraction control.

Microscopic Anatomy of Skeletal Muscle

• Sarcoplasm (Cytoplasm): Contains organelles, contractile proteins, and other specializations.

• Nuclei: Muscle fibers are multinucleated, formed from the fusion of many myoblasts during embryo development.

• Sarcolemma: The plasma membrane containing T-tubules that extend deep into the muscle cell; has voltage-gated ion channels for electrical signal conduction.

Structure and Organization of a Skeletal Muscle Fiber

• Components:

  • Myofibrils (bundles of myofilaments) enclosed in sarcoplasmic reticulum.

  • Sarcoplasmic reticulum: similar to smooth endoplasmic reticulum, contains terminal cisternae (calcium storage).

• Myofibrils: Contain thick and thin filaments.

• Thick Filaments: Comprised of many myosin protein molecules, each myosin molecule has binding sites for actin and an ATPase site.

• Thin Filaments: Mostly composed of fibrous actin (F-actin) with G-actin globules possessing myosin binding sites, troponin, and tropomyosin for regulation during contraction.

Organization of a Sarcomere

• Sarcomeres: The repeating units where myofilaments are arranged in a pattern.

• Z Discs: Specialized proteins that anchor thin filaments, delineating the ends of each sarcomere.

• I Bands: Light regions containing only thin filaments, decrease in size during contraction.

• A Bands: Dark regions containing overlapping thick and thin filaments, host the H zone and M line.

Mitochondria and Energy Production

• Muscle fibers contain abundant mitochondria for aerobic ATP production; myoglobin provides oxygen storage for aerobic metabolism; glycogen is stored for quick fuel access; creatine phosphate can quickly replenish ATP.

Innervation of Skeletal Muscle Fibers

• Motor Units: Consist of a motor neuron and all muscle fibers it controls.

• Motor Units Characteristics: Vary in size based on the number of muscle fibers they innervate (smaller units for precise control, larger units for force production).

• Neuromuscular Junction: A specialized connection where motor neurons innervate muscle fibers, containing synaptic knobs, synaptic clefts, and motor end plates.

Muscle Fibers at Rest

• Muscle fibers exhibit a resting membrane potential (RMP) of approximately -90 mV established by ion channels and pumps, preparing them for excitation.

Overview of Events in Skeletal Muscle Contraction

1. Neuromuscular Junction (Excitation): Release of ACh from synaptic vesicles and binding to ACh receptors.

2. Action Potential Propagation: Triggers muscle action potential along the sarcolemma and T-tubules to the sarcoplasmic reticulum, leading to Ca2+ release.

3. Crossbridge Cycling: Ca2+ binds to troponin, leading to contraction through sliding of myofilaments.

Role of Calcium in Contraction

• Low Ca2+ Levels: Tropomyosin blocks active sites on actin, preventing myosin attachment; muscle remains relaxed.

• High Ca2+ Levels: Ca2+ binds to troponin, exposing binding sites on actin, leading to muscle contraction.

Crossbridge Cycling Steps

1. Crossbridge formation (myosin binds to actin).

2. Power stroke (myosin pulls filament towards the center).

3. Release of myosin head (ATP binds, releasing myosin).

4. Reset myosin head (ATP is split, cocking the myosin head).

Rigor Mortis

• This state occurs 3-4 hours post-mortem, peaks at 12 hours, lasting 48-60 hours. It results from the cessation of active calcium regulation and leads to sustained contraction until the cells decay due to ATP depletion.

Sliding Filament Model of Contraction

• Force is generated without necessarily shortening the fiber. Shortening occurs when the tension from cross bridges exceeds opposing forces.

Muscle Relaxation Process

• Series of events including nerve signal termination, ACh hydrolysis, calcium return to the SR, and tropomyosin's return to covering binding sites on actin, allowing the muscle to return to its resting position.

Types of Muscle Contractions

1. Isometric Contraction

• Muscle length remains constant, tension increases but is not strong enough to move resistance.

2. Isotonic Contraction

• Muscle length changes during contraction, divided into:

  • Concentric: Muscle shortens (e.g., lifting a load).

  • Eccentric: Muscle lengthens as it contracts (e.g., lowering a load).

Principles of Muscle Mechanics

• Shortening vs. Lengthening: Tension generated must exceed opposing forces for muscle to shorten.

• Muscle Tone: A constant state of slight contraction due to spinal reflexes helps maintain muscle readiness and health.

Muscle Fiber Type

• Classified by contraction speed and metabolic pathways for ATP synthesis.

• Three Types:

  • Slow oxidative fibers (suitable for endurance).

  • Fast oxidative fibers (moderate resistance to fatigue).

  • Fast glycolytic fibers (short-term powerful movements).

Structural Characteristics of Muscle Fibers

Adaptations to Exercise

• Aerobic Exercise: Enhances muscle capillaries, mitochondrial density, and myoglobin synthesis, leading to improved endurance and strength.

• Resistance Exercise: Results in hypertrophy, increasing fiber size and strength, alongside increased storage capacity within fibers.

Homeostatic Imbalance

• Disuse Atrophy: Refers to muscle atrophy due to immobilization; can lead to drastic size reduction and tissue replacement with fibrous connective tissue.

Developmental Aspects

• Neuromuscular Coordination: Peak coordination occurs by mid-adolescence; athletic training enhances this capability.

• Gender Differences in Muscle Mass: Females have 36% and males 42% body mass in terms of skeletal muscle, with testosterone being a significant factor.

• Aging Effects: As age progresses, connective tissue increases while muscle fiber quantity decreases; regular exercise can mitigate muscle loss (sarcopenia).

Cardiac Muscle Tissue

• Structure: Cells are short, branching, usually with one or two nuclei, striated, and possess numerous mitochondria for aerobic respiration.

• Intercalated Discs: Join neighboring fibers, containing desmosomes for structural integrity and gap junctions for intercellular communication.

Smooth Muscle

• Function and Location: Found in various organ systems including blood vessels (regulates blood flow), bronchioles (controls airflow), intestines (mixes and propels material), ureters (urine transport), and in the uterus (facilitates childbirth).

• Microscopic Structure: Fusiform shape, smaller than skeletal fibers, lacks transverse tubules, with a sparse sarcoplasmic reticulum and a significant reliance on extracellular calcium sources.