Muscles and Muscle Tissue: Structure, Function, and Physiology

Muscle Tissue Overview

Introduction to Muscle Tissue

Muscle tissue comprises nearly half of the body’s mass and is essential for converting chemical energy (ATP) into mechanical energy, enabling force generation and movement. There are three main types of muscle tissue: skeletal, cardiac, and smooth, each with distinct structures and functions.

Muscle Tissue Terminology

• myo-, mys-, sarco-: Prefixes referring to muscle (e.g., sarcoplasm is muscle cell cytoplasm).

• Muscle fiber: Elongated muscle cell found in skeletal and smooth muscle, but not cardiac muscle.

Types of Muscle Tissue

• Skeletal Muscle: Attached to bones and skin, voluntary, striated, multinucleated, contracts rapidly but tires easily.

• Cardiac Muscle: Found only in the heart, involuntary, striated, single nucleus, contracts at a steady rate due to pacemaker activity.

• Smooth Muscle: Found in walls of hollow organs (e.g., stomach, bladder), involuntary, non-striated, single nucleus.

Characteristics of Muscle Tissue

• Excitability (Responsiveness): Ability to receive and respond to stimuli.

• Contractility: Ability to shorten forcibly when stimulated.

• Extensibility: Ability to be stretched.

• Elasticity: Ability to recoil to resting length.

Functions of Muscle Tissue

• Produce movement (locomotion, manipulation, digestion, pumping blood)

• Maintain posture and body position

• Stabilize joints

• Generate heat during contraction

Skeletal Muscle Structure

Organization of Skeletal Muscle

Skeletal muscle is an organ composed of muscle fibers, nerves, blood vessels, and connective tissue sheaths. Each muscle receives a nerve, artery, and veins, ensuring nutrient delivery and waste removal.

Connective Tissue Sheaths

• Epimysium: Dense irregular connective tissue surrounding the entire muscle.

• Perimysium: Fibrous connective tissue surrounding fascicles (muscle fiber bundles).

• Endomysium: Fine areolar connective tissue surrounding each muscle fiber.

Skeletal Muscle Fiber Anatomy

• Sarcolemma: Muscle fiber plasma membrane.

• Sarcoplasm: Muscle fiber cytoplasm, containing glycosomes (glycogen storage) and myoglobin (oxygen storage).

• Specialized structures: Myofibrils, Sarcoplasmic reticulum, T tubules.

Myofibrils and Sarcomeres

• Myofibrils: Densely packed, rodlike elements that make up most of the muscle cell volume.

• Striations: Alternating dark (A bands) and light (I bands) regions.

• Sarcomere: Smallest contractile unit, defined as the region between two Z discs.

• Myofilaments: Actin (thin) and myosin (thick) filaments arranged in a hexagonal pattern.

Molecular Composition of Myofilaments

• Thick filaments: Composed of myosin molecules with heads that form cross bridges during contraction.

• Thin filaments: Composed of actin, tropomyosin, and troponin. Actin subunits provide binding sites for myosin heads.

• Elastic filament (titin): Maintains alignment and elasticity of the sarcomere.

• Dystrophin: Links thin filaments to the sarcolemma.

Muscle Contraction Mechanisms

Sarcoplasmic Reticulum and T Tubules

• Sarcoplasmic reticulum (SR): Stores and releases Ca2+ for muscle contraction.

• T tubules: Invaginations of the sarcolemma that transmit action potentials deep into the muscle fiber.

• Triad: Structure formed by a T tubule and two terminal cisterns of the SR.

Sliding Filament Model of Contraction

Muscle contraction occurs when myosin heads bind to actin, forming cross bridges and pulling thin filaments toward the center of the sarcomere. This process shortens the muscle fiber without changing the length of the filaments themselves.

• During contraction: I bands shorten, Z discs move closer, H zones disappear, A bands move closer together.

Neuromuscular Junction and Muscle Fiber Excitation

• Neuromuscular junction (NMJ): Site where a motor neuron communicates with a muscle fiber via the neurotransmitter acetylcholine (ACh).

• Action potential (AP): Electrical signal that triggers muscle contraction.

• Ion channels: Chemically gated (opened by ACh) and voltage-gated (opened by changes in membrane potential).

Events at the Neuromuscular Junction

1. AP arrives at axon terminal.

2. Voltage-gated Ca2+ channels open; Ca2+ enters neuron.

3. Ca2+ triggers release of ACh into synaptic cleft.

4. ACh binds to receptors on sarcolemma, opening Na+ channels and generating an end plate potential.

5. ACh is degraded by acetylcholinesterase.

Generation and Propagation of Action Potential

• Depolarization: Na+ influx makes the inside of the sarcolemma less negative.

• Repolarization: K+ efflux restores resting membrane potential.

• Refractory period: Time during which the muscle fiber cannot be restimulated.

Excitation-Contraction (E-C) Coupling

E-C coupling links the action potential to muscle contraction. The AP travels along the sarcolemma and down T tubules, triggering Ca2+ release from the SR, which initiates contraction.

Cross Bridge Cycle

1. Cross bridge formation: Myosin head attaches to actin.

2. Power stroke: Myosin head pivots, pulling actin filament toward M line.

3. Cross bridge detachment: ATP binds to myosin, causing detachment.

4. Cocking of myosin head: ATP hydrolysis re-energizes the myosin head.

Whole Muscle Contraction

Motor Units

A motor unit consists of a motor neuron and all the muscle fibers it innervates. Smaller motor units allow for fine control, while larger units generate more force.

Muscle Twitch and Graded Contractions

• Muscle twitch: Response of a muscle to a single stimulus, consisting of latent, contraction, and relaxation phases.

• Graded contractions: Varying strength of contraction by changing stimulus frequency (temporal summation) or strength (recruitment).

Muscle Tone and Types of Contractions

• Muscle tone: Slight, constant contraction of muscles, maintaining readiness.

• Isotonic contraction: Muscle changes length (concentric: shortens; eccentric: lengthens).

• Isometric contraction: Muscle tension increases without changing length.

Muscle Metabolism and Fatigue

ATP Sources for Muscle Contraction

• Direct phosphorylation: Creatine phosphate donates phosphate to ADP to form ATP (via creatine kinase).

• Anaerobic pathway: Glycolysis and lactic acid formation (no oxygen required).

• Aerobic pathway: Glycolysis followed by aerobic respiration in mitochondria (requires oxygen).

Muscle Fatigue and Recovery

• Muscle fatigue: Inability to contract despite stimulation, often due to ionic imbalances, increased inorganic phosphate, decreased ATP, or glycogen depletion.

• Excess postexercise oxygen consumption (EPOC): Oxygen required to restore muscle to pre-exercise state (replenish ATP, remove lactate, restore glycogen).

Factors Affecting Muscle Contraction

Force of Contraction

• Number of cross bridges attached (frequency of stimulation, number of fibers recruited, size of fibers, degree of stretch).

• Muscle hypertrophy increases force production.

Velocity and Duration of Contraction

• Muscle fiber type: Slow oxidative (endurance), fast oxidative (medium intensity), fast glycolytic (short, powerful movements).

• Load: Heavier loads decrease contraction speed and duration.

• Recruitment: More motor units increase speed and duration of contraction.

Muscle Adaptation and Clinical Aspects

Exercise and Muscle Adaptation

• Aerobic exercise: Increases capillaries, mitochondria, myoglobin, endurance, and resistance to fatigue.

• Resistance exercise: Increases muscle size (hypertrophy), strength, and stores of glycogen and connective tissue.

• Disuse atrophy: Muscle degeneration due to inactivity or loss of neural stimulation.

Smooth Muscle

Structure and Function

• Found in walls of hollow organs (except heart).

• Spindle-shaped, single nucleus, non-striated, connected by gap junctions.

• Organized in longitudinal and circular layers for coordinated contraction.

Differences from Skeletal Muscle

• Less developed SR, no T tubules; Ca2+ mainly from extracellular fluid.

• Varicosities instead of neuromuscular junctions; innervated by autonomic nervous system.

• Intermediate filaments and dense bodies anchor actin filaments.

• No troponin; calmodulin binds Ca2+ for contraction regulation.

Types of Smooth Muscle

• Unitary (visceral): Most common, found in hollow organs, electrically coupled by gap junctions, can contract spontaneously.

• Multi-unit: Found in large airways, arteries, arrector pili, iris; few gap junctions, contracts in response to neural stimuli.

Developmental Aspects of Muscle

• All muscle tissues develop from embryonic mesoderm cells called myoblasts.

• Skeletal muscle cells are multinucleated due to myoblast fusion; cardiac and smooth muscle cells do not fuse but develop gap junctions.

• Muscle regeneration is limited in skeletal muscle, modest in cardiac muscle, and ongoing in smooth muscle.