Muscle Tissue: Structure, Function, and Physiology

Muscle Tissue Overview

Properties of Muscle Cells

Muscle cells, also known as myocytes, possess unique physiological properties that enable movement and force generation in the body.

• Contractility: The ability of muscle cells to shorten forcefully when stimulated, producing movement or tension.

• Excitability: The capacity to respond to stimuli, typically from nerves, by generating electrical impulses.

• Extensibility: The ability to be stretched or extended without being damaged.

• Elasticity: The ability to return to original length after contraction or extension.

Muscle Tissue Types

There are three main types of muscle tissue, each with distinct structure and function:

• Skeletal Muscle: Voluntary, striated muscle attached to bones; responsible for body movement.

• Cardiac Muscle: Involuntary, striated muscle found only in the heart; responsible for pumping blood.

• Smooth Muscle: Involuntary, non-striated muscle found in walls of hollow organs (e.g., intestines, blood vessels).

Structure of Skeletal Muscle

Organization and Components

Skeletal muscle is organized into bundles and layers, each with specialized connective tissue coverings.

• Perimysium: Surrounds bundles of muscle fibers called fascicles.

• Fascicle: A bundle of muscle fibers.

• Endomysium: Surrounds individual muscle fibers (myocytes).

• Muscle Cell (Myocyte): The basic contractile unit of muscle tissue.

Muscle Fiber (Myocyte) Structure

• Sarcolemma: The plasma membrane of a muscle cell.

• Sarcoplasm: The cytoplasm of a muscle cell, containing organelles and myofibrils.

• Transverse Tubule (T-tubule): Invaginations of the sarcolemma that help transmit action potentials.

• Sarcoplasmic Reticulum: Specialized endoplasmic reticulum that stores and releases calcium ions for contraction.

• Myofibril: Cylindrical structures within muscle fibers, composed of repeating units called sarcomeres.

Muscle Filaments and Sarcomere Structure

Filament Types

• Thin Filament: Composed primarily of actin.

• Elastic Filament: Composed of titin, providing elasticity and stability.

• Regulatory Proteins: Tropomyosin and troponin regulate actin-myosin interaction during contraction.

Sarcomere

The sarcomere is the fundamental contractile unit of muscle, defined by Z-lines and containing overlapping thick and thin filaments.

• I band: Region containing only thin filaments.

• A band: Region containing thick filaments (myosin), with overlapping thin filaments.

• Z line: Defines the boundaries of each sarcomere.

Nerve-Muscle Relationship

Neuromuscular Junction

Communication between nerves and muscles occurs at the neuromuscular junction, enabling voluntary movement.

• Motor Units: A single motor neuron and all the muscle fibers it innervates.

• Neuromuscular Junction: The synapse between a motor neuron and a muscle fiber.

• Nerve Terminal Bulb: The end of the motor neuron, where neurotransmitters are released.

• Synaptic Cleft: The gap between the nerve terminal and the muscle cell membrane (sarcolemma).

• Sarcolemma: The muscle cell membrane, containing receptors for neurotransmitters.

Neurotransmission Process

1. Action potential arrives at the nerve terminal bulb.

2. Neurotransmitter (acetylcholine) is released into the synaptic cleft.

3. Acetylcholine binds to cholinergic receptors on the sarcolemma.

4. Muscle cell membrane is activated, leading to muscle contraction.

5. If stimulation ends, esterases break down acetylcholine, terminating the signal.

Sliding Filament Model of Contraction

Phases of Muscle Contraction

Muscle contraction occurs through the interaction of actin and myosin filaments within the sarcomere.

1. Latent Period: Time between stimulus and onset of contraction; includes events at the neuromuscular junction.

2. Excitation-Contraction Coupling: Transmission of action potentials leading to myocyte activation.

3. Contraction: Cross-bridge cycling between actin and myosin, resulting in shortening of the sarcomere.

4. Relaxation: Return to resting state as calcium is reabsorbed and cross-bridges detach.

Rigor Mortis: Post-mortem muscle stiffness due to lack of ATP for relaxation.

Contraction Patterns and Muscle Response

Types of Contraction

• Twitch: A single, brief contraction followed by relaxation.

• Latent Phase: Delay before contraction begins.

• Contraction Phase: Period of active tension development.

• Relaxation Phase: Period when tension decreases and muscle returns to rest.

Frequency and Intensity

• Treppen (Staircase Effect): Gradual increase in contraction strength with repeated stimulation.

Contraction Types

• Isometric Contraction: Muscle generates tension without changing length.

• Isotonic Contraction: Muscle changes length while maintaining tension.

  • Concentric: Muscle shortens during contraction.

  • Eccentric: Muscle lengthens during contraction.

• Muscle Tone: Continuous, passive partial contraction of muscles.

Energy Sources for Muscle Contraction

ATP Production Pathways

• Creatine Phosphate: Rapid source of ATP regeneration during short, intense activity.

• Anaerobic Fermentation: ATP produced without oxygen; waste product is lactic acid.

• Aerobic Respiration: ATP produced with oxygen; waste products are carbon dioxide and water.

ATP Yield:

• Anaerobic Fermentation: $2$ ATP per glucose molecule

• Aerobic Respiration: $36$ ATP per glucose molecule

Muscle Fiber Types

Slow Twitch (Red Fibers)

• High in mitochondria and myoglobin

• Rich capillary supply

• Adapted for endurance activities

Fast Twitch (White Fibers)

• High in glycogen, low in myoglobin

• Adapted for rapid, powerful contractions

• Fatigue quickly

Muscle Vocabulary

• Hypertrophy: Increase in muscle size due to growth of individual fibers.

• Atrophy: Decrease in muscle size due to loss of fibers or inactivity.

• Hyperplasia: Increase in number of muscle fibers (rare in humans).

• Flaccid: Lacking normal muscle tone.

Muscle Disorders and Diseases

• Spasm: Sudden, involuntary muscle contraction.

• Cramp: Prolonged, painful muscle contraction.

• Tendinitis: Inflammation of a tendon (e.g., tennis elbow).

• Tendinosis: Degeneration of a tendon due to chronic overuse.

• Tear: Damage to muscle or tendon fibers.

• Paralysis: Loss of muscle function due to nerve damage.

• Myasthenia Gravis: Autoimmune disorder causing muscle weakness.

• Myopathy: General term for muscle disease (e.g., muscular dystrophy, multiple sclerosis, ALS).

• Cerebral Palsy: Neuromuscular disorder affecting movement and posture.

• Ataxia: Lack of muscle coordination.

• Apraxia: Inability to perform purposeful movements.

• Metabolic Diseases: Disorders affecting muscle metabolism (e.g., defects in carbohydrate processing).

Parts of a Skeletal Muscle

• Origin: Fixed attachment point of the muscle.

• Belly: Thick, central part of the muscle.

• Insertion: Movable attachment point of the muscle.

Major Muscle Groups

• Muscles of Facial Expression and Scalp Motion

• Muscles of Mastication

• Muscles of Swallowing / Speaking

• Muscles of Respiration

• Muscles of the Abdomen (anterior)

• Muscles of Posture (posterior)

• Muscles of the Shoulder

• Muscles of the Arm

• Muscles of the Hip

• Muscles of the Thigh

• Muscles of the Leg

Comparison of Muscle Fiber Types

Feature	Slow Twitch (Red)	Fast Twitch (White)
Myoglobin Content	High	Low
Mitochondria	Many	Few
Capillary Supply	Rich	Poor
Function	Endurance	Strength, Speed
Color	Red	White

Key Equations

• ATP Yield (Aerobic): $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + 36ATP$

• ATP Yield (Anaerobic): $C_6H_{12}O_6 \rightarrow 2C_3H_6O_3 + 2ATP$

Additional info: Some muscle group listings and disease examples were expanded for clarity and completeness. The comparison table was inferred from standard muscle physiology knowledge.