Muscle Tissue Overview

Types and Functions of Muscle Tissue

Muscle tissue is specialized for contraction, allowing movement and force generation in the body. Muscles convert chemical energy (ATP) into mechanical energy to exert force.

• 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; controls movement of substances within these organs.

Main Functions of Skeletal Muscle

• Produce movement

• Maintain posture

• Stabilize joints

• Generate heat

• Protect internal organs

• Support soft tissues

Common Properties of All Muscle Tissue

• Excitability: Ability to receive and respond to stimuli

• Contractility: Ability to shorten forcibly when stimulated

• Extensibility: Ability to be stretched or extended

• Elasticity: Ability to recoil to resting length after stretching

Connective Tissue Support

Muscle Connective Tissue Layers

• Epimysium: Surrounds entire muscle

• Perimysium: Surrounds bundles of muscle fibers (fascicles)

• Endomysium: Surrounds individual muscle fibers

Anatomy of a Skeletal Muscle Fiber

Structural Components

• Muscle fiber: Single muscle cell

• Sarcolemma: Plasma membrane of muscle fiber

• Sarcoplasm: Cytoplasm of muscle fiber

• Sarcoplasmic reticulum: Specialized endoplasmic reticulum that stores calcium ions

• Myofibrils: Rod-like structures containing contractile proteins

• Myoglobin: Oxygen-binding protein in muscle cells

• T-tubules: Invaginations of sarcolemma that transmit action potentials

Myofilaments

• Actin: Thin filament

• Myosin: Thick filament

Functional unit of contraction: The sarcomere, which is the segment between two Z-lines in a myofibril.

Muscle Contraction

Steps Initiating Muscle Contraction

1. An action potential arrives at the axon terminal.

2. Voltage-gated calcium channels open and calcium ions rush into the axon terminal.

3. Acetylcholine (ACh) is released into the synaptic cleft.

4. ACh binds to receptors on the sarcolemma of the muscle fiber.

5. Sodium channels open and sodium ions rush into the muscle fiber, initiating depolarization.

Role of Acetylcholine, Calcium, and Sodium Ions at the Neuromuscular Junction

• Acetylcholine: Neurotransmitter that transmits the nerve signal to the muscle fiber.

• Calcium ions: Released from sarcoplasmic reticulum, trigger contraction by binding to troponin.

• Sodium ions: Enter muscle fiber, causing depolarization and propagation of action potential.

Excitation-Contraction Coupling

• Action potentials sweep along the sarcolemma and down T-tubules.

• This triggers release of calcium ions from the sarcoplasmic reticulum.

• Calcium binds to troponin, causing tropomyosin to move and expose active sites on actin.

• Myosin heads bind to actin, forming cross-bridges and initiating contraction.

Steps that End a Muscle Contraction

• ACh is broken down by acetylcholinesterase in the synaptic cleft.

• Calcium ions are reabsorbed into the sarcoplasmic reticulum.

• Tropomyosin returns to its position, covering myosin binding sites on actin.

• Cross-bridges cannot form; muscle relaxes.

Muscle Twitch Phases

Phases and Cellular Events

• Latent period: Time between stimulus and onset of contraction; action potential propagates, calcium released.

• Contraction phase: Cross-bridge cycling occurs; muscle tension increases.

• Relaxation phase: Calcium reabsorbed, cross-bridges detach, muscle tension decreases.

Motor Units and Recruitment

Motor Unit Structure and Function

• Amount of tension produced depends on the number of cross-bridges pivoting in a muscle fiber.

• Factors affecting tension: frequency of stimulation, number of fibers activated, length of fiber at contraction, size of motor units.

• A motor unit consists of a single motor neuron and all muscle fibers it innervates.

• Recruitment is from small to large motor units as more force is needed.

Effects of Repeated Stimulation

• Summation: Increased force of contraction due to repeated stimuli.

• Incomplete tetanus: Muscle fibers partially relax between stimuli.

• Complete tetanus: No relaxation between stimuli; sustained contraction.

Muscle tone: Continuous, passive partial contraction of muscles.

Energy for Contraction

ATP Generation

• Direct phosphorylation: Creatine phosphate donates phosphate to ADP.

• Anaerobic glycolysis: Glucose broken down without oxygen; produces lactic acid.

• Aerobic respiration: Glucose broken down with oxygen; produces most ATP.

Fatigue occurs when ATP production cannot keep up with demand, often due to depletion of resources or accumulation of metabolic byproducts.

Muscle Adaptation & Physical Activity

Hypertrophy and Atrophy

• Hypertrophy: Increase in muscle size due to increased use.

• Atrophy: Decrease in muscle size due to disuse.

Fast-Twitch vs. Slow-Twitch Muscle Fibers

Mechanics of Movement

Types of Muscle Contraction

• Isotonic contraction: Muscle changes length; movement occurs.

• Concentric contraction: Muscle shortens as it contracts.

• Eccentric contraction: Muscle lengthens while contracting.

• Isometric contraction: Muscle tension increases, but length does not change; no movement.

Cardiac Muscle

Structure and Function

• Intercalated discs: Specialized connections between cardiac muscle cells that allow rapid transmission of electrical impulses and synchronized contraction.

• Cardiac muscle contracts about 10x slower than skeletal muscle.

• Tetanic contractions cannot occur in cardiac muscle.

• Relies solely on aerobic metabolism.

Smooth Muscle

Structure and Function

• Lacks: Striations and sarcomeres

• Found in: Walls of hollow organs (e.g., intestines, blood vessels)

• Calcium comes from: Extracellular fluid

• Contract: Slowly and can sustain contractions for long periods

Additional info: Some definitions and explanations were expanded for clarity and completeness.