Muscle Tissues: Types, Structure, and Contraction Mechanisms

Three Types of Muscle Tissue

Skeletal, cardiac, smooth.

Skeletal Muscle Characteristics

Voluntary, striated, multinucleated.

Cardiac Muscle Characteristics

Involuntary, striated, intercalated discs.

Smooth Muscle Characteristics

Involuntary, non-striated, spindle-shaped.

Epimysium

CT layer surrounding whole muscle.

Perimysium

Wraps fascicles.

Endomysium

Surrounds individual fibers.

Sarcolemma

Muscle cell membrane.

T-tubules

Transmit action potentials into muscle fiber.

Sarcoplasmic Reticulum

Stores and releases Ca²⁺.

Myofibrils

Rod-like structures containing sarcomeres.

Sarcomere

Contractile unit between Z-discs.

A Band

Contains thick filaments; constant length.

I Band

Thin filaments; shortens during contraction.

H Zone

Central region with only thick filaments; disappears on contraction.

Myosin

Thick filament; forms cross-bridges.

Actin

Thin filament; contains myosin-binding sites.

Tropomyosin

Blocks binding sites on actin.

Troponin

Binds Ca²⁺ to move tropomyosin.

Neuromuscular Junction

Synapse between motor neuron & muscle fiber.

Acetylcholine (ACh)

Neurotransmitter that triggers muscle AP.

Excitation-Contraction Coupling

Link between AP and Ca²⁺ release.

Cross-Bridge Cycle

Binding, power stroke, detachment, reactivation.

Motor Unit

One motor neuron + all fibers it innervates.

Recruitment

Increasing number of active motor units.

Isotonic Contraction

Muscle changes length (concentric/eccentric).

Isometric Contraction

Tension without length change.

Length-Tension Relationship

Max force at optimal sarcomere length.

ATP Sources

Stored ATP, creatine phosphate, glycolysis, aerobic respiration.

Type I Fibers

Slow oxidative; endurance.

Type IIa Fibers

Fast oxidative-glycolytic.

Type IIb Fibers

Fast glycolytic; power.

Intercalated Discs

Cardiac muscle junctions with gap junctions & desmosomes.

Smooth Muscle Calmodulin

Binds Ca²⁺ to activate MLCK.

MLCK

Myosin light-chain kinase; enables cross-bridge in smooth muscle.

Single-Unit Smooth Muscle

Gap junctions; synchronous contraction.

Multi-Unit Smooth Muscle

Independent fibers; precise control.

Myasthenia Gravis

Autoimmune destruction of ACh receptors.

Muscular Dystrophy

Dystrophin defect; muscle weakness.

Rigor Mortis

No ATP → locked cross-bridges.