The Muscular System: Structure, Function, and Types of Muscle Tissue

The Muscular System

Introduction to Muscle Tissue

Muscle tissue is a primary tissue in the human body, essential for movement, posture, and various physiological functions. The term muscle is derived from the Latin word for “little mouse.” There are three main types of muscle tissue: skeletal, cardiac, and smooth muscle, each with distinct locations and functions.

Properties of Muscle Tissue

• Contractility: Muscle cells shorten due to the action of myofilaments (actin and myosin).

• Excitability: Muscle cells respond to nerve signals, generating electrical impulses along their membranes.

• Extensibility: Muscles can be stretched by opposing muscles or substances within hollow organs.

• Elasticity: Muscles recoil to their original length after being stretched.

Muscle Tissue Terminology

• Myo- and mys-: Prefixes meaning “muscle.”

• Sarco-: Prefix meaning “flesh.”

• Sarcolemma: Plasma membrane of muscle cells.

• Sarcoplasm: Cytoplasm of muscle cells.

Functions of Muscle Tissue

Movement and Other Functions

• Produce Movement: Skeletal muscle moves bones; smooth muscle moves substances through hollow organs.

• Open and Close Passageways: Sphincter muscles act as valves to regulate passage of substances.

• Maintain Posture and Stabilize Joints: Muscle tone stabilizes joints and maintains posture.

• Heat Generation: Muscle contractions produce heat, helping maintain body temperature.

Types of Muscle Tissue

Skeletal Muscle Tissue

• Packaged into skeletal muscles, making up about 40% of body weight.

• Cells are striated and under voluntary control.

Cardiac Muscle Tissue

• Found only in the heart walls.

• Cells are striated and contraction is involuntary.

Smooth Muscle Tissue

• Located in the walls of hollow organs (e.g., intestines, blood vessels).

• Cells lack striations and are under involuntary control.

Gross Anatomy of Skeletal Muscle

Muscle as an Organ

Each skeletal muscle is an organ composed of muscle tissue, connective tissue, blood vessels, and nerves.

Connective Tissue Sheaths

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

• Perimysium: Fibrous connective tissue surrounding each fascicle (bundle of muscle fibers).

• Endomysium: Fine connective tissue sheath wrapping each muscle cell.

Muscle Attachments

• Most skeletal muscles attach to bones at two points: origin (less movable) and insertion (more movable).

• Attachments can be direct (to bone) or indirect (via tendons).

Microscopic and Functional Anatomy of Skeletal Muscle

Skeletal Muscle Fiber Structure

• Fibers are long, cylindrical, multinucleate cells with peripheral nuclei.

• Formed by fusion of embryonic cells; can be several centimeters long.

Myofibrils and Sarcomeres

Myofibrils are specialized contractile organelles composed of repeating units called sarcomeres, the functional unit of muscle contraction.

• Z disc (Z line): Boundary of each sarcomere.

• Thin (actin) filaments: Extend from Z disc toward the center.

• Thick (myosin) filaments: Located in the center, overlapping with thin filaments.

• A band: Full length of thick filament (anisotropic).

• I band: Region with only thin filaments (isotropic).

• H zone: Center of A band with no thin filaments.

• M line: Center of H zone, holds thick filaments together.

Sarcoplasmic Reticulum and T Tubules

• Sarcoplasmic reticulum (SR): Specialized smooth ER that stores and releases calcium ions, triggering muscle contraction.

• T tubules: Deep invaginations of the sarcolemma that transmit impulses into the muscle fiber.

• Triad: A T tubule flanked by two terminal cisterns of the SR.

Mechanism of Muscle Contraction

Types of Contraction

• Eccentric contraction: Muscle generates force as it lengthens (e.g., lowering a weight).

• Concentric contraction: Muscle shortens to do work (e.g., lifting a weight).

Sliding Filament Mechanism

Muscle contraction occurs via the sliding filament mechanism, where myosin heads attach to actin filaments and pull them toward the center of the sarcomere. This process is powered by ATP and initiated by calcium release from the SR.

• During contraction, Z discs move closer, sarcomeres shorten, I bands and H zones decrease, but A bands remain the same length.

Innervation of Skeletal Muscle

Neuromuscular Junction

• Motor neurons innervate skeletal muscle fibers at the neuromuscular junction.

• Terminal boutons (axon terminals) store neurotransmitters (e.g., acetylcholine, ACh).

• Synaptic cleft: Space between axon terminal and muscle fiber membrane (sarcolemma).

Motor Units

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

• Axons branch to form neuromuscular junctions with multiple muscle fibers.

Types of Skeletal Muscle Fibers

Classification by Energy Use and Contraction Speed

• Oxidative fibers: Produce ATP aerobically (using oxygen).

• Glycolytic fibers: Produce ATP anaerobically (without oxygen, via glycolysis).

Three Main Classes

• Slow oxidative fibers (Type I): Red, high myoglobin, many mitochondria, fatigue-resistant, small diameter.

• Fast glycolytic fibers (Type IIb): White, low myoglobin, few mitochondria, fatigue quickly, large diameter.

• Fast oxidative fibers (Type IIa): Intermediate properties, contract quickly, moderate fatigue resistance.

Fiber Type	Main Energy Source	Fatigability	Mitochondria	Glycogen	Color
Red Slow (Type I)	Oxidative	Resistant to fatigue	High	Low	Red (high myoglobin)
Intermediate (Type IIa)	Oxidative & Glycolytic	Medium	Medium	Medium	Red (medium myoglobin)
White Fast (Type IIb)	Glycolytic	Fatigable	Few	High	White (low myoglobin)

Comparison of Skeletal, Cardiac, and Smooth Muscle

Characteristic	Skeletal	Cardiac	Smooth
Body Location	Attached to bones or skin	Walls of the heart	Walls of hollow organs
Cell Shape & Appearance	Long, cylindrical, multinucleate, striated	Branching chains, uni- or binucleate, striated	Fusiform, uninucleate, no striations

Disorders of Muscle Tissue

Muscular Dystrophy

• Inherited diseases causing muscle degeneration and replacement by fat/connective tissue.

• Types include Duchenne muscular dystrophy and myotonic dystrophy.

Other Disorders

• Myofascial pain syndrome: Pain from tightened muscle bands.

• Fibromyalgia: Chronic pain syndrome, mainly in women, with fatigue and musculoskeletal pain.