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," reflecting the appearance of muscles under the skin. There are three main types of muscle tissue: skeletal, cardiac, and smooth muscle, each with distinct locations and functions.

Properties of Muscle Tissue

Key Properties

• Contractility: Muscle cells shorten due to the action of myofilaments, which are composed of actin and myosin proteins.

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

• Extensibility: Muscles can be stretched by opposing muscles or by substances within hollow organs (e.g., food in the stomach).

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

Muscle Tissue Terminology

• Myo- and mys-: Prefixes meaning "muscle" (e.g., myofibril, myosin).

• Sarco-: Prefix meaning "flesh" (e.g., sarcolemma, sarcoplasm).

• Sarcolemma: The plasma membrane of muscle cells.

• Sarcoplasm: The cytoplasm of muscle cells.

Functions of Muscle Tissue

Major Functions

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

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

• Maintain Posture and Stabilize Joints: Muscle tone helps maintain posture and stabilize synovial joints.

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

Types of Muscle Tissue

Skeletal, Cardiac, and Smooth Muscle

• Skeletal Muscle: Attached to bones, voluntary control, striated, makes up about 40% of body weight.

• Cardiac Muscle: Found only in the heart, involuntary control, striated.

• Smooth Muscle: Found in walls of hollow organs, involuntary control, non-striated.

Characteristic	Skeletal	Cardiac	Smooth
Location	Attached to bones or skin	Walls of heart	Walls of hollow organs
Control	Voluntary	Involuntary	Involuntary
Striations	Yes	Yes	No

Gross Anatomy of Skeletal Muscle

Connective Tissue Organization

Skeletal muscle is an organ composed of muscle tissue, connective tissue, blood vessels, and nerves. Connective tissue sheaths organize and protect muscle fibers:

• 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

• Origin: The less movable attachment of a muscle.

• Insertion: The more movable attachment.

• Most skeletal muscles span joints and attach to bones in at least two places.

Microscopic and Functional Anatomy of Skeletal Muscle

Muscle Fiber Structure

• Muscle fibers are long, cylindrical, multinucleate cells formed by the fusion of embryonic cells.

• Nuclei are located at the periphery of the cell.

• Myofibrils are specialized contractile organelles within muscle fibers, making up about 80% of the cytoplasm.

Myofibrils and Sarcomeres

Myofibrils are composed of repeating segments called sarcomeres, the functional units of muscle contraction. The arrangement of thick (myosin) and thin (actin) filaments within sarcomeres creates the striated appearance of skeletal muscle.

• 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, overlap 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

Calcium Storage and Release

• Sarcoplasmic reticulum (SR): Specialized smooth endoplasmic reticulum that stores calcium ions.

• Calcium is released from the SR when the muscle is stimulated, triggering contraction.

• T tubules: Deep invaginations of the sarcolemma that conduct 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

The sliding filament mechanism explains how muscles contract concentrically. Myosin heads attach to actin in the thin 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 together, sarcomeres shorten, I bands and H zones decrease, but A bands remain the same length.

Innervation of Skeletal Muscle

Neuromuscular Junction

• Motor neurons: Nerve cells that stimulate muscle fibers to contract.

• Neuromuscular junction: The site where a motor neuron meets a muscle fiber.

• Terminal boutons: Axon terminals that store neurotransmitters (e.g., acetylcholine).

• Synaptic cleft: The space between the axon terminal and the sarcolemma.

Motor Units

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

• Motor units allow for graded control of muscle contraction.

Types of Skeletal Muscle Fibers

Classification by Metabolism and Contraction Speed

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

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

• Fast oxidative fibers (Type IIa): Intermediate characteristics, aerobic metabolism, moderately fatigue-resistant.

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

Disorders of Muscle Tissue

Common Disorders

• Muscular dystrophy: Inherited diseases causing muscle degeneration and replacement by fat/connective tissue. Types include Duchenne and myotonic dystrophy.

• Myofascial pain syndrome: Pain caused by tightened muscle bands.

• Fibromyalgia: Chronic pain syndrome with fatigue, sleep issues, and widespread musculoskeletal pain, more common in women.