The Muscular System: Structure and Function

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MUSCULAR SYSTEM

Overview of Muscle Tissue

The muscular system is one of the four major tissue types in the human body, alongside epithelial, connective, and neural tissues. Muscle tissue is specialized for contraction, enabling movement and other vital functions. There are three types of muscle tissue: skeletal muscle, smooth muscle, and cardiac muscle. Understanding their differences is essential for anatomy and physiology students.

Skeletal Muscle: Also known as voluntary or striated muscle, it is the largest muscle type. Fibers can be up to 14 inches long, are multinucleated, and possess striations. Skeletal muscle accounts for up to 40% of body weight and is primarily responsible for movement.
Smooth Muscle: Also called involuntary or visceral muscle, it has a single nucleus and tapered ends. Fibers are much smaller (rarely exceeding 200 μm) and lack striations. Smooth muscle lines internal organs and blood vessels.
Cardiac Muscle: The smallest muscle cells (rarely exceeding 100 μm), usually with a single nucleus (sometimes two). Fibers are branched, have faint striations, and contain unique intercalated discs. Cardiac muscle is found only in the heart and possesses intrinsic rhythmicity (self-contraction).

Example: Skeletal muscle enables voluntary movements such as walking, smooth muscle controls involuntary actions like digestion, and cardiac muscle contracts rhythmically to pump blood.

Functions of the Muscular System

Muscles perform several essential functions in the body:

Movement: Muscles work with the skeletal system to produce movement.
Protection: Muscles protect internal organs and smaller structures like blood vessels.
Heat Production: Muscle contraction generates heat, helping maintain body temperature.
Communication: Muscles enable facial expressions and gestures, conveying information nonverbally.

Muscle Terminology

Specialized Terms in Muscle Tissue

Muscle tissue has unique terminology reflecting its specialized structure and function:

Muscle Fiber: The term "muscle fiber" is used instead of "muscle cell" for the cells that make up muscle tissue.
Sarcolemma: The cell membrane of a muscle fiber.
Sarcoplasm: The cytoplasm within a muscle fiber.
Sarcoplasmic Reticulum: The specialized endoplasmic reticulum in muscle fibers, serving as a reservoir for calcium ions.
Transverse Tubules (T Tubules): Membrane-bound extensions of the sarcolemma that run perpendicular to it, forming passageways deep into the muscle fiber and containing extracellular fluid.

Example: During muscle contraction, calcium ions are released from the sarcoplasmic reticulum, triggering the interaction between actin and myosin.

Structure of Skeletal Muscle

Connective Tissue Layers

Skeletal muscle is organized and protected by several layers of connective tissue:

Superficial Fascia (Hypodermis): Separates muscle from skin, composed of areolar and adipose tissue. Provides pathways for nerves, blood vessels, and lymphatic vessels.
Deep Fascia: Dense irregular connective tissue that holds muscles together.
Epimysium: Outermost layer encircling the entire muscle, composed of dense irregular connective tissue.
Perimysium: Surrounds groups of muscle fibers, separating them into fascicles. Also composed of dense irregular connective tissue.
Endomysium: Thin sheath of areolar connective tissue that separates individual muscle fibers within a fascicle.

These layers are continuous with each other and with connective tissues that attach muscles to other structures (e.g., tendons, ligaments, bones).

Muscle Fiber Structure

Each skeletal muscle is composed of thousands of muscle fibers. Between bundles of muscle fibers are collagen, elastic fibers, nerves, and blood vessels. Each muscle fiber contains its own nerve supply.

Myofibrils and Myofilaments

Muscle fibers contain numerous myofibrils, which are the main intracellular structures responsible for contraction. Each myofibril is composed of myofilaments, which are bundles of protein filaments.

F Actin: Filamentous actin, a contractile protein made of two rows of G actin. G actin contains myosin binding sites. F actin is a major component of thin filaments (~6 nm diameter).
Nebulin: Filamentous protein that helps hold F actin fibers together.
Myosin: Contractile protein, major component of thick filaments (~12 nm diameter). Each myosin molecule has a rodlike tail and two globular heads (cross bridges).
Tropomyosin: Regulatory protein that maintains the shape of thin filaments and covers myosin binding sites in resting muscle.
Troponin: Regulatory protein that holds tropomyosin in place. Differences in troponin between skeletal and cardiac muscle are clinically significant for diagnosing heart attacks.
Titin (Connectin): Accessory protein, major component of elastic filaments. Titin stabilizes contractile filaments and returns stretched muscles to resting length. It is the largest known protein.

Example: During muscle contraction, myosin heads bind to actin, forming cross bridges and pulling the filaments past each other.

Sarcomere Structure

Each myofibril is divided into repeating segments called sarcomeres (about 10,000 per myofibril). A sarcomere is the segment between two successive Z lines (Z discs). It is the smallest contractile unit of a muscle fiber. The Z disc serves as the attachment site for actin. Cross bridges (myosin heads) connect thick and thin filaments, enabling contraction.

Sarcomere: Functional contractile unit of muscle fiber.
Z Disc: Protein structure marking the boundary of each sarcomere and serving as the attachment site for actin.
Cross Bridges: Myosin heads that connect thick and thin filaments during contraction.

Example: The sliding filament model describes how actin and myosin filaments slide past each other to shorten the sarcomere and produce muscle contraction.

Summary Table: Types of Muscle Tissue

Type	Location	Cell Structure	Striations	Nucleus	Function
Skeletal	Attached to bones	Long, cylindrical, multinucleated	Present	Multiple	Voluntary movement
Smooth	Walls of organs, blood vessels	Small, tapered, single nucleus	Absent	Single	Involuntary movement
Cardiac	Heart	Branched, small, single or double nucleus	Faint	Single or two	Rhythmic contraction

Key Terms and Definitions

Muscle Fiber: The basic unit of muscle tissue, also called a muscle cell.
Sarcolemma: The plasma membrane of a muscle fiber.
Sarcoplasm: The cytoplasm of a muscle fiber.
Sarcoplasmic Reticulum: Specialized endoplasmic reticulum in muscle fibers, stores calcium ions.
Myofibril: Intracellular structure composed of myofilaments, responsible for contraction.
Myofilament: Protein filaments (actin, myosin, etc.) within myofibrils.
Sarcomere: The smallest contractile unit of a muscle fiber.
Fascicle: Bundle of muscle fibers surrounded by perimysium.
Epimysium, Perimysium, Endomysium: Connective tissue layers surrounding muscle structures.

Additional info:

Muscle contraction is initiated by the release of calcium ions from the sarcoplasmic reticulum, which allows myosin to bind to actin.
Troponin levels in the blood are used clinically to diagnose and monitor heart attacks, as cardiac muscle troponin differs from skeletal muscle troponin.
The sliding filament model explains muscle contraction: actin and myosin filaments slide past each other, shortening the sarcomere.