The Muscular System: Structure, Function, and Major Muscles

The Muscular System

Introduction

The muscular system is essential for movement, posture, and many physiological processes. This chapter focuses on the structure, classification, and function of skeletal muscles, as well as the major muscle groups of the human body.

Structure of Skeletal Muscles

Gross Anatomy of Skeletal Muscle

• Skeletal Muscle Fibers: Long, thin cells surrounded by a thin layer of extracellular matrix called the endomysium.

• Fascicle: A bundle of 10–100 muscle fibers, surrounded by perimysium.

• Epimysium: Connective tissue that surrounds all fascicles in a muscle.

• Fascia: The most superficial connective tissue sheath, continuous with the epimysium.

• Tendons: Attach muscle to bone or other structures.

• Rich Blood and Nerve Supply: Necessary for muscle function.

• Voluntary Control: Skeletal muscle contraction is under conscious control and requires nervous stimulation.

Fascicle Patterns and Muscle Shapes

• Parallel: Evenly spaced fascicles; e.g., Sartorius.

• Convergent: Broad at one end, tapers to a single tendon; e.g., Pectoralis Major.

• Circular (Sphincters): Encircle openings; e.g., Orbicularis Oculi.

• Fusiform: Thick in the middle, tapered at ends; e.g., Biceps Brachii.

• Pennate: Fascicles attach at an angle to the tendon.

  • Unipennate: One side; e.g., Flexor Pollicis Longus.

  • Bipennate: Both sides; e.g., Rectus Femoris.

  • Multipennate: Multiple tendons; e.g., Deltoid.

Naming Muscles

Principles of Muscle Naming

• Size: Major, Minor, Longus, Brevis, Vastus.

• Location: Directional (Superior, Inferior, Medial, Lateral) or regional terms.

• Attachment: Named for bones or structures attached (e.g., Sternocleidomastoid).

• Function: Flexor, Extensor, Levator, Adductor, Abductor.

• Depth: Profundus (deep), Superficialis (superficial).

Functions of Skeletal Muscles

Muscle Tension and Movement

• Muscle Tension: Force generated by contraction.

• Body Movements: Actions produced by muscle tension.

• Heat Generation: Conversion of ATP chemical energy to mechanical energy produces heat (e.g., shivering).

Functional Groups of Muscles

• Agonist (Prime Mover): Main muscle producing movement.

• Antagonist: Opposes or slows movement; located opposite the agonist.

• Synergist: Assists the agonist, stabilizes movement.

• Fixator: Stabilizes the origin of the agonist for efficient movement.

Muscle Origin and Insertion

• Origin (O): More fixed attachment point.

• Insertion (I): More moveable attachment point.

• Muscle contraction typically moves the insertion toward the origin.

• Origin and insertion can switch depending on movement context.

Lever Systems in Body Movements

• Lever: Bone that moves on a fixed point (fulcrum).

• Load: Object or body part being moved.

• Force: Muscle tension applied to the lever.

• Fulcrum: Joint acting as the pivot point.

• First-Class Lever: Fulcrum between load and force (e.g., atlanto-occipital joint).

• Second-Class Lever: Load between fulcrum and force (e.g., metatarsophalangeal joints).

• Third-Class Lever: Force between load and fulcrum (e.g., elbow joint with biceps brachii); most common in the body.

Mechanical Advantage and Disadvantage

• Mechanical Advantage: Small force moves a large load (fulcrum close to load).

• Mechanical Disadvantage: Greater force required to move the load (fulcrum farther from load).

Muscle Injuries and Clinical Applications

Muscle Knots (Myofascial Trigger Points)

• Localized, irritable spots in muscle fascia causing pain and sustained contraction.

• Caused by repetitive exercise, trauma, stress, disease, or trapped nerves.

• Treatment: Massage, anti-inflammatory medications, muscle relaxants, passive stretching.

Muscle Strains

• Occurs when muscle is overstretched or overloaded, causing tears.

• Symptoms: Pain, limited movement, swelling, bruising.

• Treatment: PRICE method (Protect, Rest, Ice, Compression, Elevation), medications, physical therapy, or surgery.

Studying Muscles: Overview of Major Muscle Groups

Major Muscle Groups

• Muscles of the head, neck, and vertebral column

• Muscles of the trunk and pelvic floor

• Muscles of the pectoral girdle and upper limb

• Muscles of the hip and lower limb

Summary Table: Common Terms in Muscle Anatomy

Term	Meaning
Major	Large
Minor	Small
Longus	Long
Brevis	Short
Vastus	Broad
Profundus	Deep
Superficialis	Superficial
Flexor	Decreases angle at a joint
Extensor	Increases angle at a joint
Adductor	Moves limb toward midline
Abductor	Moves limb away from midline

Key Equations

• Mechanical Advantage: $\text{Mechanical Advantage} = \frac{\text{Length of Effort Arm}}{\text{Length of Load Arm}}$

Conclusion

The muscular system is highly organized, with muscles classified by structure, function, and location. Understanding muscle anatomy and physiology is essential for comprehending movement, injury mechanisms, and clinical treatments.