Chapter 9: The Muscular System – Structure, Function, and Classification of Skeletal Muscles

Structure of Skeletal Muscles

Gross Anatomy of a Skeletal Muscle

Skeletal muscles are composed of specialized cells and connective tissue layers that organize and support muscle function.

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

• Fascicle: A bundle of 10–100 muscle fibers, surrounded by Perimysium connective tissue.

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

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

Additional Structures

• Tendons: Attach muscle to bone or other structures, transmitting force for movement.

• Blood and Nerve Supply: Skeletal muscles are highly vascularized and innervated.

• Voluntary Control: Skeletal muscle contraction is under conscious control, requiring nervous system stimulation.

Muscle Knots (Myofascial Trigger Points)

Localized areas within the fascia that are irritable or inflamed, often causing pain and sustained contraction.

• Causes: Repetitive exercise, trauma, stress, disease, or trapped nerves.

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

Fascicle Patterns and Muscle Shapes

Classification by Fascicle Arrangement

• Parallel Muscle: Fascicles run parallel to the tendon; e.g., Sartorius.

• Convergent Muscle: Broad origin, tapers to a single tendon; e.g., Pectoralis Major.

• Circular Muscle (Sphincter): Encircles openings; e.g., Orbicularis Oculi.

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

• Pennate Muscle: Fascicles attach at an angle to the tendon, resembling a feather.

  • Unipennate: Fascicles on one side; e.g., Flexor Pollicis Longus.

  • Bipennate: Fascicles on both sides; e.g., Rectus Femoris.

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

Naming Muscles

Criteria for Muscle Names

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

• Location: Directional terms (Superior, Inferior, Medial, Lateral), anatomical regions.

• Attachment: Named for structures they attach to; e.g., Sternocleidomastoid.

• Function: Flexors, Extensors, Levators, Adductors, Abductors.

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

Table: Common Terms in Muscle Anatomy

Term and Meaning	Example
Brevis—short	Fibularis brevis muscle
Longus—long	Adductor longus muscle
Vastus—wide/large	Vastus lateralis muscle
Anterior—toward the front	Tibialis anterior muscle
Flexor—decreases angle between bones	Flexor digitorum longus muscle
Extensor—increases angle between bones	Extensor digitorum longus muscle
Biceps—two heads	Biceps brachii muscle
Deltoid—triangular	Deltoid muscle
Trapezius—trapezoid shape	Trapezius muscle

Functions of Skeletal Muscles

Muscle Tension and Actions

The primary function of skeletal muscle is to contract and generate muscle tension, enabling movement and heat production.

• Actions: Movement of body parts.

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

Functional Groups of Muscles

• Agonists (Prime Movers): Main force provider for movement.

• Antagonists: Oppose or slow the action of agonists.

• Synergists: Assist agonists, ensuring smooth movement.

• Fixators: Stabilize bones, improving movement efficiency and reducing injury risk.

Muscle Origin and Insertion

• Origin (O): Attachment site that remains stationary.

• Insertion (I): Attachment site that moves during contraction.

• Example: Biceps brachii originates at the scapula and inserts at the radius.

• Origin and insertion may switch depending on movement context (e.g., Iliopsoas).

Lever Systems in Body Movements

Muscles and bones interact as levers to produce movement, with four main components:

• Lever: Bone

• Load: Object/body part being moved

• Force: Muscle tension

• Fulcrum: Joint

Types of Levers

• First-Class Lever: Fulcrum in the middle; e.g., Atlanto-Occipital Joint.

• Second-Class Lever: Load in the middle; e.g., Metatarsophalangeal Joints.

• Third-Class Lever: Force in the middle; e.g., Elbow Joint (Biceps brachii).

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 Strains

Definition and Treatment

• Muscle Strain: Overstretching or excessive contraction causes muscle fiber tears, ranging from minor to complete rupture.

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

• PRICE Method:

  • Protect from further injury

  • Rest to allow healing

  • Ice and Compression to reduce inflammation

  • Elevation to drain fluid

• Additional treatments: Anti-inflammatory medications, physical therapy, surgical repair.

Studying Muscles

Muscle Group Classification

• 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

Familiarization with superficial muscles (anterior and posterior views) is recommended for foundational study.

Key Equations

Lever System Equation

The mechanical advantage of a lever can be expressed as:

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

Where the effort arm is the distance from the fulcrum to the point where force is applied, and the load arm is the distance from the fulcrum to the load.

Summary Table: Muscle Naming Terms

Term	Meaning	Example
Flexor	Decreases angle between bones	Flexor digitorum longus
Extensor	Increases angle between bones	Extensor digitorum longus
Abductor	Moves limb away from midline	Abductor pollicis longus
Adductor	Moves limb toward midline	Adductor magnus
Biceps	Two heads	Biceps brachii
Deltoid	Triangular shape	Deltoid muscle

Additional info: Tables have been reconstructed and expanded for clarity. Lever system equations and muscle group classifications have been supplemented for academic completeness.