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

9.1 Structure of Skeletal Muscles

Gross Anatomy of a Skeletal Muscle

Skeletal muscles are complex organs composed of muscle fibers, connective tissue, blood vessels, and nerves. Understanding their structure is essential for grasping how muscles generate movement and force.

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

• Fascicle: A bundle of 10–100 muscle fibers grouped together, 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.

Attachment and Innervation

• Tendons: Attach muscle to bone or other structures to facilitate movement.

• Blood and Nerve Supply: Skeletal muscles are richly supplied with blood vessels and nerves, essential for function and repair.

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

Muscle Knots (Myofascial Trigger Points)

Myofascial trigger points, or "muscle knots," are localized, irritable spots within the fascia that can cause pain and sustained contraction.

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

• Symptoms: Pain and sustained muscle contraction.

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

Fascicle Patterns and Muscle Shapes

Muscles are classified by the arrangement of their fascicles, which affects their function and appearance.

• Parallel: Evenly spaced fascicles, tendon same width as muscle (e.g., Sartorius).

• Convergent: Broad at one end, tapers to a single tendon (e.g., Pectoralis major).

• Circular (Sphincters): Encircle openings, constrict when contracted (e.g., Orbicularis oculi).

• Fusiform: Thick in the middle, tapered at ends (e.g., Biceps brachii).

• Pennate: Fascicles attach to tendon at an angle, feather-like appearance:

  • Unipennate: Fascicles on one side (e.g., Flexor pollicis longus).

  • Bipennate: Fascicles on both sides (e.g., Rectus femoris).

  • Multipennate: Several tendons, fascicles join like multiple feathers (e.g., Deltoid).

9.1 Naming Muscles

Principles of Muscle Naming

Muscles are named based on various characteristics to aid in identification and understanding of their function.

• Size: Terms like major, minor, longus, brevis, vastus.

• Location: Directional (superior, inferior, medial, lateral) and regional anatomical terms (e.g., Pectoralis minor).

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

• Function: Flexors, extensors, levators, adductors, abductors.

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

Common Terms in Muscle Anatomy

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

9.1 Functions of Skeletal Muscles

Muscle Tension and Movement

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

• Muscle Tension: Force generated by muscle contraction.

• Body Movements: Actions produced by muscle tension.

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

Functional Groups of Muscles

Muscles work in groups to coordinate movement efficiently and safely.

• Agonists (Prime Movers): Provide most of the force for a movement.

• Antagonists: Oppose and slow the action of agonists, allowing controlled movement.

• Synergists: Assist agonists and ensure smooth movement.

• Fixators: Stabilize bones, making movement more efficient and reducing injury risk.

Muscle Origin and Insertion

Muscles attach to bones at two points: the origin (more fixed) and the insertion (moves during contraction).

• Origin (O): Attachment to the stationary bone.

• Insertion (I): Attachment to the bone that moves.

• Example: Biceps brachii—origin at scapula, insertion at radius.

• Origin and insertion can switch depending on the movement (e.g., iliopsoas during forward bending).

Lever Systems Used in Body Movements

Muscles and bones interact as lever systems to produce movement. There are three classes of levers in the body:

• Lever: The bone that moves.

• Load: The object or body part being moved.

• Force: Muscle tension applied to the lever.

• Fulcrum: The joint or hinge 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; rare in body).

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

Mechanical Advantage and Disadvantage

Levers can provide a mechanical advantage (move a large load with less force) or disadvantage (require more force to move a load).

• Mechanical Advantage: Fulcrum close to load; small force moves large load.

• Mechanical Disadvantage: Fulcrum far from load; greater force needed to move load.

Muscle Strains

A muscle strain occurs when a muscle is overstretched or subjected to excessive force, resulting in tearing of muscle fibers.

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

• Treatment: PRICE method—Protect, Rest, Ice, Compression, Elevation; may include medications, physical therapy, or surgery.

9.1 Studying Muscles

Muscle Groups of the Human Body

Muscles are categorized by their location and function in the body. Understanding these groups aids in learning muscle anatomy.

• 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

Familiarity with the anterior and posterior superficial muscles provides a foundation for further study.