Muscular System: Structure, Function, and Major Muscles

MUSCULAR SYSTEM

General Characteristics of Muscle Tissue

Muscle tissue is specialized for contraction, enabling movement and force generation in the body. Muscle cells, also known as muscle fibers, are elongated and contractile, shortening and thickening during contraction to pull on their attachment points and produce movement.

• Contractility: Ability to shorten forcefully.

• Three Types of Muscle Tissue:

  • Skeletal Muscle: Voluntary, striated, attached to bones.

  • Cardiac Muscle: Involuntary, striated, found in the heart.

  • Smooth Muscle: Involuntary, non-striated, found in walls of hollow organs.

SKELETAL MUSCLE: STRUCTURE

Composition and Connective Tissue Coverings

Skeletal muscle is composed of muscle tissue, nervous tissue, blood, and connective tissue. Several connective tissue layers organize and protect muscle fibers:

• Fascia: Dense connective tissue that separates muscles and holds them in position.

  • Deep fascia: Dense CT surrounding muscles.

  • Subcutaneous fascia: Beneath the skin.

  • Subserous fascia: Beneath serous membranes.

• Tendons: Cord-like projections of fascia that attach muscle to bone (periosteum).

• Aponeuroses: Broad, flat sheets of connective tissue attaching adjacent muscles.

Connective Tissue Layers Within Muscle

• Epimysium: Surrounds the entire muscle.

• Perimysium: Surrounds bundles of muscle fibers called fascicles.

• Endomysium: Covers each individual muscle fiber within a fascicle.

HUMAN MUSCLE ANATOMY

Muscle Fiber Structure

• Sarcolemma: The muscle cell membrane.

• Myofibrils: Parallel, threadlike structures within muscle fibers, composed of bundles of myofilaments.

  • Actin: Thin protein filament.

  • Myosin: Thick protein filament.

• Sarcomere: The functional unit of contraction, defined by alternating light (I band) and dark (A band) regions, bordered by Z lines.

Sarcomere Structure

• Z line: Boundary of each sarcomere.

• I band: Contains only thin filaments (actin).

• A band: Contains thick filaments (myosin) and overlapping thin filaments.

• H zone: Center of A band, only thick filaments.

MUSCLE ORGANIZATION AND FUNCTION

Fascicles and Muscle Arrangement

Muscle cells are organized into bundles called fascicles. The arrangement of fascicles affects the muscle's power, range, and speed of movement.

Classification of Skeletal Muscles

• Based on fascicle arrangement and relationship to tendons.

CLASSIFICATION OF MUSCLE SHAPES

Parallel Muscles

• Fibers run parallel to the long axis (e.g., biceps brachii).

• Muscle thickens in the center when contracted.

• Contracts about 30% of its length.

• Tension: Depends on total number of myofibrils and cross-sectional area. Example: 1 in2 (6.45 cm2) cross section develops 50 lb (23 kg) of tension.

Convergent Muscles

• Broad area converges on a single attachment site (e.g., tendon, aponeurosis, or raphe).

• Fibers pull in different directions depending on stimulation (e.g., pectoralis major).

Pennate Muscles

• Fibers form an angle with the tendon.

• Do not move as far as parallel muscles but generate more tension.

• Types:

  • Unipennate: Fibers on one side of tendon (e.g., extensor digitorum).

  • Bipennate: Fibers on both sides of tendon (e.g., rectus femoris).

  • Multipennate: Tendon branches within muscle (e.g., deltoid).

Circular Muscles

• Also called sphincters.

• Open and close to guard entrances of the body (e.g., orbicularis oris).

MUSCLE FUNCTION

• Enable movement, maintain posture, and generate heat.

• Skeletal muscles attach to the skeleton and produce motion.

• Type of muscle attachment affects power, range, and speed of movement.

ENERGY FOR MUSCLE CONTRACTION

ATP and Cellular Respiration

• Energy is stored in muscles as ATP.

• ATP is produced from glucose breakdown during cellular respiration in the mitochondria.

• When muscles are fatigued, contraction is impaired due to lack of oxygen.

• Energy sources:

  • Aerobic respiration: Requires oxygen, produces more ATP.

  • Anaerobic respiration: Does not require oxygen, produces less ATP and lactic acid.

EXERCISE AND MUSCLE CONTRACTIONS

Isometric vs. Isotonic Contractions

• Isometric: Tension increases, muscle length remains the same. Important for posture (e.g., antigravity muscles).

• Isotonic: Tension remains the same, muscle shortens, used in movement. Most normal exercise is isotonic.

ORIGIN AND INSERTION

Muscle Attachments

• Origin: End of muscle attached to an immovable or fixed part.

• Insertion: Moveable end, undergoes most movement.

• When a muscle contracts, insertion is pulled toward origin.

• Some muscles have more than one origin (e.g., biceps brachii).

INTERACTIONS OF SKELETAL MUSCLES

• Muscles function in groups:

• Prime Mover (Agonist): Main muscle responsible for an action (e.g., deltoid for arm abduction).

• Synergists: Assist prime mover, stabilize movement.

• Antagonists: Oppose prime mover, produce opposite movement.

• Fixator: Stabilizes origin of prime mover for efficient movement.

• If both prime mover and antagonist contract simultaneously, the structure remains rigid.

MUSCLE MOVEMENTS

Types of Movements

• Flexion: Decreases angle between bones (e.g., bending elbow).

• Extension: Increases angle between bones (e.g., straightening elbow).

• Hyperextension: Extension beyond anatomical position.

• Abduction: Movement away from midline.

• Adduction: Movement toward midline.

• Circumduction: Circular movement combining flexion, extension, abduction, and adduction.

• Rotation: Movement around longitudinal axis.

NAMES OF SKELETAL MUSCLES

Muscle Naming Criteria

• About 650 muscles in the human body.

• Names may indicate size, shape, location, action, number of attachments, or direction of fibers.

Examples

• Pectoralis major: Large, pectoral region.

• Deltoid: Shaped like delta/triangle.

• Extensor digitorum: Extends digits of fingers/toes.

• Biceps brachii: Two heads, located in brachium/arm.

• Sternocleidomastoid: Attached to sternum, clavicle, and mastoid process.

• External oblique: Near outside, fibers run obliquely/slant direction.

SUPERFICIAL MUSCLES

Anterior and Posterior Views

Superficial muscles are those closest to the skin and are visible in anatomical diagrams. They include major muscles responsible for movement and posture.

MUSCLES OF FACIAL EXPRESSION

Major Muscles and Functions

Muscle	Function
Epicranius	Raises eyebrows, wrinkles forehead
Orbicularis oculi	Closes eyelids, aids flow of tears
Orbicularis oris	Closes and protrudes lips (kissing)
Buccinator	Compresses cheek, holds food during chewing
Zygomaticus	Draws corner of mouth upward (smiling/laughing)
Platysma	Pulls angle of mouth downward, helps lower mandible

MUSCLES OF MASTICATION

Major Muscles and Actions

Muscle	Origin	Insertion	Action
Masseter	Zygomatic arch	Mandible	Elevates jaw
Temporalis	Side of skull	Mandible	Elevates and retracts jaw
Medial Pterygoid	Sphenoid, palatine, maxillary bones	Mandible	Closes jaw, moves side to side
Lateral Pterygoid	Region below mandibular condyle	Sphenoid bone	Opens mouth, protrudes and moves jaw side to side

MUSCLES THAT MOVE HEAD AND VERTEBRAL COLUMN

Key Muscles

• Sternocleidomastoid: Flexes and rotates head; origin at manubrium of sternum and clavicle, insertion at mastoid process.

• Splenius capitis, Semispinalis capitis, Erector spinae: Extend, rotate, and laterally flex the head and vertebral column.

MUSCLES THAT MOVE THE PECTORAL GIRDLE

Key Muscles

• Trapezius: Moves scapula, extends head.

• Rhomboideus major: Retracts scapula.

• Levator scapulae: Elevates scapula.

• Serratus anterior: Protracts scapula.

• Pectoralis minor: Depresses and protracts scapula.

Additional info: These notes expand on the provided slides with definitions, examples, and tables for clarity and completeness.