The Muscular System

Overview

The muscular system is responsible for movement, posture, and heat production in the human body. Muscles are classified by their shapes, functions, and anatomical locations. Understanding muscle terminology, groupings, and functions is essential for studying anatomy and physiology.

Muscle Shapes

Types of Muscle Shapes

• Parallel: Muscle fibers run parallel to the long axis of the muscle (e.g., sartorius).

• Convergent: Broad origin with fibers converging toward a single tendon (e.g., pectoralis major).

• Pennate: Short fibers attach obliquely to a central tendon; increases power.

  • Unipennate: Fibers insert into one side of the tendon (e.g., extensor digitorum longus).

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

  • Multipennate: Multiple tendons with fibers attaching obliquely (e.g., deltoid).

• Circular: Fibers arranged in concentric rings; control openings (e.g., orbicularis oculi).

• Fusiform: Spindle-shaped with a thick middle and tapered ends (e.g., biceps brachii).

Functional Groups of Muscles

Roles in Movement

• Agonist (Prime Mover): Main muscle responsible for a specific movement.

• Antagonist: Muscle that opposes the action of the agonist.

• Synergist: Assists the agonist by adding force or reducing unnecessary movement.

• Fixator: Stabilizes the origin of the agonist to allow efficient movement.

Origin and Insertion

Definitions

• Origin: The fixed attachment point of a muscle, usually proximal.

• Insertion: The movable attachment point, usually distal; moves toward the origin during contraction.

Root Terms in Muscle Names

Common Prefixes, Suffixes, and Roots

• Brevis: Short

• Longus: Long

• Vastus: Large

• Anterior: Front

• External/Internal: Outer/Inner

• Infra/Supra: Below/Above

• Intercostal: Between the ribs

• Posterior: Back

• Profundus/Superficialis: Deep/Superficial

• Abductor/Adductor: Moves limb away from/toward midline

• Depressor/Levator: Lowers/Raises a body part

• Erector: Straightens a part

• Extensor/Flexor: Increases/Decreases joint angle

• Pronator/Supinator: Turns palm down/up

• Abdominis, Brachii, Capitis, Carpi, Cervicis, Digitorum/Digiti, Femoris, Gluteal, Hallucis, Oculi, Oris, Pectoralis, Pollicis: Indicate location or structure acted upon (e.g., brachii = arm, femoris = thigh).

• Oblique, Orbicular, Rectus, Transversus: Fiber direction (e.g., rectus = straight, oblique = angled).

• Biceps, Triceps, Quadriceps: Number of origins (2, 3, 4 respectively).

• Deltoid, Maximus, Minimus/Minimi, Minor, Quadratus, Rhomboid, Serratus, Trapezius: Shape or size descriptors.

Major Muscles to Identify on Diagrams

Key Muscles and Locations

• Facial Muscles: Frontalis, Occipitalis, Orbicularis Oculi, Levator Labii Superioris, Risorius, Orbicularis Oris, Buccinator, Platysma

• Mastication: Masseter, Temporalis

• Neck and Back: Sternocleidomastoid, Trapezius, Levator Scapulae

• Thorax and Abdomen: Diaphragm, Rectus Abdominis, External Oblique, Internal Oblique, Transversus Abdominis, Serratus Anterior, Pectoralis Major, Latissimus Dorsi

• Shoulder and Arm: Deltoid, Supraspinatus, Infraspinatus, Subscapularis, Biceps Brachii, Brachialis, Brachioradialis, Triceps Brachii

• Hand and Foot: Lumbricals, Palmar Interossei, Dorsal Interossei, Plantar Interossei

• Hip and Thigh: Tensor Fascia Lata, Sartorius, Rectus Femoris, Gluteus Maximus

• Leg: Gastrocnemius

Functions of Selected Muscles

Facial Expression

• Frontalis: Raises eyebrows, wrinkles forehead.

• Occipitalis: Pulls scalp posteriorly.

• Orbicularis Oculi: Closes eyelids.

• Levator Labii Superioris: Elevates upper lip.

• Risorius: Draws corner of mouth laterally.

• Orbicularis Oris: Closes and protrudes lips.

• Buccinator: Compresses cheek (as in blowing).

• Platysma: Tenses skin of neck, depresses mandible.

Eye Movement

• Superior/Inferior/Medial/Lateral Rectus: Move eyeball up, down, medially, laterally.

• Superior/Inferior Oblique: Rotate eyeball and assist in looking up/down and laterally.

Mastication

• Masseter: Elevates mandible (closes jaw).

• Temporalis: Elevates and retracts mandible.

• Medial/Lateral Pterygoid: Side-to-side movement of jaw.

Tongue Movement

• Genioglossus: Protrudes tongue.

• Hyoglossus: Depresses tongue.

• Styloglossus: Retracts and elevates tongue.

Ventilation

• Diaphragm: Prime mover of inspiration; flattens to enlarge thoracic cavity.

• External Intercostals: Elevate ribs during inspiration.

• Internal Intercostals: Depress ribs during forced expiration.

Abdominal Muscles

• Rectus Abdominis: Flexes vertebral column, compresses abdomen.

• External/Internal Oblique: Rotate and laterally flex trunk; compress abdomen.

• Transversus Abdominis: Compresses abdominal contents.

Rotator Cuff

• Supraspinatus: Abducts arm.

• Infraspinatus: Laterally rotates arm.

• Subscapularis: Medially rotates arm.

• Teres Minor: Laterally rotates arm.

Upper Limb

• Biceps Brachii: Flexes elbow, supinates forearm.

• Brachialis: Flexes elbow.

• Brachioradialis: Flexes forearm at elbow.

• Triceps Brachii: Extends elbow.

• Pronator Teres: Pronates forearm.

• Supinator: Supinates forearm.

Hand and Foot

• Lumbricals: Flex metacarpophalangeal joints, extend interphalangeal joints.

• Palmar/Plantar Interossei: Adduct fingers/toes.

• Dorsal Interossei: Abduct fingers/toes.

Lower Limb

• Rectus Femoris, Vastus Lateralis, Vastus Medialis, Vastus Intermedius: Extend knee (Quadriceps group).

• Biceps Femoris, Semimembranosus, Semitendinosus: Flex knee, extend hip (Hamstring group).

• Gastrocnemius: Plantar flexes foot, flexes knee.

• Soleus: Plantar flexes foot.

Muscle Group Classifications

Major Muscle Groups and Their Members

Summary Table: Muscle Actions and Locations

Additional info:

• Muscle names often reflect their shape, size, location, action, or number of origins.

• Understanding the root terms helps in identifying muscle function and location.

• Muscle groups work together to produce coordinated movements and maintain posture.