Chapter 9: The Muscular System – Structure, Organization, and Major Muscles

Overview of Skeletal Muscle

Structure of Skeletal Muscle

Skeletal muscle is composed of elongated cells called muscle fibers, which are organized into bundles and surrounded by layers of connective tissue. This organization supports muscle function and force transmission.

• Muscle fiber: Also known as a muscle cell; surrounded by endomysium.

• Fascicle: Bundle of muscle fibers (typically 10–100 fibers); surrounded by perimysium.

• Muscle: Composed of multiple fascicles; surrounded by epimysium.

• Connective tissues converge to form a tendon, which attaches muscle to bone.

Types of Skeletal Muscle Based on Physical Organization

The arrangement of fascicles determines muscle shape, strength, and function.

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

• Parallel: Long, evenly spaced fascicles (e.g., Rectus abdominis, Sartorius); can shorten more but less forceful.

• Convergent (Triangular): Broad origin, fascicles converge to a single tendon (e.g., Pectoralis major).

• Pennate: Fascicles attach obliquely to a tendon; feather-like appearance.

  • Unipennate: Fascicles on one side (e.g., Palmar interosseous).

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

  • Multipennate: Multiple feather-like regions (e.g., Deltoid).

• Circular (Sphincters): Fascicles encircle an opening (e.g., Orbicularis oculi); contraction closes the opening.

Naming Muscles

• Muscles are named based on shape, size, position, number of heads, points of attachment, or action (e.g., flexor, extensor).

• Some names are historical (e.g., Achilles tendon).

Functions of Skeletal Muscles

• Movement (locomotion, breathing)

• Stability (posture, joint stabilization)

• Communication (facial expression, speech)

• Swallowing

• Control of body openings and passages

• Heat production (thermogenesis)

Functional Groups of Muscles

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

• Antagonist: Opposes the agonist; located on the opposite side of the joint.

• Synergist: Assists the agonist; adds force or reduces unwanted movement.

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

Origin and Insertion of Muscles

• Origin: Less movable attachment, usually proximal or closer to the trunk.

• Insertion: More movable attachment, usually distal or further from the trunk.

• Some muscles cross multiple joints; origin/insertion may vary with movement.

Muscles of the Head, Neck, and Vertebral Column

Muscles of Facial Expression

• Epicranius: Includes Frontalis (forehead), Occipitalis (back of head), connected by the epicranial aponeurosis; raises eyebrows.

• Orbicularis oculi: Encircles eye; closes eyelids.

• Zygomaticus major & minor: From zygomatic bone to mouth; raises corners of mouth (smiling).

• Levator labii superioris: Above upper lip; elevates upper lip (grimace/sneer).

• Orbicularis oris: Encircles mouth; controls lip movements.

• Mentalis: Over chin; pulls lower lip down.

• Buccinator: Compresses cheeks (whistling, blowing, chewing).

• Platysma: Broad neck muscle; depresses jaw ("jaw dropping").

Extrinsic Eye Muscles

• Six muscles control eye movement; four are rectus (straight), two are oblique (angled).

• Superior rectus: Elevates eye.

• Inferior rectus: Depresses eye.

• Medial rectus: Moves eye medially.

• Lateral rectus: Moves eye laterally.

• Superior oblique: Depresses and laterally rotates eye.

• Inferior oblique: Elevates and laterally rotates eye.

Selected Muscles of the Head and Neck

• Masseter: Elevates jaw (chewing).

• Temporalis: Elevates and retracts mandible.

• Sternocleidomastoid: Rotates head to opposite side; flexes neck.

• Trapezius: Elevates head; moves scapula.

• Splenius capitis: Extends and rotates head to same side.

• Semispinalis capitis: Extends head; rotates to opposite side.

Muscles of the Vertebral Column

• Erector spinae group: Three columns along vertebral column; maintain posture and extend spine.

  • Spinalis: Along spinous processes; extends vertebral column.

  • Longissimus: Along transverse processes; extends and laterally bends vertebral column.

  • Iliocostalis: From ribs to cervical vertebrae; extends and laterally bends vertebral column.

Muscles of the Trunk and Pelvic Floor

Selected Muscles of the Trunk

• Diaphragm: Separates thoracic and abdominal cavities; contraction increases thoracic volume (inhalation).

• External intercostals: Elevate ribcage (inhalation).

• Internal intercostals: Depress ribcage (exhalation).

Selected Abdominal Muscles

• Rectus abdominis: Flexes trunk; compresses abdomen.

• External oblique: Flexes/laterally bends trunk; compresses abdomen.

• Internal oblique: Flexes/laterally bends trunk; compresses abdomen.

• Transverse abdominis: Compresses abdomen.

Muscles of the Pectoral Girdle and Upper Limb

Selected Muscles of the Shoulder and Arm

• Serratus anterior: Protracts and rotates scapula upward.

• Trapezius: Elevates, retracts, depresses, and rotates scapula.

• Levator scapulae: Elevates scapula.

• Rhomboid major/minor: Retract scapula.

• Pectoralis minor: Protracts and depresses scapula.

• Pectoralis major: Flexes, adducts, and medially rotates arm.

• Latissimus dorsi: Extends, adducts, and medially rotates arm.

• Deltoid: Abducts, flexes, and extends arm.

• Teres major: Adducts, extends, and medially rotates arm.

• Coracobrachialis: Flexes and adducts arm.

• Supraspinatus: Assists abduction; stabilizes shoulder.

• Infraspinatus: Laterally rotates and stabilizes shoulder.

• Teres minor: Laterally rotates and stabilizes shoulder.

• Subscapularis: Medially rotates and adducts arm; stabilizes shoulder.

Selected Muscles of the Arm, Forearm, and Hand

• Biceps brachii: Flexes elbow; supinates forearm.

• Brachialis: Flexes elbow.

• Brachioradialis: Assists elbow flexion.

• Triceps brachii: Extends elbow.

• Anconeus: Assists elbow extension.

• Pronator teres: Pronates forearm.

• Flexor carpi radialis: Flexes wrist; abducts hand.

• Palmaris longus: Flexes wrist; tenses palm.

• Flexor carpi ulnaris: Flexes wrist; adducts hand.

• Flexor pollicis longus: Flexes thumb.

• Extensor carpi radialis longus: Extends wrist; abducts hand.

• Extensor digitorum: Extends fingers and wrist.

• Extensor carpi ulnaris: Extends wrist; adducts hand.

• Supinator: Supinates forearm.

Muscles of the Hip and Lower Limb

Selected Muscles of the Thigh and Knee

• Iliacus: Flexes thigh.

• Psoas major: Flexes thigh; laterally bends vertebral column.

• Sartorius: Flexes thigh and knee; abducts and laterally rotates thigh.

• Adductor magnus/longus/brevis: Adduct and medially rotate thigh; longus and brevis also flex thigh.

• Pectineus: Adducts and flexes thigh; medially rotates thigh.

• Gracilis: Adducts thigh; flexes and medially rotates leg.

• Quadriceps group (anterior thigh):

  • Rectus femoris: Extends knee; flexes thigh.

  • Vastus lateralis: Extends knee; stabilizes patella.

  • Vastus medialis: Extends knee; stabilizes patella.

  • Vastus intermedius: Extends knee.

• Gluteus maximus: Extends, laterally rotates, and abducts thigh.

• Gluteus medius/minimus: Abduct and medially rotate thigh; stabilize pelvis during walking.

• Piriformis: Abducts and laterally rotates thigh.

• Hamstring group (posterior thigh):

  • Biceps femoris: Extends thigh; flexes knee.

  • Semitendinosus: Extends thigh; flexes knee.

  • Semimembranosus: Extends thigh; flexes knee.

Selected Muscles of the Lower Leg and Foot

• Tibialis anterior: Dorsiflexes and inverts foot.

• Extensor digitorum longus: Extends toes; dorsiflexes foot.

• Extensor hallucis longus: Extends great toe; dorsiflexes foot.

• Fibularis (peroneus) longus: Everts and plantar flexes foot.

• Gastrocnemius: Plantar flexes foot; flexes knee.

• Soleus: Plantar flexes foot.

• Plantaris: Plantar flexes foot; assists knee flexion.

• Tibialis posterior: Inverts and plantar flexes foot; stabilizes foot.

• Flexor digitorum longus: Flexes toes; stabilizes foot.

• Flexor hallucis longus: Flexes great toe.

• Popliteus: Flexes and rotates leg at knee.

Table: Major Muscle Groups and Their Primary Actions

Muscle Group	Location	Primary Action	Example Muscle(s)
Facial Expression	Head/Face	Move facial skin, expression	Orbicularis oculi, Zygomaticus
Mastication	Jaw	Chewing	Masseter, Temporalis
Neck	Neck	Head movement	Sternocleidomastoid, Trapezius
Trunk	Thorax/Abdomen	Breathing, trunk movement	Diaphragm, Rectus abdominis
Shoulder	Shoulder	Move scapula/arm	Deltoid, Pectoralis major
Upper Limb	Arm/Forearm	Move elbow, wrist, hand	Biceps brachii, Triceps brachii
Hip/Thigh	Pelvis/Thigh	Move thigh, stabilize pelvis	Gluteus maximus, Quadriceps
Lower Leg/Foot	Leg/Foot	Move ankle, toes	Tibialis anterior, Gastrocnemius

Additional info:

• Muscle contraction is based on the sliding filament theory, involving actin and myosin filaments (see Chapter 10 for details).

• Muscle actions are often coordinated in groups for smooth movement and joint stability.