The Muscular System: Major Muscles, Locations, and Actions

Chapter 10: The Muscular System

Overview of the Muscular System

The muscular system is responsible for movement, posture, and heat production in the human body. Muscles are classified based on their location, structure, and function. This chapter focuses on the major skeletal muscles, their anatomical locations, and their primary actions.

Muscles of the Face and Neck

Facial Muscles

• Frontalis: Covers the forehead and dome of the skull; raises eyebrows and wrinkles the forehead.

• Orbicularis oculi: Encircles the eye; responsible for closing the eyelids, blinking, and squinting.

• Zygomaticus: Raises the lateral corners of the mouth; known as the "smiling muscle."

• Risorius: Tenses lips and draws the corners of the lips laterally.

• Levator labii superioris: Elevates the upper lip and opens the lips.

• Depressor anguli oris: Draws the corners of the mouth downward and laterally, as in a grimace.

• Depressor labii inferioris: Draws the lower lip downward, producing a pout.

• Orbicularis oris: Closes and purses the lips; involved in kissing and whistling.

• Buccinator: Compresses the cheek; important for whistling, sucking, and maintaining food between teeth during chewing.

Neck Muscles

• Sternocleidomastoid: Flexes and laterally rotates the head; prominent muscle on the side of the neck.

• Platysma: Superficial muscle covering the sternocleidomastoid; tenses the skin of the neck.

Muscles of Mastication (Chewing)

• Temporalis: Elevates and retracts the mandible; assists in closing the jaw.

• Masseter: Elevates the mandible; primary muscle for jaw closure.

• Pterygoid muscles: Medial and lateral pterygoids allow side-to-side (lateral) movement of the mandible.

Muscles Involved in Swallowing

• Stylohyoid: Elevates the hyoid bone during swallowing.

• Thyrohyoid: Elevates the larynx and closes the glottis.

• Pharyngeal constrictors: Contract sequentially to push food down the throat (pharynx) during swallowing.

Muscles of Respiration

• Diaphragm: Dome-shaped muscle separating the thoracic and abdominal cavities; primary muscle of respiration.

• Intercostal muscles: Located between the ribs; assist with expansion and contraction of the rib cage during breathing.

Muscles of the Abdomen

• External oblique: Most superficial abdominal muscle; aids in trunk rotation and lateral flexion.

• Internal oblique: Middle layer; functions similarly to the external oblique.

• Transverse abdominis: Deepest abdominal muscle; compresses abdominal contents.

• Rectus abdominis: Runs vertically along the anterior wall; known as the "six-pack" muscle; flexes the vertebral column.

Muscles of the Back

• Trapezius: Stabilizes, elevates, retracts, and rotates the scapula.

• Latissimus dorsi: Prime mover of arm extension; adducts and medially rotates the arm at the shoulder.

• Erector spinae: Prime mover of back extension; maintains posture and straightens the spine.

Muscles Acting on the Scapula (Shoulder Girdle)

• Pectoralis minor: Protracts and depresses the scapula; lifts ribs during forced expiration.

• Serratus anterior: Abducts and rotates or depresses the scapula; important for pushing and throwing movements.

• Trapezius: Rotates scapula upward; retracts and depresses scapula.

• Levator scapulae: Elevates the scapula.

• Rhomboideus (major and minor): Elevates and retracts the scapula.

Muscles Acting on the Humerus (Shoulder Joint)

• Pectoralis major: Flexes, adducts, and medially rotates the arm; forms the bulk of the chest muscles.

• Deltoid: Major abductor of the arm; forms the rounded contour of the shoulder.

• Teres major: Extends, adducts, and medially rotates the arm.

Rotator Cuff Muscles

The rotator cuff is a group of four muscles that stabilize and strengthen the shoulder joint.

• Supraspinatus: Initiates abduction of the arm; reinforces the shoulder joint capsule.

• Infraspinatus: Laterally rotates the arm; reinforces the joint capsule.

• Teres minor: Laterally rotates the arm; reinforces the joint capsule.

• Subscapularis: Medially rotates the arm; reinforces the joint capsule.

Muscles Acting on the Elbow and Forearm

• Biceps brachii: Flexes the elbow and supinates the forearm.

• Brachialis: Primary flexor of the elbow.

• Brachioradialis: Synergistic flexor of the elbow.

• Triceps brachii: Prime extensor of the elbow.

• Supinator muscles: Rotate the forearm so the palm faces anteriorly (supination).

• Pronator muscles: Rotate the forearm so the palm faces posteriorly (pronation).

Muscles of the Forearm and Hand

• Digitorum muscles: Insert into the fingers; responsible for finger movements.

• Carpi muscles: Insert into the carpal bones; control wrist movements.

• Pollicis muscles: Insert into the thumb; control thumb movements.

• Thenar group: Muscles at the base of the thumb; control fine thumb movements.

Muscles of the Buttocks (Gluteal Region)

• Gluteus maximus: Largest gluteal muscle; extends and laterally rotates the hip.

• Gluteus medius: Abducts and medially rotates the thigh; stabilizes the pelvis during walking.

• Gluteus minimus: Deepest gluteal muscle; assists in hip abduction.

Muscles of the Thigh

Quadriceps Femoris Group (Anterior Thigh)

The quadriceps femoris is a group of four muscles on the anterior thigh, responsible for extending the knee and flexing the hip. They are essential for climbing, jumping, running, and rising from a seated position.

• Vastus intermedius

• Rectus femoris

• Vastus lateralis

• Vastus medialis

Hamstring Muscles (Posterior Thigh)

• Biceps femoris (long and short head): Flexes the knee and extends the hip.

• Semitendinosus: Flexes the knee and extends the hip.

• Semimembranosus: Flexes the knee and extends the hip.

Muscles of the Leg

Anterior Side

• Tibialis anterior: Dorsiflexes and inverts the foot.

• Extensor hallucis longus: Extends the big toe and dorsiflexes the ankle.

• Extensor digitorum longus: Extends the toes and dorsiflexes the ankle.

Posterior Side

• Gastrocnemius: Calf muscle; flexes the knee and plantarflexes the ankle.

• Soleus: Located beneath the gastrocnemius; plantarflexes the ankle.

Summary Table: Major Muscles and Their Actions

Muscle	Location	Primary Action
Frontalis	Forehead	Raises eyebrows
Orbicularis oculi	Around eye	Closes eyelids
Masseter	Jaw	Elevates mandible
Sternocleidomastoid	Neck	Flexes and rotates head
Diaphragm	Thoracic/abdominal border	Inspiration (breathing)
Rectus abdominis	Abdomen	Flexes vertebral column
Trapezius	Upper back	Moves scapula
Deltoid	Shoulder	Abducts arm
Biceps brachii	Upper arm	Flexes elbow
Triceps brachii	Upper arm	Extends elbow
Gluteus maximus	Buttocks	Extends hip
Quadriceps femoris	Anterior thigh	Extends knee
Hamstrings	Posterior thigh	Flex knee, extend hip
Gastrocnemius	Calf	Plantarflexes ankle

Key Terms

• Flexion: Decreasing the angle between two bones (e.g., bending the elbow).

• Extension: Increasing the angle between two bones (e.g., straightening the knee).

• Abduction: Movement away from the midline of the body.

• Adduction: Movement toward the midline of the body.

• Rotation: Movement around an axis (e.g., turning the head).

• Supination: Rotating the forearm so the palm faces up.

• Pronation: Rotating the forearm so the palm faces down.

Example Application

• When rising from a seated position, the quadriceps femoris group powerfully extends the knee, while the gluteus maximus extends the hip.

• Throwing a ball involves coordinated action of the deltoid, pectoralis major, triceps brachii, and rotator cuff muscles.

Additional info: The above notes expand on the brief muscle listings by providing definitions, anatomical context, and examples of muscle actions. The summary table is constructed to facilitate quick review and comparison of major muscles and their functions.