Muscular System I: Identification of Major Muscles

Muscular System I: Identification of Major Muscles

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 section focuses on the identification and anatomical location of major skeletal muscles, which are essential for practical examinations in anatomy and physiology.

Muscles of the Head

Major Muscles and Their Functions

• Frontalis (frontal belly): Raises the eyebrows and wrinkles the forehead.

• Occipitalis (occipital belly): Pulls the scalp posteriorly.

• Corrugator supercilii: Draws the eyebrows downward and medially, producing frowning.

• Orbicularis oculi: Closes the eyelids; involved in blinking and squinting.

• Risorius: Draws the corner of the mouth laterally (as in smiling).

• Levator labii superioris: Elevates the upper lip.

• Zygomaticus major and minor: Elevate the corners of the mouth (smiling muscles).

• Masseter: Elevates the mandible; primary muscle of mastication (chewing).

• Buccinator: Compresses the cheek (as in blowing or sucking).

• Orbicularis oris: Closes and protrudes the lips (kissing muscle).

• Mentalis: Protrudes the lower lip and wrinkles the chin.

• Depressor labii inferioris: Depresses the lower lip.

• Depressor anguli oris: Depresses the corner of the mouth (frowning).

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

Muscles of the Neck

Major Muscles and Their Functions

• Platysma: Tenses the skin of the neck and depresses the mandible.

• Sternocleidomastoid: Flexes the neck; rotates the head to the opposite side.

• Styloglossus: Retracts and elevates the tongue.

• Hyoglossus: Depresses and retracts the tongue.

• Genioglossus: Protracts the tongue.

• Sternohyoid: Depresses the hyoid bone after swallowing.

• Stylohyoid: Elevates and retracts the hyoid bone.

• Mylohyoid: Elevates the floor of the mouth and hyoid bone.

• Digastric: Depresses the mandible and elevates the hyoid bone.

Anterior Muscles of the Trunk

Major Muscles and Their Functions

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

• Deltoid: Abducts the arm; assists in flexion and extension of the shoulder.

• Rectus abdominis: Flexes the vertebral column; compresses the abdomen.

• Transverse abdominis: Compresses abdominal contents.

• External & Internal obliques: Flex and rotate the vertebral column; compress the abdomen.

• Intercostals: Elevate and depress the ribs during breathing.

• Serratus anterior: Protracts and stabilizes the scapula.

Posterior Muscles of the Trunk

Major Muscles and Their Functions

• Trapezius: Elevates, retracts, and rotates the scapula; extends the neck.

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

• Rhomboid major and minor: Retract and rotate the scapula.

• Levator scapulae: Elevates the scapula.

Muscles of the Arm

Major Muscles and Their Functions

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

• Triceps brachii: Extends the elbow.

• Brachialis: Flexes the elbow.

• Pronator teres: Pronates the forearm.

• Brachioradialis: Flexes the elbow, especially when the forearm is in mid-position.

• Flexor carpi radialis: Flexes and abducts the wrist.

• Palmaris longus: Flexes the wrist.

• Flexor carpi ulnaris: Flexes and adducts the wrist.

• Extensor carpi radialis longus & brevis: Extend and abduct the wrist.

• Extensor carpi ulnaris: Extends and adducts the wrist.

• Extensor digitorum: Extends the fingers.

• Extensor pollicis brevis: Extends the thumb.

Muscles of the Leg

Major Muscles and Their Functions

• Rectus femoris: Extends the knee and flexes the hip (part of quadriceps femoris group).

• Vastus lateralis & Vastus medialis: Extend the knee (quadriceps group).

• Sartorius: Flexes, abducts, and laterally rotates the hip; flexes the knee.

• Tensor fasciae latae: Abducts and medially rotates the thigh.

• Gracilis: Adducts the thigh and flexes the knee.

• Adductor longus: Adducts and medially rotates the thigh.

• Fibularis (peroneus) longus & brevis: Evert and plantarflex the foot.

• Tibialis anterior: Dorsiflexes and inverts the foot.

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

• Gluteus maximus: Extends and laterally rotates the hip.

• Gluteus medius: Abducts and medially rotates the hip.

• Biceps femoris, Semitendinosus, Semimembranosus: Extend the hip and flex the knee (hamstring group).

• Gastrocnemius: Plantarflexes the foot and flexes the knee.

• Soleus: Plantarflexes the foot.

Example: Functional Grouping of Muscles

Muscles are often grouped by their primary actions. For example, the quadriceps femoris group (rectus femoris, vastus lateralis, vastus medialis, and vastus intermedius) all extend the knee, while the hamstring group (biceps femoris, semitendinosus, semimembranosus) flex the knee and extend the hip.

Table: Selected Muscles and Their Primary Actions

Muscle	Location	Primary Action
Masseter	Head (jaw)	Elevates mandible (chewing)
Sternocleidomastoid	Neck	Flexes neck, rotates head
Pectoralis major	Anterior trunk	Flexes, adducts arm
Trapezius	Posterior trunk	Elevates, retracts scapula
Biceps brachii	Arm	Flexes elbow, supinates forearm
Rectus femoris	Leg (anterior thigh)	Extends knee, flexes hip
Gastrocnemius	Leg (posterior calf)	Plantarflexes foot, flexes knee

Additional info:

• Muscle names often reflect their location, shape, size, direction of fibers, or function (e.g., flexor indicates a muscle that flexes a joint).

• Understanding muscle attachments (origin and insertion) is crucial for predicting their actions.

• For practical exams, students should be able to identify these muscles on models, diagrams, or cadavers and describe their primary actions.