The Muscular System: Structure, Function, and Major Muscles

The Muscular System

Introduction to the Muscular System

The muscular system is responsible for movement, posture, and heat production in the human body. It consists of skeletal, cardiac, and smooth muscles, but this chapter focuses on skeletal muscles, which are under voluntary control and attach to bones to facilitate movement.

Naming Muscles

Principles of Muscle Naming

Muscles are named based on several criteria, including size, location, shape, appearance, position, number of origins, and function. Understanding these conventions helps in identifying and learning the muscles of the body.

• Size: Terms such as major, minor, longus (long), brevis (short), and vastus (broad) are used.

• Location: Directional and regional terms (e.g., pectoralis minor, superior, inferior, medial, lateral).

• Attachment: Named for bones or structures to which they attach (e.g., sternocleidomastoid).

• Function: Named for their action (e.g., flexor, extensor, adductor, abductor).

Functional Groups of Muscles

Agonists, Antagonists, Synergists, and Fixators

Muscles work in groups to produce coordinated movements. Each group has a specific role:

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

• Antagonist: Opposes the action of the agonist, usually located on the opposite side of the joint.

• Synergist: Assists the agonist in performing its action.

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

Overview of Major Muscle Groups

Regional Organization

Skeletal muscles are organized into regional groups for study:

• Muscles of the head, neck, and vertebral column

• Muscles of the trunk and pelvic floor

• Muscles of the pectoral girdle and upper limb

• Muscles of the hip and lower limb

Muscles of Facial Expression

Key Muscles and Their Actions

Muscles of facial expression are responsible for movements such as smiling, frowning, and blinking. They are unique in that they insert into the skin rather than bone.

• Frontalis: Elevates the eyebrows.

• Occipitalis: Pulls the scalp posteriorly.

• Orbicularis oculi: Closes the eyelids.

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

• Orbicularis oris: Controls the lips for eating, drinking, and puckering.

• Buccinator: Pulls the cheek inward (sucking).

Muscle	Origin/Insertion/Nerve	Action
Frontalis	Epicranial aponeurosis/skin of eyebrows; Facial nerve	Raises eyebrows, wrinkles forehead
Orbicularis oculi	Frontal and maxillary bones/skin around eyelids; Facial nerve	Closes eye
Zygomaticus major	Zygomatic bone/skin at corner of mouth; Facial nerve	Elevates lateral corners of mouth
Orbicularis oris	Maxilla and mandible/skin around mouth; Facial nerve	Closes and protrudes lips
Buccinator	Maxilla and mandible/orbicularis oris; Facial nerve	Compresses cheek

Muscles of Mastication

Major Muscles Involved in Chewing

Muscles of mastication move the mandible for chewing and speaking.

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

• Temporalis: Elevates and retracts the mandible.

• Medial and Lateral Pterygoids: Move the mandible side to side and assist in grinding movements.

Muscle	Origin/Insertion/Nerve	Action
Masseter	Zygomatic arch/mandible; Mandibular nerve	Elevates mandible
Temporalis	Temporal fossa/coronoid process of mandible; Mandibular nerve	Elevates and retracts mandible
Medial pterygoid	Pterygoid plate/mandible; Mandibular nerve	Elevates and protracts mandible, side-to-side movement
Lateral pterygoid	Pterygoid plate/mandible; Mandibular nerve	Protracts and depresses mandible, side-to-side movement

Muscles of Swallowing

Key Muscles and Their Functions

Swallowing involves coordinated action of several muscles in the neck and pharynx.

• Omohyoid and Sternohyoid: Depress the hyoid bone during swallowing.

• Mylohyoid, Geniohyoid, Stylohyoid: Elevate the hyoid and floor of the mouth.

Muscle	Origin/Insertion/Nerve	Action
Omohyoid	Scapula/hyoid bone; Cervical nerves	Depresses hyoid
Sternohyoid	Sternum/hyoid bone; Cervical nerves	Depresses hyoid
Mylohyoid	Mandible/hyoid bone; Trigeminal nerve	Elevates hyoid and floor of mouth

Muscles That Move the Head and Neck

Major Muscles and Their Actions

These muscles are responsible for head rotation, flexion, and extension.

• Sternocleidomastoid: Rotates and flexes the head.

• Trapezius: Extends the head and neck.

• Splenius capitis: Rotates and extends the head and neck.

Muscle	Origin/Insertion/Nerve	Action
Sternocleidomastoid	Sternum and clavicle/mastoid process; Accessory nerve	Flexes and rotates head
Trapezius	Occipital bone, vertebrae/scapula, clavicle; Accessory nerve	Extends head and neck
Splenius capitis	Vertebrae/mastoid process; Cervical nerves	Extends, rotates head

Muscles of the Vertebral Column

Erector Spinae and Associated Muscles

The muscles of the vertebral column maintain posture and allow for extension, flexion, and rotation of the spine.

• Erector spinae group: Includes longissimus, spinalis, and iliocostalis muscles; extends and laterally flexes the vertebral column.

• Longissimus thoracis: Assists in extending or rotating the head and neck.

Muscle	Origin/Insertion/Nerve	Action
Erector spinae	Vertebrae/ribs, vertebrae, skull; Spinal nerves	Extends and laterally flexes spine
Longissimus thoracis	Vertebrae/ribs; Spinal nerves	Extends, rotates head and neck

Muscles of Ventilation

Diaphragm and Intercostal Muscles

Muscles of ventilation are essential for breathing by changing the volume of the thoracic cavity.

• Diaphragm: Contracts to increase thoracic volume during inspiration.

• External intercostals: Elevate the ribs during inspiration.

• Internal intercostals: Depress the ribs during forced exhalation.

Muscle	Origin/Insertion/Nerve	Action
Diaphragm	Sternum, ribs, vertebrae/central tendon; Phrenic nerve	Increases thoracic cavity volume (inhalation)
External intercostals	Ribs/ribs; Intercostal nerves	Elevate ribs (inspiration)
Internal intercostals	Ribs/ribs; Intercostal nerves	Depress ribs (forced expiration)

Abdominal Muscles

Major Muscles and Their Functions

Abdominal muscles support the trunk, allow movement, and hold organs in place by regulating internal abdominal pressure.

• Rectus abdominis: Compresses the abdomen and flexes the trunk.

• External and internal obliques: Rotate and laterally flex the trunk.

• Transversus abdominis: Compresses the abdominal cavity.

Muscle	Origin/Insertion/Nerve	Action
Rectus abdominis	Pubic bone/ribs, sternum; Intercostal nerves	Flexes trunk, compresses abdomen
External oblique	Ribs/iliac crest; Intercostal nerves	Flexes, rotates trunk
Internal oblique	Iliac crest/ribs; Intercostal nerves	Flexes, rotates trunk
Transversus abdominis	Ribs, iliac crest/linea alba; Intercostal nerves	Compresses abdomen

Muscles That Move the Scapula at the Pectoral Girdle

Key Muscles and Their Actions

These muscles stabilize and move the scapula, allowing for a wide range of shoulder movements.

• Serratus anterior: Protracts the scapula.

• Pectoralis minor: Depresses the scapula.

• Trapezius: Extends the head and neck, elevates, retracts, and depresses the scapula.

• Rhomboid major and minor: Retract the scapula.

Muscle	Origin/Insertion/Nerve	Action
Serratus anterior	Ribs/scapula; Long thoracic nerve	Protracts scapula
Pectoralis minor	Ribs/coracoid process; Medial pectoral nerve	Depresses scapula
Trapezius	Occipital bone, vertebrae/scapula, clavicle; Accessory nerve	Elevates, retracts, depresses scapula
Rhomboid major/minor	Vertebrae/scapula; Dorsal scapular nerve	Retracts scapula

Muscles That Move the Arm at the Shoulder Joint

Major Muscles and Rotator Cuff

These muscles are responsible for the wide range of motion at the shoulder joint.

• Pectoralis major: Adducts the arm.

• Coracobrachialis: Assists with flexion of the humerus.

• Deltoid: Abducts the arm.

• Latissimus dorsi: Extends the arm.

• Teres major: Extends and adducts the arm.

• Rotator cuff muscles: Supraspinatus (abducts), infraspinatus (rotates), teres minor (extends, adducts), subscapularis (rotates).

Muscle	Origin/Insertion/Nerve	Action
Pectoralis major	Sternum, clavicle/humerus; Pectoral nerves	Adducts, flexes arm
Deltoid	Clavicle, scapula/humerus; Axillary nerve	Abducts arm
Latissimus dorsi	Vertebrae, iliac crest/humerus; Thoracodorsal nerve	Extends, adducts arm
Rotator cuff	Scapula/humerus; Suprascapular, axillary, subscapular nerves	Stabilizes, moves shoulder

Muscles That Move the Forearm and Hand

Flexors, Extensors, and Pronators

These muscles control movements at the elbow, wrist, and fingers.

• Biceps brachii, brachialis, brachioradialis: Flex the forearm.

• Triceps brachii: Extends the forearm.

• Flexor carpi radialis, palmaris longus, flexor carpi ulnaris: Flex the hand.

• Pronator teres: Pronates the forearm.

• Extensor carpi radialis longus/brevis, extensor carpi ulnaris: Extend the hand and wrist.

• Extensor digitorum: Extends the fingers.

Muscles of the Hip, Thigh, Knee, and Leg

Anterior, Medial, and Posterior Muscle Groups

These muscles are responsible for movements of the hip, thigh, knee, and leg, including walking, running, and maintaining posture.

• Iliopsoas (iliacus and psoas major): Flex the thigh.

• Adductor group (magnus, longus, brevis), gracilis: Adduct the thigh.

• Sartorius: Flexes and laterally rotates the thigh.

• Quadriceps femoris group (rectus femoris, vastus lateralis/intermedius/medialis): Extend the leg at the knee.

• Gluteal group (maximus, medius, minimus): Extend and abduct the thigh.

• Piriformis: Laterally rotates the thigh.

• Hamstring group (semitendinosus, semimembranosus, biceps femoris): Extend the thigh and flex the leg.

Muscles of the Ankle, Foot, and Toes

Key Muscles and Their Actions

These muscles control movements such as dorsiflexion, plantarflexion, inversion, eversion, and toe flexion/extension.

• Tibialis anterior, extensor digitorum longus: Dorsiflex the foot.

• Fibularis longus: Plantarflexes and everts the foot.

• Gastrocnemius, soleus: Plantarflex the foot.

• Flexor digitorum longus: Flexes the toes.

Additional info: For each muscle group, understanding the origin, insertion, innervation, and action is essential for clinical and academic applications. Muscle tables provide a concise summary for review and exam preparation.