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 muscles, which are under voluntary control, and is organized into functional groups based on their anatomical location and action.

Naming Muscles

Muscle Naming Conventions

Muscles are named according to several criteria, which help describe their characteristics and functions:

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

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

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

• Function: Named for their action, such as flexor, extensor, adductor, or abductor.

• Number of Heads: Indicates the number of proximal attachments (e.g., biceps has two heads).

Functional Groups of Skeletal Muscles

Muscle Roles in Movement

Muscles work together to produce coordinated movements. They are classified into functional groups:

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

• Antagonist: Lies on the opposite side of the joint from the agonist and opposes its action.

• Synergist: Assists the agonist in performing its action.

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

Overview of Major Muscle Groups

Muscle Group Classification

Muscles are categorized based on their anatomical location:

• 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

Familiarity with the superficial muscles (both anterior and posterior) is essential for understanding muscle function and identification.

Muscles of Facial Expression

Key Muscles and Actions

Muscles of facial expression are responsible for movements of the face, such as smiling, frowning, and blinking:

• Frontalis: Elevates the eyebrows.

• Occipitalis: Pulls the scalp posteriorly.

• Orbicularis oculi: Closes the eyelids (blinking/winking).

• 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(s)
Frontalis	Epicranial aponeurosis/skin of eyebrows/facial nerve	Raises eyebrows, wrinkles forehead
Orbicularis oculi	Medial orbital margin/skin around eyelids/facial nerve	Closes eyelids
Zygomaticus major	Zygomatic bone/skin at angle of mouth/facial nerve	Elevates corner of mouth
Buccinator	Alveolar processes of maxilla and mandible/orbicularis oris/facial nerve	Compresses cheek

Muscles of Mastication

Key Muscles and Actions

Muscles of mastication are responsible for movements of the jaw during chewing:

• Masseter: Elevates the mandible (closes the jaw).

• Temporalis: Elevates the mandible.

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

Muscle	Origin/Insertion/Nerve	Action(s)
Temporalis	Temporal fossa/coronoid process of mandible/mandibular nerve	Elevates and retracts mandible
Masseter	Zygomatic arch/mandibular ramus/mandibular nerve	Elevates mandible
Lateral pterygoid	Lateral pterygoid plate/mandibular condyle/mandibular nerve	Protracts and depresses mandible
Medial pterygoid	Medial pterygoid plate/mandibular angle/mandibular nerve	Elevates and protracts mandible

Muscles of Swallowing

Key Muscles and Actions

Muscles involved in swallowing coordinate the movement of the hyoid bone and larynx:

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

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

Muscle	Origin/Insertion/Nerve	Action(s)
Omohyoid	Superior border of scapula/hyoid bone/ansa cervicalis	Depresses hyoid bone
Sternohyoid	Manubrium/hyoid bone/ansa cervicalis	Depresses hyoid bone

Muscles That Move the Head and Neck

Key Muscles and Actions

These muscles are responsible for movements such as rotation, flexion, and extension of the head and neck:

• Sternocleidomastoid: Rotates the head laterally and flexes the neck.

• Trapezius: Extends the head and neck.

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

Muscle	Origin/Insertion/Nerve	Action(s)
Sternocleidomastoid	Sternum and clavicle/mastoid process/accessory nerve	Flexes and rotates head
Trapezius	Occipital bone, spinous processes of C7-T12/clavicle, scapula/accessory nerve	Extends head and neck
Splenius capitis	Spinous processes of cervical vertebrae/mastoid process/cervical spinal nerves	Extends, rotates head

Muscles of the Vertebral Column

Key Muscles and Actions

These muscles maintain posture and allow movement of the vertebral column:

• Erector spinae group: Extends and laterally flexes the vertebral column.

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

Muscle	Origin/Insertion/Nerve	Action(s)
Erector spinae	Iliac crest, sacrum, lumbar vertebrae/ribs, thoracic and cervical vertebrae/spinal nerves	Extends vertebral column

Muscles of Ventilation

Key Muscles and Actions

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(s)
Diaphragm	Xiphoid process, lower ribs/central tendon/phrenic nerve	Increases thoracic cavity volume (inhalation)
External intercostals	Inferior border of rib above/superior border of rib below/intercostal nerves	Elevate ribs (inspiration)
Internal intercostals	Superior border of rib below/inferior border of rib above/intercostal nerves	Depress ribs (forced expiration)

Abdominal Muscles

Key Muscles and Actions

Abdominal muscles support the trunk, allow movement, and hold organs in place:

• 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(s)
Rectus abdominis	Pubic crest, symphysis/xiphoid process, costal cartilages/thoracoabdominal nerves	Flexes trunk, compresses abdomen
External oblique	Lower eight ribs/iliac crest, linea alba/thoracoabdominal nerves	Flexes, rotates trunk
Transversus abdominis	Lower six ribs, iliac crest/linea alba/thoracoabdominal nerves	Compresses abdomen

Muscles That Move the Scapula at the Pectoral Girdle

Key Muscles and 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 and retracts the scapula.

• Rhomboid major and minor: Retract the scapula.

Muscle	Origin/Insertion/Nerve	Action(s)
Serratus anterior	Ribs 1-8/medial border of scapula/long thoracic nerve	Protracts scapula
Pectoralis minor	Ribs 3-5/coracoid process/medial pectoral nerve	Depresses scapula
Trapezius	Occipital bone, spinous processes of C7-T12/clavicle, scapula/accessory nerve	Elevates, retracts scapula
Rhomboid major/minor	Spinous processes of C7-T5/medial border of scapula/dorsal scapular nerve	Retracts scapula

Muscles That Move the Arm at the Shoulder Joint

Key Muscles and Actions

These muscles are responsible for movements of the arm 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: Stabilize and move the shoulder joint (includes supraspinatus, infraspinatus, teres minor, subscapularis).

Muscle	Origin/Insertion/Nerve	Action(s)
Pectoralis major	Clavicle, sternum/humerus/medial and lateral pectoral nerves	Adducts, flexes arm
Deltoid	Clavicle, scapula/humerus/axillary nerve	Abducts arm
Latissimus dorsi	Spinous processes of lower thoracic vertebrae/humerus/thoracodorsal nerve	Extends, adducts arm
Rotator cuff muscles	Scapula/humerus/suprascapular, axillary, subscapular nerves	Stabilize shoulder

Muscles That Move the Forearm and Hand

Muscles That Move the Forearm at the Elbow Joint

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

• Triceps brachii: Extends the forearm.

Muscle	Origin/Insertion/Nerve	Action(s)
Biceps brachii	Scapula/radius/musculocutaneous nerve	Flexes forearm, supinates forearm
Brachialis	Humerus/ulna/musculocutaneous nerve	Flexes forearm
Brachioradialis	Humerus/radius/radial nerve	Flexes forearm
Triceps brachii	Scapula, humerus/ulna/radial nerve	Extends forearm

Muscles That Move the Hand and Fingers

• Flexors: Located on the anterior and medial forearm; flex the hand (e.g., flexor carpi radialis, palmaris longus, flexor carpi ulnaris).

• Pronator teres: Pronates the forearm.

• Extensors: Located on the posterior and lateral forearm; extend the hand and wrist (e.g., extensor carpi radialis longus, extensor carpi ulnaris, extensor digitorum).

Muscles of the Hip, Thigh, Knee, and Leg

Anterior and Medial Muscles

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

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

• Gracilis: Adducts the thigh.

• Sartorius: Flexion and lateral rotation of the thigh.

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

Muscle	Origin/Insertion/Nerve	Action(s)
Iliopsoas	Iliac fossa, lumbar vertebrae/femur/femoral nerve	Flexes thigh
Adductor magnus	Ischial tuberosity/femur/obturator nerve	Adducts thigh
Quadriceps femoris	Ilium, femur/patella, tibia/femoral nerve	Extends leg

Posterior Muscles

• 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.

Muscle	Origin/Insertion/Nerve	Action(s)
Gluteus maximus	Ilium, sacrum/femur/inferior gluteal nerve	Extends thigh
Hamstrings	Ischial tuberosity/tibia, fibula/sciatic nerve	Extends thigh, flexes leg

Muscles of the Ankle, Foot, and Toes

Key Muscles and Actions

• Tibialis anterior, extensor digitorum longus: Dorsiflexion of the foot.

• Fibularis longus: Plantarflexion and eversion of the foot.

• Gastrocnemius, soleus: Plantarflexion of the foot.

• Flexor digitorum longus: Flexes the digits (toes).

Muscle	Origin/Insertion/Nerve	Action(s)
Tibialis anterior	Tibia/medial cuneiform, first metatarsal/deep fibular nerve	Dorsiflexes foot
Gastrocnemius	Femur/calcaneus/tibial nerve	Plantarflexes foot
Flexor digitorum longus	Tibia/distal phalanges of toes/tibial nerve	Flexes toes

Additional info: For each muscle group, understanding the origin, insertion, innervation, and action is essential for clinical and academic applications. Tables referenced above provide detailed anatomical and functional information for each muscle.