Muscles of the Head, Neck, Thorax, Upper Limb, and Abdominal Wall: Origins, Insertions, Actions, and Innervations

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Muscles of Facial Expression

Overview

Muscles of facial expression are responsible for producing movements of the face, such as smiling, frowning, and blinking. These muscles are primarily innervated by the facial nerve (cranial nerve VII).

Muscle	Origin	Insertion	Action	Innervation
Frontalis	Galea aponeurotica	Skin of eyebrows and root of nose	Raises eyebrows; wrinkles forehead	Facial nerve (CN VII)
Occipitalis	Occipital and temporal bone	Galea aponeurotica	Pulls scalp posteriorly	Facial nerve (CN VII)
Orbicularis oculi	Frontal and maxillary bones	Tissue of the eyelid	Produces blinking, squinting, and draws eyebrows inferiorly	Facial nerve (CN VII)
Buccinator	Maxilla and mandible	Orbicularis oris	Compresses the cheek (as in whistling)	Facial nerve (CN VII)
Orbicularis oris	Arises indirectly from the maxilla and mandible	Encircles the mouth; inserts into skin and muscles at lateral corners of mouth	Closes the lips; purses the lips, as in whistling and kissing	Facial nerve (CN VII)
Mentalis	Mandible	Skin of the chin	Protrudes lower lip; wrinkles the chin	Facial nerve (CN VII)

Key Points

Facial muscles are unique in that they insert into the skin rather than bone, allowing for subtle facial expressions.
All listed muscles are innervated by the facial nerve (CN VII).

Example

The orbicularis oculi is essential for blinking and protecting the eye from foreign objects.

Muscles of Mastication (Chewing)

Overview

Muscles of mastication are responsible for movements of the jaw during chewing. All are innervated by the trigeminal nerve (cranial nerve V).

Muscle	Origin	Insertion	Action	Innervation
Masseter	Zygomatic arch	Mandible	Elevates mandible	Trigeminal nerve (CN V)
Temporalis	Occipital and temporal bone	Mandible	Elevates mandible	Trigeminal nerve (CN V)

Key Points

Masseter is the primary muscle for jaw elevation (closing the mouth).
Temporalis assists in jaw elevation and retraction.

Example

The masseter is one of the strongest muscles in the body relative to its size.

Muscles of the Neck

Overview

The neck muscles facilitate movement of the head and support breathing and swallowing. The sternocleidomastoid is a key muscle for head rotation and flexion.

Muscle	Origin	Insertion	Action	Innervation
Sternocleidomastoid	Sternum and clavicle	Mastoid process of the temporal bone	Acting together: flex the head; acting alone: rotates head toward the opposite shoulder	Accessory nerve (CN XI)

Key Points

Sternocleidomastoid is important for turning the head and flexing the neck.

Example

Contraction of one sternocleidomastoid muscle turns the head to the opposite side.

Muscles of the Thorax Relating to Respiratory Movements

Overview

These muscles are essential for breathing, facilitating inspiration and expiration. Most are innervated by intercostal nerves, except the diaphragm.

Muscle	Origin	Insertion	Action	Innervation
External intercostals	Inferior border of the rib above	Superior border of the rib below	Elevates rib cage, assists in inspiration	Intercostal nerves
Internal intercostals	Superior border of the rib below	Inferior border of the rib above	Depresses rib cage, assists in forced expiration	Intercostal nerves
Respiratory diaphragm	Rib cage and lumbar vertebrae	Central tendon	Prime mover of inspiration	Phrenic nerve

Key Points

Diaphragm is the primary muscle for breathing.
Intercostal muscles assist in expanding and contracting the rib cage.

Example

During inhalation, the diaphragm contracts and flattens, increasing thoracic volume.

Muscles of the Thorax Relating to Movements of the Scapula

Overview

These muscles stabilize and move the scapula, allowing for shoulder mobility and upper limb movement.

Muscle	Origin	Insertion	Action	Innervation
Pectoralis minor	Ribs 3-5	Scapula	Stabilizes scapula by drawing it forward and down	Pectoral nerve
Levator scapulae	Vertebrae C1 through C4	Scapula	Elevates, retracts, and rotates scapula	Cervical spinal nerves and dorsal scapular nerve
Rhomboids (major & minor)	Vertebrae C7 through T1	Scapula	Retracts and stabilizes scapula	Dorsal scapular nerve

Key Points

Pectoralis minor is important for scapular stabilization.
Levator scapulae and rhomboids help with elevation and retraction of the scapula.

Example

The rhomboids are used when pulling the shoulders back.

Muscles Acting on the Shoulder and Arm

Overview

These muscles move the shoulder joint and upper arm, allowing for a wide range of movements. They are innervated by various nerves, including the axillary, suprascapular, and thoracodorsal nerves.

Muscle	Origin	Insertion	Action	Innervation
Pectoralis major	Sternum, costal cartilages, and clavicle	Humerus	Flexes, adducts, and medially rotates arm	Pectoral nerves
Deltoid (anterior fibers)	Clavicle (lateral and posterior fibers)	Deltoid tuberosity of the humerus	Flexes, extends, abducts, and medially rotates arm	Axillary nerve
Biceps brachii (long head and short head)	Scapula	Radius	Weak shoulder flexion (also flexes the elbow and supinates forearm)	Musculocutaneous nerve
Deltoid (lateral and posterior part)	Scapula	Humerus	Flexes, extends, abducts, and laterally rotates arm	Axillary nerve
Supraspinatus	Scapula	Greater tubercle of the humerus	Assists in arm abduction, stabilizes shoulder joint	Suprascapular nerve
Infraspinatus	Scapula	Greater tubercle of the humerus	Laterally rotates arm, stabilizes shoulder joint	Suprascapular nerve
Teres minor	Scapula	Greater tubercle of the humerus	Laterally rotates arm, stabilizes shoulder joint	Axillary nerve
Subscapularis	Scapula	Lesser tubercle of the humerus	Medially rotates arm, stabilizes shoulder joint	Subscapular nerve
Teres major	Scapula	Humerus	Extends, adducts, and medially rotates arm	Subscapular nerve
Triceps brachii (long head)	Scapula	Ulna	Extends and adducts arm	Radial nerve
Latissimus dorsi	Thoracic and lumbar vertebrae, ribs, iliac crest	Humerus	Extends, adducts, and medially rotates arm	Thoracodorsal nerve

Key Points

Rotator cuff muscles (supraspinatus, infraspinatus, teres minor, subscapularis) stabilize the shoulder joint.
Deltoid is the main abductor of the arm.

Example

The latissimus dorsi is used in movements such as swimming and climbing.

Muscles Acting on the Forearm

Overview

Forearm muscles control movements of the wrist, hand, and fingers. They are divided into flexor and extensor compartments, with different nerve supplies.

Anterior Compartment (Flexors)

Muscle	Origin	Insertion	Action	Innervation
Biceps brachii	Scapula	Radius	Flex and supinate forearm	Musculocutaneous nerve
Brachialis	Humerus	Ulna	Flex forearm	Musculocutaneous nerve
Pronator teres	Humerus and proximal ulna	Mid-radius	Pronates forearm	Median nerve
Brachioradialis	Humerus	Radius	Flexes forearm	Radial nerve
Palmaris longus	Medial epicondyle of the humerus	Palmar aponeurosis	Flex wrist	Median nerve
Flexor carpi ulnaris	Medial epicondyle of the humerus and ulna	Fifth metacarpal	Flexes wrist, adducts wrist	Ulnar nerve
Flexor carpi radialis	Medial epicondyle of the humerus	Second and third metacarpal	Flexes wrist and abducts wrist	Median nerve
Flexor digitorum superficialis	Medial epicondyle of the humerus, ulna, and radius	Phalanges of fingers 2-5	Flexes wrist and middle phalanges of fingers 2-5	Median nerve
Flexor digitorum profundus	Ulna	Phalanges of fingers 2-5	Flexes distal and proximal interphalangeal joint, metacarpophalangeal joint, and wrist	Median (lateral half) and ulnar (medial half) nerves

Posterior Compartment (Extensors)

Muscle	Origin	Insertion	Action	Innervation
Triceps brachii	Long head: Scapula; Lateral head: Humerus; Medial head: Humerus	Ulna	Extends forearm at elbow	Radial nerve
Supinator	Humerus and ulna	Mid-radius	Supinates forearm	Radial nerve
Extensor carpi radialis longus	Humerus	Second metacarpal	Extends and abducts wrist	Radial nerve
Extensor carpi radialis brevis	Humerus	Third metacarpal	Extends and abducts wrist	Radial nerve
Extensor carpi ulnaris	Lateral epicondyle of the humerus	Fifth metacarpal	Extends and adducts wrist	Radial nerve
Extensor digitorum	Humerus	Phalanges of fingers 2-5	Prime mover of finger extension, extends wrist	Radial nerve

Key Points

Flexor muscles are generally located on the anterior side of the forearm.
Extensor muscles are located on the posterior side.

Example

The brachioradialis is unique in that it flexes the forearm but is innervated by the radial nerve, which typically supplies extensors.

Muscles of the Anterior and Lateral Abdominal Wall

Overview

These muscles support the trunk, allow movement, and help maintain abdominal pressure. All are innervated by the intercostal nerves.

Muscle	Origin	Insertion	Action	Innervation
External abdominal oblique	Lower 8 ribs	Linea alba	Rotates vertebral column and compresses abdominal contents	Intercostal nerves
Internal abdominal oblique	Iliac crest	Linea alba	Rotates vertebral column and compresses abdominal contents	Intercostal nerves

Key Points

Oblique muscles are important for trunk rotation and lateral flexion.
They also play a role in forced expiration and maintaining intra-abdominal pressure.

Example

The external abdominal oblique is activated during twisting movements of the torso.

Additional info

Muscle actions are determined by their origin, insertion, and the direction of muscle fibers.
Innervation is crucial for muscle function; damage to the nerve can result in paralysis or weakness of the muscle.
Understanding muscle anatomy is essential for clinical assessment and treatment of musculoskeletal disorders.