Muscle Attachments, Nerve Supply & Actions

1. Latissimus Dorsi

Attachments

• Vertebral part: Spinous processes of lower six thoracic vertebrae (T7-T12) and their interspinous ligaments
• Thoracolumbar fascia: Via this fascia to the spinous processes of all lumbar vertebrae, sacrum, and related interspinous ligaments
• Iliac part: Posterior third of the iliac crest
• Costal part: Lower 3-4 ribs (9th to 12th ribs)
• Scapular part: Inferior angle of the scapula (small slip)

• Floor of the intertubercular (bicipital) groove of the humerus

The tendon twists 180° before inserting so that superior fibers insert inferiorly and vice versa.

Nerve Supply

• Thoracodorsal nerve (C6, C7, C8) - a branch of the posterior cord of the brachial plexus
• Blood supply: Thoracodorsal artery (primary), dorsal branches of posterior intercostal and lumbar arteries

Actions

• Extension of the arm at the glenohumeral joint
• Adduction of the arm
• Medial (internal) rotation of the arm
• Depression of the shoulder (prevents upward movement)
• Assists in expiration (as a "cough muscle") when the upper limb is fixed
• Pulls the body upward when arms are fixed (as in climbing)

2. Pectoralis Major

Attachments

• Clavicular head: Medial half of the anterior surface of the clavicle
• Sternocostal head: Anterior surface of the sternum (manubrium + body) and costal cartilages of ribs 2-6
• Abdominal part: Anterior layer of the rectus sheath (variable, may be absent)

• Crest of the greater tubercle (lateral lip of the intertubercular sulcus) of the humerus
• The tendon forms a bilaminar fold - fibers from the clavicular head (superior origin) insert lower, and fibers from the sternocostal head (inferior origin) insert higher, creating a twist

Nerve Supply

• Lateral pectoral nerve (C5, C6, C7) - primarily to the clavicular head
• Medial pectoral nerve (C8, T1) - primarily to the sternocostal head
• Both arise from the brachial plexus; combined spinal levels: C5-T1

Actions

• Adduction of the arm at the glenohumeral joint (entire muscle)
• Medial (internal) rotation of the arm (entire muscle)
• Flexion of the arm (clavicular + sternocostal parts)
• Clavicular head only: Flexes the arm from an extended position
• Sternocostal head only: Extends the arm from a flexed position (especially against resistance)
• Assists in respiration (elevates ribs) when the shoulder girdle is fixed

3. Serratus Anterior

Attachments

• Lateral surfaces (outer surfaces) of ribs 1-9, and the intervening deep fascia over the related intercostal spaces

• Superior part: Costal and dorsal surfaces of the superior angle of the scapula
• Intermediate part: Costal surface of the medial border of the scapula
• Inferior part (largest): Costal and dorsal surfaces of the inferior angle of the scapula, and costal surface of the medial border

Nerve Supply

• Long thoracic nerve (C5, C6, C7) - arises from the roots of the brachial plexus, runs vertically down the external surface of serratus anterior along the medial wall of the axilla, just deep to skin and superficial fascia

Injury to the long thoracic nerve causes paralysis of serratus anterior, leading to "winging of the scapula" (medial border lifts off the posterior thoracic wall).

Actions

• Protracts the scapula - pulls/draws the scapula laterally forward over the thoracic wall (essential for pushing and reaching movements)
• Holds the scapula closely opposed to the thoracic wall (prevents winging)
• Rotates the scapula - the inferior part rotates the inferior angle of the scapula laterally and anteriorly, turning the glenoid cavity superiorly; this is essential for arm elevation above 90°
• Superior part: Lowers the raised arm (antagonist to the inferior part)
• Assists in respiration (elevates ribs) when the shoulder girdle is fixed

4. Brachioradialis

Attachments

• Proximal part of the lateral supraepicondylar ridge (lateral surface of the distal humerus)
• Lateral intermuscular septum

• Lateral side of the distal end of the radius, just proximal to the radial styloid process (styloid process of the radius)

Nerve Supply

• Radial nerve (C5, C6) - the radial nerve emerges from the posterior compartment of the arm just deep to the brachioradialis in the distal arm

Notable: The brachioradialis lies in the posterior compartment of the forearm but acts as a flexor of the elbow - an apparent paradox explained by its anterior position relative to the elbow joint axis.

Actions

• Flexion of the elbow joint (primary action; acts as an accessory flexor, most efficient when forearm is in mid-prone position)
• Semipronation - brings the forearm to the midprone (neutral/handshake) position from full supination
• Forms the lateral boundary of the cubital fossa
• Produces a prominent bulge when acting against resistance with the forearm midprone

• Gray's Anatomy for Students (Tables 2.1, 7.5; Chapters 2 & 7)
• THIEME Atlas of Anatomy - General Anatomy and Musculoskeletal System, Sections 2.6, 2.7