Shoulder and Axilla Muscles

Shoulder and Axilla Muscles

Scapular Muscles

Trapezius (CN XI + C3/C4): upper fibres elevate the scapula, middle fibres retract, lower fibres depress — together they rotate the glenoid upward for full arm elevation. Levator scapulae (C3–C5): elevates the scapula. Rhomboid minor and major (dorsal scapular nerve, C4–C5): retract and slightly elevate the scapula. Serratus anterior (long thoracic nerve, C5–C7): protracts the scapula and rotates the glenoid upward; stabilises the scapula against the chest wall. Long thoracic nerve palsy (e.g., from mastectomy, carrying heavy loads) causes winging of the scapula.

Rotator Cuff and Glenohumeral Muscles

SITS muscles (Supraspinatus, Infraspinatus, Teres minor, Subscapularis) form a muscular cuff around the glenohumeral joint. Supraspinatus (suprascapular nerve, C5–C6) passes beneath the coracoacromial arch and initiates abduction; it is the most commonly torn rotator cuff muscle. Impingement of supraspinatus between the greater tuberosity and acromion produces a painful arc syndrome (pain 60–120° of abduction). Infraspinatus and teres minor (suprascapular and axillary nerves respectively) laterally rotate the arm. Subscapularis (upper and lower subscapular nerves) medially rotates and adducts.

Deltoid (axillary nerve, C5–C6): clavicular fibres flex and medially rotate; acromial (middle) fibres abduct (primary abductor above 15°); spinal fibres extend and laterally rotate.

Teres major (lower subscapular nerve): extends, adducts, and medially rotates — functional synergist of latissimus dorsi (described as lat's little helper).

Pectoralis major: clavicular head flexes the arm; sternocostal head extends the flexed arm; both heads adduct and medially rotate. Latissimus dorsi: adducts, extends, and medially rotates — the powerful swimmer's and crutch-walking muscle.

Axilla and Brachial Plexus

The axilla is a pyramidal space with four walls (anterior = pec major/minor; posterior = subscapularis/teres major/latissimus dorsi; medial = serratus anterior/ribs; lateral = intertubercular groove of humerus), an apex (between clavicle/first rib/scapula) and a floor (skin/fascia of armpit).

The brachial plexus (C5–T1) passes through the axilla: roots → trunks (upper C5/6, middle C7, lower C8/T1) → divisions (anterior/posterior) → cords (lateral, medial, posterior) → terminal branches. The cords are named for their relationship to the axillary artery. Terminal branches: musculocutaneous (lateral cord), axillary (posterior cord), radial (posterior cord), median (medial + lateral cords), ulnar (medial cord).

The axillary artery (continuation of subclavian artery at lateral border of first rib) has three parts relative to pectoralis minor. The anterior and posterior circumflex humeral arteries arise from the third part and supply the humeral head.