Upper Limb

The upper limb is attached to the axial skeleton mainly through the pectoral girdle, with the clavicle acting as a strut that positions the scapula and upper limb away from the trunk. The pectoral region contains muscles that move and stabilize the shoulder girdle and humerus. Deep to this region, the axilla is a pyramidal passage between neck and upper limb. Its walls are formed by muscles, bones and fascia, and it transmits the axillary artery and vein, cords and branches of the brachial plexus, lymphatics and fat.

Learning the axilla as a three-dimensional space is more useful than memorizing a contents list. The artery and plexus change relationships as they pass through the region, and axillary lymph nodes receive drainage from the upper limb and important areas of the thoracic wall and breast. This is why anatomy of the axilla matters in vascular access, nerve injury and breast surgery.

The brachial plexus reorganizes anterior rami, classically C5–T1, into a network that distributes motor and sensory fibers to the upper limb. The organizational sequence is roots, trunks, divisions, cords and terminal branches. Roots combine into upper, middle and lower trunks; each trunk divides into anterior and posterior divisions; divisions regroup into cords named by their relationship to the axillary artery. Major terminal nerves include musculocutaneous, axillary, radial, median and ulnar nerves.

The reason for this apparently complicated design is redistribution: fibers from one spinal level can reach multiple peripheral nerves, and one peripheral nerve may contain fibers from several spinal levels. Therefore, a root lesion and a peripheral nerve lesion produce different patterns. Clinical localization becomes a reasoning exercise: identify weak movements, sensory changes and reflex findings, then ask where those affected fibers travel together.

The anatomical arm extends from shoulder to elbow and is divided by fascial septa into anterior and posterior compartments. The anterior compartment is dominated by flexor muscles and is primarily supplied by the musculocutaneous nerve; the posterior compartment contains the major extensors and is supplied by the radial nerve. The brachial artery descends through the arm and divides near the elbow into radial and ulnar arteries.

The cubital fossa is a triangular transition region anterior to the elbow. Its boundaries and layered contents make it clinically important. Superficial veins are commonly used for venepuncture, while deeper structures include the brachial artery and median nerve. The elbow itself balances stability with movement through the humeroulnar, humeroradial and proximal radioulnar relationships.

The forearm contains radius and ulna plus muscle groups that move the wrist, hand and digits and rotate the forearm. Anterior muscles are broadly flexor-pronator in function and are mainly supplied by the median nerve, with important ulnar-nerve exceptions. Posterior muscles are broadly extensor-supinator and are supplied by the radial nerve and its branches. The radius rotates around the ulna during pronation and supination, so radial and ulnar relationships are often more informative than simply medial and lateral.

Neurovascular structures travel between muscle layers and through anatomical tunnels. Understanding these paths explains entrapment syndromes and why injury at different levels produces different deficits. Rather than memorizing every muscle independently, group muscles by compartment, action, innervation and depth, then add exceptions.

The hand combines long extrinsic tendons from the forearm with intrinsic muscles located within the hand. This arrangement allows both power grip and fine precision. Thenar muscles position the thumb for opposition and manipulation; hypothenar muscles act on the little finger; lumbricals and interossei coordinate complex finger movements. Median and ulnar nerves divide the intrinsic motor supply in a clinically important pattern, while cutaneous sensory territories provide additional localization clues.

The arches of the hand and mobile thumb make the hand mechanically adaptable. Tendon sheaths reduce friction but can also provide pathways for spread of infection. Because many tendons, nerves and vessels occupy limited spaces, small injuries can have major functional consequences. Examination should test specific movements rather than relying on a vague impression of 'hand strength.'

The shoulder complex provides exceptional mobility at the cost of reduced bony stability. Glenohumeral stability therefore depends heavily on the labrum, capsule, ligaments, rotator-cuff muscles and coordinated scapular movement. The elbow is more constrained, while radioulnar joints permit forearm rotation. Wrist and hand joints distribute motion across multiple articulations.

Applied anatomy follows directly from relationships. A shoulder dislocation may threaten the axillary nerve; a humeral shaft fracture may endanger the radial nerve; injuries near the medial epicondyle can affect the ulnar nerve. These are not arbitrary associations: each follows the normal course of the structure next to bone or joint. Imaging should be read by tracing alignment, cortical continuity and joint congruence.