Why does the radial nerve primarily receive input from C8 and T1 nerve roots, while the axillary nerve, also originating from the posterior cord, primarily receives input from C5 and C6 nerve roots?

Anatomical Basis for Differential Nerve Root Contributions

The radial nerve receives predominant input from C7-C8 (not primarily C8-T1 as stated), while the axillary nerve receives predominant input from C5-C6 because these nerves originate from different anatomical divisions of the brachial plexus—the radial nerve forms from the posterior cord after contributions from all three trunks, whereas the axillary nerve branches from the posterior cord before the lower trunk contributions are fully integrated.

Brachial Plexus Organization

The key to understanding this apparent paradox lies in the sequential formation of the brachial plexus:

• The axillary nerve branches from the posterior cord at a more proximal level, receiving fibers predominantly from the upper trunk (C5-C6) before significant contributions from the middle and lower trunks are incorporated 1, 2

• The radial nerve forms more distally from the posterior cord, after all three trunks (upper, middle, and lower) have contributed their posterior divisions, allowing it to receive fibers from C5 through T1, with predominant contributions from C7-C8 3, 4

Specific Nerve Root Contributions

Axillary Nerve (C5-C6 Predominant)

• C6 contributes the majority of fibers to the deltoid muscle in 35% of cases, while C5 contributes equally with C6 in only 10% of cases 1

• C5 was never the sole component of axillary nerve branches to the deltoid, and C6 contributed 15-99% of fibers across all specimens studied 1

• The axillary nerve branches off the posterior cord before lower trunk (C8-T1) contributions significantly influence the cord's composition 2

Radial Nerve (C7-C8 Predominant)

• The radial nerve is the largest branch of the brachial plexus and receives contributions from all nerve roots (C5-T1), but with predominant contributions from the middle and lower portions 3

• In lower root avulsions (C8-T1), radial nerve function is severely impaired, demonstrating its dependence on these lower roots for hand and wrist extension 4

• The anatomical classification for Pancoast tumors designates invasion as T4 stage if there is involvement of C8 or higher nerve roots affecting the brachial plexus cords, indicating the clinical significance of C8 contributions to distal nerve structures 5

Clinical Implications

Injury Patterns

• C5-C6 injuries (Erb's palsy) primarily affect the axillary nerve, resulting in deltoid weakness and shoulder abduction deficits 6, 1

• C8-T1 injuries (Klumpke paralysis) severely impair radial nerve function in the hand and wrist, with preservation of more proximal functions 4

• Superior sulcus tumors invading the lower brachial plexus (C8-T1) present with radicular pain or neurologic findings of the ulnar hand, consistent with radial nerve distribution 5

Diagnostic Considerations

• Motor testing of deltoid function primarily assesses C5-C6 integrity via the axillary nerve 1

• Wrist and finger extension deficits indicate C7-C8 involvement affecting the radial nerve 4

• The presence of hand swelling with C5-T1 nerve root compression suggests vascular compromise or plexopathy rather than simple radiculopathy, as nerve root compression alone does not typically cause hand swelling 6