Ulnar Nerve Root Origins in the Brachial Plexus
The ulnar nerve originates from the C8 and T1 nerve roots of the brachial plexus, arising from the medial cord after these roots form the lower trunk. 1, 2
Anatomical Organization
The ulnar nerve formation follows a specific anatomical sequence through the brachial plexus:
The nerve roots C8 and T1 form the lower trunk of the brachial plexus, which then continues as the anterior division and ultimately becomes the medial cord 1, 2
The medial cord gives rise to the ulnar nerve as one of its terminal branches in the infraclavicular region, specifically at the lateral margin of the pectoral minor muscle 1
The C8-T1 nerve roots pass between the anterior and middle scalene muscles alongside the subclavian artery before forming the plexus structures 1
Functional Motor Innervation
T1 is the predominant contributor to ulnar nerve function, particularly for intrinsic hand muscle innervation:
Electrophysiological studies demonstrate that T1 primarily forms the ulnar nerve to innervate the intrinsic muscles of the hand, including the abductor digiti minimi and other small hand muscles 3
C8 contributes more heavily to median nerve function for finger flexion, while T1 dominates ulnar nerve innervation 3
Clinical Significance
Understanding the precise root origins has critical implications for diagnosis and surgical planning:
Lesions involving C8 or higher nerve roots in the brachial plexus (including the cords) are classified as T4 tumors in lung cancer staging, while T1-T2 root involvement is classified as T3 4
Tumors invading the lower brachial plexus (C8-T1) present with radicular pain or neurologic findings affecting the ulnar distribution of the hand 4
Differentiating between preganglionar (nerve root) and postganglionar (plexus/cord) lesions is essential because treatment approaches differ significantly 1
In brachial plexus injuries with C8 and T1 root avulsion, the ulnar nerve from the contralateral side can be used as a vascularized nerve graft for reconstruction 5
Anatomical Variations
While the standard C8-T1 origin is most common, variations exist:
Occasional contributions from C7 to the ulnar nerve have been documented, though C7 primarily innervates the radial nerve and triceps muscle 3
The brachial plexus may receive contributions from C4 and/or T2 in some individuals, though these are less common 1
Communicating branches between the ulnar nerve and lateral cord have been reported in cadaveric studies, occurring in approximately 5% of cases 6