The inferior (lower) trunk forms from the C8 and T1 roots and lies on the first rib behind the subclavian artery. It carries the fibres for the small muscles of the hand and the long finger flexors, and splits behind the clavicle into an anterior division (forming the medial cord) and a posterior division (joining the posterior cord).
| Origin | Union of the C8 and T1 roots |
|---|---|
| Course | Lies on the upper surface of the first rib behind the subclavian artery and divides behind the clavicle into anterior and posterior divisions |
| Motor |
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| Sensory |
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Its position on the first rib makes the inferior trunk the part of the plexus most affected in neurogenic thoracic outlet syndrome, often from a cervical rib or fibrous band, and by an apical lung (Pancoast) tumour. Upward traction on the arm, as in grabbing a branch during a fall or a breech delivery, injures it to produce Klumpke palsy. Hand wasting with numbness along the medial forearm and little finger points to the lower trunk rather than the ulnar nerve alone.
Traction on an abducted arm damages C8-T1 or the inferior trunk, causing weakness of the intrinsic hand muscles and finger flexors with a claw hand. Horner syndrome may accompany it when the T1 root is avulsed.
Compression of the lower trunk over a cervical rib or band causes medial forearm and hand numbness and wasting, classically of the thenar muscles (the Gilliatt-Sumner hand). Surgical decompression is considered when there is objective weakness.
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