Posterior external arcuate fibers

The posterior external arcuate fibers are second-order neurons originating in the accessory cuneate nucleus, a gray matter structure located in the dorsolateral medulla oblongata, lateral to the cuneate nucleus of the dorsal column-medial lemniscal (DC-ML) pathway. The accessory cuneate nucleus serves as the upper limb equivalent of Clarke's column, mediating the transmission of unconscious proprioceptive information from the upper extremities to the cerebellum. Since Clarke's column is absent above the C8 spinal segment, the posterior external arcuate fibers replace the spinocerebellar tract at these higher levels.

Proprioceptive input is initially carried by first-order neurons from spinal nerves C1–C4, which ascend through the fasciculus cuneatus to synapse within the accessory cuneate nucleus. The posterior external arcuate fibers, as second-order neurons, then project from the accessory cuneate nucleus to the vermal and paravermal regions of the cerebellum via the inferior cerebellar peduncle. In the cerebellum, these fibers play a critical role in supporting motor coordination and balance.