Variability of anterior external arcuate fibers in the human medulla oblongata

Anterior external arcuate fibers (AEAF) are efferents of the arcuate nuclei, which are located on the ventral surface of pyramids. Several types of fibre bundles superficial to the pyramids have been described in early and mid 20th century. Recently, few of these have been studied in detail. Objective of present study was to observe the morphology of AEAF in the Indian population. Distinct AEAF were noted in 13 out of 50 brain specimens. Based on their relation to olive, AEAF were further classified as supraolivary, preolivary and supraolivary fibers and their prevalence noted as 25%, 15%, and 9% respectively. Supraolivary and preolivary fibers were present together in 9 brainstem sides, while co-presence of preolivary and circumolivary fibers was noted in only 1 side. All three types of fibres were observed together in 5 brainstem sides. When present bilaterally, supraolivary and preolivary fibers were seen in 92.30% and 66.66% of brainstem respectively, while circumolivary fibers were seen bilaterally in 28.57% of brainstem. Supraolivary and circumolivary fibers exhibited variable morphology as single, double and multiple fiber bundles. Morphometric analysis revealed presence of thicker supraolivary fiber bundle on right side, while thicker circumolivary fiber bundles were noted on left side. Present study will add to knowledge of this variable fiber bundle pattern, which has been reported to play an important role in regulation of crucial physiological functions such as breathing and cardiorespiratory mechanisms. These observations open avenues for further research into developmental factors involved in migration of neurons from the rhombic lip.