Gross Anatomy and Cervical Ventral Branch Variations of the Phrenic Nerve in Akkaraman Sheep (Ovis aries)

Abstract

Sheep models are important in peripheral nerve research due to anatomical similarities to humans. The phrenic nerve, as the sole motor nerve of the diaphragm, represents a crucial structure for understanding peripheral nerve anatomy. This study investigated the gross anatomical characteristics and cervical ventral branch variations of the phrenic nerve in Akkaraman sheep to provide fundamental data for peripheral nerve research. Seven one-year-old male Akkaraman sheep underwent neurological examination. Following euthanasia and perfusion fixation with paraformaldehyde-glutaraldehyde solution, systematic dissection was performed to identify phrenic nerve origins, course and branching patterns. Phrenic nerve formation showed species-specific variations. In four animals (57.1%), the nerve was formed by ventral branches from C5, C6 and C7, whereas in the remaining three animals (42.9%), formation involved ventral branches from C5 and C6 only. Intrathoracic formation occurred in six animals (85.7%) and extrathoracic in one animal (14.3%). Consistent bilateral symmetry of the phrenic nerve was observed in all specimens, accompanied by variations in cervical ventral branch contributions and formation levels. No macroscopically detectable branches supplying the pericardium were identified along the intrathoracic course of the phrenic nerve. These findings support the use of sheep as large animal models for peripheral nerve research and emphasize the need to consider anatomical variability in experimental design.