An Exploration of the Sympathetic Component of the Right Phrenic Nerve

Abstract

Introduction The phrenic nerve is a mixed nerve that arises from the anterior primary rami of C3-C5 bilaterally and is the main motor supply to the diaphragm. The diaphragm muscle has a multifaceted impact on body functions- including breathing, visceral health, CSF flow, and emotional regulation. According to anatomical textbooks, the phrenic nerve receives catecholaminergic fibers from the middle and lower cervical ganglia. However, previous studies reveal that both phrenic nerves lack the sympathetic component at the diaphragm level, recovering it in the abdominal region supposedly from communication with a celiac plexus. Thus, the aim of the current study was to analyze the constitution of the supradiaphragmatic and abdominal divisions of the right phrenic nerve, targeting the presence of sympathetic fibers. Methods Six formalin-preserved adult human cadavers were dissected at the MSSU human dissection laboratory. The anatomical course of the supra and subdiaphragmatic portions of the right phrenic nerve was documented before obtaining tissue samples for histological examination. Samples were collected at the root of the neck, near the inferior vena caval diaphragmatic hiatus, where the phrenic nerve splits into two main trunks. Thoracic posterior trunk (TPT) enters the abdominal cavity, and thoracic anterolateral trunk (TALT) supplies the right thoracic hemidiaphragm. Samples were also collected from identified abdominal branches. All sections were prepared using H&E and Luxol Fast Blue staining. Antibody staining for tyrosine hydroxylase was used to determine the presence of catecholaminergic fibers. The results were processed with ImageJ software for quantitative analysis. Results Lack of communication was observed between the cervical phrenic nerve and sympathetic cervical ganglia. No TH activity was detected, indicating an absence of catecholaminergic fibers in this area. The TALT showed an absence of TH activity throughout the effective surface area. Whereas TPT displayed 15% and the abdominal phrenic nerve 27% of catecholaminergic fibers. A branch to IVC of the Celio–Phrenic communication was identified in three specimens with an effective surface area of 0.049mm2, and 46-71% of this area contained TH-positive fibers. Conclusions Immunohistochemical analysis revealed an absence of catecholaminergic activity in the cervical area. This finding contradicts previous knowledge regarding cervical phrenic nerve anatomy and requires further investigation. TH positive fibers were only observed in the posterior trunk and absent in the TALT. Also, the prevalence of sympathetic fibers in the abdominal course of the phrenic nerve was evident. Thus, the lack of apparent communication between TALT and abdominal sympathetic ganglia resulted in the absence of sympathetic activity in the branch. These findings further support previous studies regarding the celiac plexus being a primary contributor of catecholaminergic fibers to the right phrenic nerve. Significance Understanding of the function requires profound expertise of the structure. Knowledge of sympathetic component within the phrenic nerve would clarify the autonomic role of the nerve, emphasizing the impact of proper respiration on overall health.