Sympathetic fibers of the right phrenic nerve implicate a role for respiratory modulation

This investigation utilized immunohistochemical analysis to study sympathetic catecholaminergic fibers within supradiaphragmatic and abdominal regions of the right phrenic nerve. The phrenic nerve consists of motor, sensory, and autonomic components and is the primary source of the diaphragm’s somatic motor and sensory innervation. The autonomic component contains sympathetic catecholaminergic fibers that vary by location. Despite extensive research, no clear understanding of the autonomic role of the phrenic nerve has been proposed. Knowledge of phrenic nerve morphology on a gross and histological scale is crucial to understanding its autonomic function and multifaceted impacts on the diaphragm, visceral organs, and mechanisms of slow breathing. Six (n = 6) formalin-preserved human cadavers were dissected at Missouri Southern State University. Samples were collected from the root of the neck, intradiaphragmatic branches, and abdominal communicating branches. Histological sections were prepared using H&E, Luxol Fast Blue(LFB), and anti-tyrosine hydroxylase antibody. Quantitative analysis measured effective surface area(ESA) and tyrosine hydroxylase(TH)-positive, myelinated, and unmyelinated regions. Results showed that the phrenic branch below the diaphragm through the caval opening established anastomoses with abdominal sympathetic structures, acquiring sympathetic fibers. While our findings confirm presence of sympathetic catecholaminergic fibers within the subdiaphragmatic phrenic nerve described previously, we extend this understanding by characterizing fascicular distribution and regional differences in myelination and tyrosine hydroxylase expression. These findings suggest that sympathetic integration within the right phrenic nerve contributes to respiratory modulation through autonomic–somatic interactions influencing diaphragmatic control and visceral function, supporting an anatomical basis for respiratory–sympathetic coordination during slow, controlled breathing.