Projections of supraspinal structures to the phrenic motor nucleus in cats studied by a horseradish peroxidase microinjection method.

The activity of phrenic motor neurons is influenced by the cardiovascular control system of the supraspinal structures. In order to obtain the basic data for analyzing the anatomical relations between the cardiovascular and the pulmonary control system, supraspinal structures projecting to the phrenic motor nucleus of the cat spinal cord were studied using a horseradish peroxidase method. A double-barrel coaxial electrode was employed. To determine the site of the phrenic motor neurons, the inner barrel electrode, filled with 3 M NaCl solution, was used for recording the activity of these neurons. The outer barrel electrode, filled with a 20% HRP solution, was used for injecting HRP iontophoretically into the phrenic motor nucleus. Out of 13 experiments, 5 showed that the HRP-injection sites were centered in and almost confined to the phrenic motor nucleus. Some 1798 HRP-labeled cells were thus identified in the selected 5 experiments. They were distributed in the medulla oblongata (93.5%), pons (6.0%) and midbrain (0.5%). The majority were concentrated in the nucleus para-ambiguus (48.9%), nucleus tractus solitarii (21.5%) and in or around the nucleus retrofacialis (9.8%). A few labeled cells were scattered throughout the nucleus raphe (1.1%) and the parabrachial and Koelliker-Fuse nuclei (0.3%), suggesting that these nuclei may be, if any, only minor sources of input to the phrenic motor nucleus.