Projections from the ventral respiratory group to phrenic and intercostal motoneurons in cat: an autoradiographic study.

Anterograde transport of tritiated amino acids (leucine, lysine, and proline) was used to examine the spinal projections of respiratory premotor neurons in the ventral respiratory group (VRG) of cats. This population of neurons corresponds anatomically with the nucleus ambiguus-retroambigualis. Small volumes (20 to 50 nl) of tritiated amino acids were pressure ejected into the middle of the VRG through a micropipette which permitted simultaneous recording of respiratory modulated activity. In two cats injections were made caudal to the obex in regions which contained expiratory modulated neurons. In five cats injections were made rostral to the obex in regions containing inspiratory neurons. After a 2-week survival period, cats were anesthetized and perfused. The entire neuraxis was removed and processed using standard autoradiographic techniques. Transport of tritiated amino acids revealed a marked bilateral projection to lamina IX of the spinal cord at the C4 to C6 level and a primarily contralateral projection to laminae VIII and IX in the thoracic spinal cord. Distinct descending pathways to the phrenic motor neurons were observed in the lateral funiculus and in the ventral funiculus; descending fibers to the intercostal motoneurons in the thoracic cord appeared to be restricted to the ventral funiculus. Labeling of axon terminals in both the cervical and thoracic cords was confined to ventral horn regions which contain motoneurons. These results suggest that monosynaptic projections from brainstem bulbo-spinal neurons to spinal motoneurons are important in controlling respiratory movements of the diaphragm and intercostal muscles.