The phrenic nucleus of th albino rat: a correlative HRP and Golgi study

The phrenic nucleus of the adult albino rat was studied by utilizing the O-dianisidine method for the retrograde transport of horseradish peroxidase in conjunction with the zinc chromate modification of the Golgi technique. Application of HRP to the transected phrenic nerve in the neck labeled a column of phrenic motor neurons from C3 to C5 in the ipsilateral spinal cord. However, when HRP was applied to the phrenic nerve intrathoracically, labeled neurons were found from C3 to C6. The long axis of the column of phrenic neurons was oriented tangentially from rostral to caudal poles. There was a gradual shift of the column from posterior to anterior and from lateral to medial positions in the ventral horn. The peroxidase material was also used to localize impregnated phrenic motor neurons in the Golgi sections and to provide quantitative data on phrenic motor neurons. In Golgi-impregnated material two types of phrenic neurons were distinguished on the basis of dendritic morphology and orientation. These neurons were designated (1) large neurons with smooth, radially oriented dendrites, and (2) smaller neurons with varicose, tangentially oriented dendrites. Both types of neurons had a small number of spines and bulbous appendages issuing from the dendritic trunks and branches. The dendritic fields of adjacent phrenic neurons overlapped extensively with one another and with dendrites of more distally placed ventral horn motor neurons. In peroxidase-labeled sagittal sections the dendrites of phrenic neurons were primarily oriented in the rostrocaudal plane. The mean total number of peroxidase-labeled neurons in the phrenic nucleus was 415.75 +/- 18.36 cells. In sagittal sections the mean long axis diameter of phrenic cell bodies was 34.5 micrometers. In frontal sections the mean long axis diameter of phrenic cell bodies was 22.5 micrometers. Thus, from direct measurement, the phrenic neurons were 34% longer in the sagittal plane than in the frontal plane. In the present study each phrenic nucleus contributed fibers only to the ipsilateral phrenic nerve, and no evidence for peripheral crossing of fibers was found.