Development of the rat phrenic nerve and the terminal distribution of phrenic afferents in the cervical cord

The development of the right phrenic nerve and the distribution of phrenic nerve afferents to the spinal cord have been examined with the aid of elect ron microscopy and carbocyanine dye retrograde diffusion along the phrenic nerve, respectively. The formation of; fascicles in the right phrenic nerve commenced at E15, while Schwann cells penetrated the nerve from E17 and my elination began at PO. The total number of axons in the right phrenic nerve decreased from E15 (943, 965 in two animals) to E19 (539, 582), remained s teady until PO (564, 594) before rising to almost adult values by P7 (689, 934). The postnatal rise in number of axons appears to be due to a large in flux of unmyelinated axons. Carbocyanine dye tracing revealed that at E13, neurons in dorsal root ganglia C-2 to C-6 contributed peripheral processes to the phrenic nerve. Phrenic afferents arrived in the spinal cord by E13 a nd penetrated the dorsal horn at E14. Three terminal fields for phrenic aff erents became apparent by E17. These were:(1) in the central parts of lamin ae I to V, (2) medially in laminae V to VII or adjacent area X near the cen tral canal, (3) in laminae VIII and IX, around the differentiating phrenic motoneurons. Around the time of birth, some phrenic afferents in the second group were distributed across the midline and could be seen to approach th e ventromedial dendritic bundle of phrenic motoneurons on the contralateral side, but these were no longer seen by P4. Just before birth (E21), affere nts in the third group divided into two further subsets, supplying the dors olateral and ventromedial groups of phrenic motoneuron dendritic bundles, r espectively. Our findings strongly suggest that phrenic afferent differenti ation is largely complete by birth.