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

Abstract

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 electron 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 myelination began at P0. The total number of axons in the right phrenic nerve decreased from E15 (943, 965 in two animals) to E19 (539, 582), remained steady until P0 (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 influx 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 and penetrated the dorsal horn at E14. Three terminal fields for phrenic afferents became apparent by E17. These were:(1) in the central parts of laminae I to V, (2) medially in laminae V to VII or adjacent area X near the central 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 the ventromedial dendritic bundle of phrenic motoneurons on the contralateral side, but these were no longer seen by P4. Just before birth (E21), afferents in the third group divided into two further subsets, supplying the dorsolateral and ventromedial groups of phrenic motoneuron dendritic bundles, respectively. Our findings strongly suggest that phrenic afferent differentiation is largely complete by birth.

Neuromuscular Development in the Absence of Programmed Cell Death: Phenotypic Alteration of Motoneurons and Muscle

“…9B). However, because unmyelinated axons of sensory and autonomic origin are also normally present in WT mixed nerves, it was not possible to definitively determine the extent to which these represented atrophied ␣-MNs (Song et al, 1999). The unmyelinated VR axons were observed both as individual axons and in small and large clusters (Remak-like bundles).…”

Spinal Activation of Serotonin 1A Receptors Enhances Latent Respiratory Activity After Spinal Cord Injury

“…There are anatomical reports that place phrenic afferents in close proximity to phrenic motor neurons (31) as well as the presence of axo-axonic synapses on phrenic motor neurons (32), indicating a possible monosynaptic inhibition of phrenic motor neurons by phrenic afferents. Physiological studies also support that phrenic motor neurons are presynaptically inhibited.…”

Morphometric analysis of the phrenic nerve in male and female Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR)

“…Possible explanations of lateral asymmetry could be: 1) an asymmetrical innervation of the diaphragm, 2) differences in the mechanoreceptor afferents from the pericardium, hepatic parenchyma, hepatic veins, and inferior vena cava in each phrenic nerve, or 3) differences in the post-ganglionic visceral efferents (29). There are suggestions that the right phrenic nerve innervates a larger diaphragm area compared to the left one (30,31), a fact that might result in more myelinated and unmyelinated fibers, including efferents and afferents on the right side nerve.…”