Morphology and fine structure of the feline neonatal medullary raphe nuclei.

A light and electron microscopic study of the caudal medullary raphe nuclei of neonatal kittens reveals that these nuclei are composed of three size classes of neurons with several possible subclasses. Internuclearly projecting dendritic arborizations in the transverse plane and intranuclear projections in the sagittal plane are common features of large and medium size class neurons of raphe nuclei magnus and obscurus though not for the cells of nucleus raphe pallidus. A positive correlation exists between neuron size and density of axosomatic and axodendritic synapses, which suggests that the large class neurons are the first to receive input in synaptogenesis, which is occurring at this time. A wide variety of synaptic forms and integration is also a characteristic feature within these nuclei, though it is not clear whether this morphological variance represents a phylogenic and/or ontogenic trend or just an expression of the multifunctional nature of this region.