Observations on the fine structure of the phrenic nucleus in the cervical spinal cord of the cat with special reference to its dendritic bundles.

The structure of the cat phrenic nucleus in the cervical spinal cord was studied by light and electron microscopy. Neurons of this nucleus are medium sized and fusiform and are arranged longitudinally. The most unique feature of this nucleus is the extremely well organised longitudinal dendritic bundles. In the dendritic bundles there are dendrodendritic casual appositions without specialised structures and with desmosome-like contacts. Gap junctions, however, are not found at the site dendrodendritic contact. Neurons of the phrenic nucleus possess many axodendritic synapses whereas axosomatic synapses are infrequent. In most axon terminals there are round, clear, synaptic vesicles, but on rare occasions terminals containing either cored or flat vesicles are observed. The significance of these features of the nucleus, especially of the large dendritic bundles, is briefly discussed.