White-matter dendrites in the upper cervical spinal cord of the adult cat: a light and electron microscopic study

The organization and structure of dendrites penetrating into the white matter of upper cervical spinal segments have been examined by means of Golgi staining techniques, intracellular horseradish peroxidase (HRP) injections, and ultrastructural studies. The Golgi studies established that several groups of neurons located in intermediate and ventral laminae of the upper cervical spinal cord have a substantial part of their dendritic tree extending into adjacent ventral and lateral funiculi. Most dendrites in white matter showed irregular varicosities along their length. They were devoid of spines and followed relatively direct paths. In contrast, grey matter dendrites were occasionally observed with spines and complex appendages frequently followed tortuous paths. The size and location of some Golgi stained neurons suggested that white matter dendrites might originate from neck muscle motoneurons. This possibility was confirmed using intracellular HRP injections. These studies also showed that the distribution of white matter dendrites of neck muscle motoneurons depended on the location of the motoneuron soma. White matter dendrites of neck muscle motoneurons located deep in the ventral horn projected into all regions of white matter surrounding the ventral horn. Other neck muscle motoneurons, located in the spinal accessory nucleus, had white matter dendrites largely confined to the lateral funiculus. White matter dendrites of motoneurons in the commissural nucleus were found to project across the ventral commissure into the contralateral spinal cord. Light microscopic studies of semi-thin sections stained with toluidine blue and electron microscopic studies of thin sections revealed that white matter dendrites were confined to special regions of the white matter. These regions resembled the grey matter neuropil and contained dendrites and unmyelinated and small diameter myelinated axons. Axon terminals were also found in white matter. These terminals contained either flattened or spherical vesicles and formed synaptic contacts on white matter dendrites. White matter dendrites, by virtue of their frequency of occurrence, distribution, and type of synaptic contacts may represent a means by which descending or ascending spinal systems can influence spinal neurons without recourse to axon collaterals which terminate in grey matter.