Electron microscope study of Golgi-stained cells in lamina II of the rat spinal dorsal horn.

Seven Golgi-stained cells in lamina II of the rat spinal dorsal horn were examined by electron microscopy. One of the cells studied was an islet cell, two were stalked cells, and the remaining four cells could not be classed into either group. The islet cell and three of the unclassified cells possessed presynaptic dendrites and formed synapses onto a variety of dendritic spines and shafts within lamina II. The axons of the islet cell and of one of the unclassified cells formed symmetric axodendritic synapses mainly onto dendritic shafts. The two stalked cells and the remaining unclassified cell did not possess vesicle-containing dendrites. This last cell bore some resemblance to a stalked cell and may have represented an atypical example of one. Most of the synapses involving the cells took place outside synaptic glomeruli, but all seven cells were postsynaptic to central axons within glomeruli and in most cases to both type I and type II central axons, suggesting a monosynaptic input from myelinated and unmyelinated primary afferent axons. In addition, most of the cells were postsynaptic to vesicle-containing dendrites. It is concluded that certain cells, which do not belong to the stalked or islet classes, possess presynaptic dendrites and function as presumed inhibitory interneurones within lamina II. The target of the cells with presynaptic dendrites includes other cells within lamina II and may also include cells in deeper laminae of the dorsal horn. Further evidence will be needed in order to determine whether all cells in lamina II that do not possess presynaptic dendrites form a single functional class.