Terminations of olfactory afferents on layer II and III neurons in the entorhinal area: degeneration-Golgi-electron microscopic study in the rat.

The degeneration-Golgi-electron microscopy technique was used to study the termination of olfactory afferents in the entorhinal area (EA) in the rat. Following resection of the olfactory bulb, degenerating axon terminals can be observed in a superficial lamina of layer I of both medial (MEA) and lateral (LEA) part of EA. Twenty-one Golgi-impregnated cells of 7 neuronal types located in LEA were investigated with the electron microscope. Of these cells, two layer II spinous multipolar neurons and one layer III spinous pyramidal cell, all located in the ventral part of LEA (VLEA), and one layer II sparsely spined multipolar cell, located in the dorsal part of LEA (DLEA), were observed to form synaptic contact with degenerating axon terminals. Based on these findings the possibility of a 3-member neuronal chain is discussed: mitral cell of the olfactory bulb - layer II or III neuron of EA-CA3 cell or granule cell in the hippocampus.