An electron-microscopic study of the termination of the afferent fibres to the olfactory bulb from the cerebral hemisphere.

An experimental investigation has been made of the site and mode of termination of the 3 groups of afferent fibres to the olfactory bulb which come from more caudal parts of the cerebral hemisphere. Lesions have been placed in the relevant parts of the brain of the rat and the resulting degeneration of axon terminals in the olfactory bulb studied with the electron microscope. All 3 groups of these extrinsic afferent fibres end in asymmetrical synapses upon the granule cells, and they have a differential termination upon its various processes. The possibility that these fibres also end upon other cells in the bulb (particularly the short-axon and penglomerular cells) cannot be excluded. The centrifugal fibres end upon gemmules in the deep half of the external plexiform layer only; no degenerating terminals were found in relation to the glomeruh although degenerating centrifugal axons are present here. The fibres of the anterior commissure terminate upon spines and vancosities of the deep dendntes and upon somatic spines of the granule cells. After lesions of the anterior olfactory nucleus, degenerating terminals were found in the ipsilateral olfactory bulb, which could not be ascribed to the centrifugal fibres or to the fibres of the anterior commissure, as they ended upon the spines of peripheral processes in the granule cell layer, and upon gemmules in the superficial as well as in the deep half of the external plexiform layer. It is proposed that these terminals are those of the axon collaterals from the ipsilateral anterior olfactory nucleus. The axons which form symmetrical synapses, and many which form asymmetrical synapses, do not degenerate even after a lesion immediately behind the olfactory bulb, and are therefore intrinsic to the bulb. It is suggested that the axons which are associated with symmetrical synapses are those of the short-axon cells, and the asymmetrical synapses are formed by the axon collaterals of the mitral and tufted cells.