Experimental Studies on the Axons Intrinsic to the Glomerular Layer of The Olfactory Bulb

Lesions have been made in the superficial layers of the dorsal surface of the rat olfactory bulb; the resulting axonal degeneration has been studied with light and electron microscopy, in order to determine the mode and extent of interconnexion of the intrinsic interneurons of the glomerular layer. Although the lesions necessarily involve extrinsic pathways, the axonal distribution of each of the intrinsic neurons may be deduced from this evidence and a knowledge of normal and experimental material. Tufted cell collaterals extend for 6-12 glomeruli, periglomerular cell axons for 4-5 glomeruli and superficial short-axon cell axons for 2-3 glomeruli; these figures correspond well with observations of Golgi-impregnated material. The experimental approach to the study of intrinsic connexions and its applicability to other sites is considered, and the results are discussed from the point of view of neural coding and odour specificity.

The Neuropil of the Periglomerular Region of the Olfactory Bulb

The periglomerular region of the olfactory bulb, apart from containing the somata and stem dendrites of the cells contributing to the glomeruli, is the sole region of distribution of the periglomerular cell thin dendrites and the short-axon cell dendrites. It is also the major site of termination of all axons to the glomerular layer except the olfactory axons - i.e. tufted cell collaterals, periglomerular cell and short-axon cell axons and centrifugal fibres. Its characteristic neuropil has been studied with the electron microscope to define the cells of origin of the types of neuronal process and their synaptic relationships. Three types of axon terminals have been found: those with spherical, large flattened and small flattened vesicles, which are deduced to derive from tufted cell collaterals or centrifugal fibres, periglomerular cell and short axon-cell axons respectively; those with spherical vesicles are consistently associated with asymmetrical membrane thickenings and those with either type of flattened vesicles with symmetrical thickenings. The thin periglomerular cell dendrites are very irregular and often have a somewhat dense cytoplasm, rich in ribosomes; they may become extended into very attenuated glia-like sheets that surround the mitral or tufted cell stem dendrites, from which they may receive synaptic contacts. Such dendrites also receive some synapses from all 3 types of axon in the periglomerular region. The short-axon cell dendrites are thick and varicose and show no sign of synaptic specialization oriented from them; they have few spines but receive many asymmetrical-type synapses on their shafts. Both axon terminal types synapsing with symmetrical thickenings are also found on the shafts. The evidence obtained from the study of normal material is summarized and the various cellular roles considered. In the light of observations on the olfactory bulb, it is suggested that dendrites may be divided into 2 major classes: those that only receive synapses (Class A) and those that make synaptic contacts as well as receiving them (Class B). Further comparisons with the deep layers of the bulb and physiological implications are discussed.

The Neuropil of the Glomeruli of the Olfactory Bulb

The neuropil of the glomeruli of the rat olfactory bulb has been studied with the electron microscope with a view to elucidating the type of processes involved - dendrites, appendages and axons - their cellular identity, and the synaptic relationships they establish. The problems encountered in defining these are considered and criteria based on the previous study of neuron types and on examination of serial sections are put forward. The glomeruli are large structures containing many thousands of processes and are the sole site of termination of the olfactory receptor axons. The terminals of the latter are characteristically electron-dense, allowing identification in normal material; they run through the glomeruli making many synapses by means of spherical vesicles and asymmetrical thickenings on to all types of dendritic profile. The glomerular dendritic arborizations of mitral and tufted cells, which are indistinguishable from each other, start as large, fairly regular, pale profiles but become increasingly varicose as they branch and diminish in size. They regularly show groups of spherical vesicles, often in association with asymmetrical synaptic thickenings directed from the dendrite; these are typically associated with return, reciprocal, synapses of the symmetrical type from profiles containing large flattened vesicles. These latter profiles are those of the dendrites and gemmules of periglomerular cells; the dendrites are of irregular outline and give rise to many appendages, mostly gemmules making synaptic contact with mitral or tufted cell dendrites. A small number of pale axon terminals containing either small or large flattened vesicles, derived from short-axon and periglomerular cells respectively, synapse with symmetrical thickenings on to the periglomerular cell dendritic processes. Close associations of particular types of axo-dendritic and dendro-dendritic synapses on interconnecting processes, termed synaptic patterns, are described and their significance considered. The nature of the glomerular interactions is discussed and then placed in the context of other, smaller glomeruli in the central nervous system; certain common principles of glomeruli are suggested.