The Synaptology of the Granule Cells of the Olfactory Bulb

The synapses related to the granule cells of the olfactory bulb of rat brain have been studied in aldehyde-fixed material. The synapses can be divided into three classes: (1) those which have asymmetrical synaptic membrane thickenings and spheroidal synaptic vesicles; (2) those with symmetrical synaptic thickenings and flattened vesicles; and (3) the reciprocal synapses, one half of which (from mitral to granule cell) has an asymmetrical synaptic thickening associated with spheroidal vesicles, while the other half (from granule to mitral cell) has a symmetrical synaptic thickening and flattened vesicles. Qualitative observations, supported by preliminary quantitative measurements, suggest that it may be possible to divide both the spheroidal and flattened-vesicle types into two further varieties, on the basis of size, The smaller variety of spheroidal vesicles is found in most axon terminals, while the larger spheroidal vesicles are present in mitral cell dendrites and in some of the axon terminals. The flattened vesicles associated with symmetrical synapses which are oriented on to the granule cells are smaller than the spheroidal vesicles, but the flattened vesicles in the spines and gemmules of the granule cells are the same size or larger than the spheroidal vesicles. The division of flattened vesicles into two sizes is supported by statistical analysis of measurements of these vesicles, but because of difficulty in identifying the axon terminals with asymmetrical synapses there is no quantitative evidence for such a division of spheroidal vesicles. The asymmetrical synapses are found predominantly on spines, gemmules, and dendritic varicosities, although they are occasionally present on shafts of dendrites and on the cell somata. The symmetrical synapses are almost completely restricted to the shafts of the peripheral processes and the deep dendrites, and to the cell somata; only very rarely are synapses of this type found on spines, and then always in conjunction with an asymmetrical synapse.

Electron microscopic study of the somatic sensory cortex of the cat. II. The fine structure of layers I and II

Layers I and II of the somatic sensory cortex are clearly distinguishable with the electron microscope because of characteristic differences in the number, type and orientation of neurons and dendritic and axonal ramifications. Layer I may be subdivided into: (i) a subpial astrocytic layer immediately deep to the basement membrane of the cerebral surface; (ii) a superficial quarter consisting of bundles of small myelinated axons and large numbers of small axon terminals which contain spherical vesicles and end in asymmetrical synaptic complexes mainly on large dendritic spines. Most of these terminals are derived from a dense feltwork of fine unmyelinated axons which are especially concentrated at the junction of the superficial and deep parts of layer I; (iii) a deeper three quarters with similar features to the above but with the additional characteristic of many obliquely orientated large dendrites which are the diverging branches of apical dendrites ascending from deeper layers. Small pyramidal neurons dominate layer II, but among them are a small number of non-pyramidal neurons whose beaded dendrites are covered with axon terminals. Large apical dendrites traverse this layer, and in addition to the typical asymmetrical synapse on dendritic spines, a few symmetrical types appear. These are derived from thin unmyelinated axons orientated horizontally within the layer, and the terminals contain many small flattened or pleomorphic synaptic vesicles.