The cytology of neurons and their dendrites in the simple mammalian nucleus lateralis an electron microscope study

Summary

Profiles of large neurons in the lateral nucleus range from 16 μm to 35 μm in diameter with dimpled nuclei, large Nissl bodies, and well developed Golgi apparatus. Two types of perikarya are distinguished, those that are smooth and those with irregular somatic and dendritic protuberances. About 86% of all large neuronal somata are covered with axosomatic synapses, predominantly with terminals of Purkinje axons and a few belonging to axons of the small neurons. The remaining 14% have no axosomatic synapses. The thick, fleshy dendrites of these cells are covered with terminals, the majority of which synapse directly upon the dendritic shaft. A few are present on spines. The initial segment of the large neuron is thick and robust and receives synapses upon its shaft or upon a spinous projection. The small neurons measure less than 12 μm in diameter and have very lobulated nuclei in a sparse cytoplasm characterized by small Nissl bodies and a poorly elaborated Golgi apparatus. About 52% of all small neuronal somata bear no synapses whereas the remaining 48% are covered with axosomatic synapses, mainly from the axons of Purkinje cells and a few axons of other small cells. The slender long dendrites of both large and small cells bear synapses with six classes of axons in the neuropil. Synaptic protuberances of two varieties occur on the surfaces of both perikarya and dendrites, (a) dome-shaped ones capped with a pronounced asymmetrical synaptic junction and (b) ones with thin long necks and bulbous heads having synapses on both parts. Frond-like dendritic excrescences are borne on the processes of some small and large neurons and they are postsynaptic to many axon terminals clustered around them.