Dendritic and somatic appendages of identified rubrospinal neurons of the cat

Giant neurons of the red nucleus of the cat were stained intracellularly with horseradish peroxidase and examined using light microscopy, electron microscopy of thin sections, and high voltage electron microscopy of thick sections (2-5 microns). Special attention was paid to the arrangement of dendritic spines and other appendages relative to the distribution of synaptic contacts from known sources. In the region of the neuron known to receive synaptic contacts from the nucleus interpositus of the cerebellum (soma and proximal 200-300 microns of dendrites), the dendrites were relatively unbranched, and free of long spines or complex appendages. The surface of the neurons in this region was covered with a dense layer of short thin appendages that invaginated or penetrated between the synaptic terminals that cover this part of the cells. The small spines received synapses of the types associated both with the cerebellar afferent fibers and with the local inhibitory interneurons. These same terminals made synaptic contacts directly onto the surface of the neurons and onto the lateral surfaces of the spines, suggesting that the spines may serve primarily to increase the available synaptic surface area. The more distal portion of the dendritic field, where cerebellar afferents do not make synaptic contacts, exhibited a dramatically different appearance. The dendrites were much more branched, and exhibited many and varied dendritic appendages. The appendages were of three general types. One was a large protrusion with a cup-shaped head that formed the principal postsynaptic component of a glomerular arrangement also involving an axon terminal and usually a presynaptic dendrite. A second was a long thin filiform process that usually occurred around the glomeruli. This appendage was occasionally postsynaptic. The third was a spherical appendage containing many lysosomal organelles resembling residual bodies. The glomerular dendritic protrusions were very common in the distal portion of the dendritic field, numbering at least 1000 per cell. At least some of the glomeruli are specialized for receipt of synaptic input from the corticorubral pathway, since lesions of sensorimotor cortex resulted in degeneration of the central synaptic terminal in some glomeruli on horseradish peroxidase-injected rubrospinal neurons. These specializations of dendritic structure may contribute to the differences in excitatory postsynaptic potential wave shape between cortical and cerebellar inputs, and they may play a role in the changes in the cortical excitatory postsynaptic potential that develop after lesions of cerebellar inputs.(ABSTRACT TRUNCATED AT 250 WORDS)