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Evidence suggesting that both the cortico pontine and cerebello pontine systems are each composed of 2 separate neuronal populations an electron microscopic and horseradish peroxidase study in the rat


, : Evidence suggesting that both the cortico pontine and cerebello pontine systems are each composed of 2 separate neuronal populations an electron microscopic and horseradish peroxidase study in the rat. Journal of Comparative Neurology 195(2): 221-242

Evidence from horseradish peroxidase (HRP) and EM studies suggests that both the corticopontine and cerebellopontine systems each arise from 2 separate populations of neurons within their respective sites of origin. Following injection of HRP into the pontine nuclei using a ventral approach, labeled neurons in sensorimotor cortex included the large pyramidal cells deep in layer Vb and small-to-intermediate-sized neurons located more superficially in Vb. Previous studies (Wise and Jones) showed that these same large pyramidal neurons give rise to fibers of the corticospinal system while smaller more superficial cells project either to various brainstem locations or to the spinal cord. HRP-positive neurons were observed in visual cortical areas where they were again restricted to layer Vb, which is the source of cells projecting to the superior colliculus and pulvinar (Gilbert and Kelley; Lund, et al.). The corticopontine projection apparently consists of collateral terminations of the corticospinal system as well as branches of a corticofugal system which arises from neurons in sensorimotor and visual cortices and projects primarily to certain brainstem locations. EM studies revealed that following sensorimotor or visual cortex lesions both filamentous and dark degenerating boutons were evident in the pons. The small dark boutons terminated distally on pontine neurons and predominated after sensorimotor lesions while the larger filamentous endings contacted more proximal parts of pontine neurons and predominated after visual cortex lesions. Similar results were obtained in studies of the cerebellopontine system. Basilar pontine HRP injections resulted in the labeling of cells in each of the cerebellar nuclei, the majority of which were classified as medium-to-large multipolar neurons while a lesser number appeared to be smaller fusiform or spindle-shaped cells. After interruption of cerebellopontine axons in the brachium conjunctivum, many large filamentous degenerating boutons were observed in contact with a characteristic cluster of dendritic spines as well as somata and dendritic shafts. A lesser number of small boutons undergoing the typical dark type of degeneration were in opposition to singular, small dendritic shafts. The combination of HRP and EM observations suggests that the cerebellopontine system, like the corticopontine system, consists of 2 neuronal elements: larger multipolar neurons giving rise to the large boutons which form the central element in a characteristic synaptic complex and undergo filamentous degeneration, and smaller neurons whose axons terminate as the small boutons which contact only small dendritic profiles and do not participate in the synaptic complexes.

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