Evidence suggesting that both the corticopontine and cerebellopontine systems are each composed of two separate neuronal populations: An electron microscopic and horseradish peroxidase study in the rat
Authors: Mihailoff Gregory A. , Watt C. B. , Burne R. A. 
Published: January 2005

Evidence from horseradish peroxidase (HRP) and electron microscopic (EM) studies was interpreted to suggest that both the corticopontine and cerebellopontine systems each arise from two 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, '77) have shown 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 also to the spinal cord. In these same cases, HRP‐positive neurons were also observed in visual cortical areas where they were again restricted to layer Vb, which has been shown to be the source of cells projecting to the superior colliculus and pulvinar (Gilbert and Kelley, '75; Lund et al., 75). These findings suggest that the corticopontine projection 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. In support of this notion, 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 conjuctivum, many large filamentous degenerating boutons were observed in contact with a characteristic cluster of dendritic spines as well as somata and dendritic shafts. In addition, a lesser number of small boutons undergoing the typical dark type of degeneration were found in apposition to singular, small dendritic shafts. Thus, the combination of HRP and EM observations has been interpreted to suggest that the cerebellopontine system, like the corticopontine system, consists of two neuronal elements: (1) larger multipolar neurons giving rise to the large boutons which form the central element in a characteristic synaptic complex and undergo filamentous degeneration, and (2) smaller neurons whose axons terminate as the small boutons which contact only small dendritic profiles and do not participate in the synaptic complexes.