The pattern and mode of termination of afferents to the red nucleus of the rat were investigated with the electron microscope. Lesions were placed by electrocautery in the sensorimotor cortex or were placed electrolytically in the deep cerebellar nuclei and brachium conjunctivum using a stereotaxic approach. With both types of lesion, degenerating fibres of passage, preterminal axons, and synaptic terminals were observed in greatest numbers on the third post-operative day. Following cerebellar lesions, degenerating terminals occurred on the cell bodies and proximal dendrites of large, multipolar neurons in the magnocellular portion of the red nucleus, and on intermediate and small dendrites in the parvocellular portion. It is concluded that the former are interpositus terminals while the latter are dentate (lateralis) terminals ending on rubrospinal and rubrobulbar neurons respectively. Following lesions of the sensorimotor cortex, small degenerating terminals were observed on the distal dendrites and dendritic spines of parvocellular, rubrobulbar neurons. Large terminals containing round vesicles did not undergo degeneration following either type of lesion. These findings suggest the existence of an interpositorubro-spinal pathway in which the interpositus terminals exert a strong influence on the large, caudally placed rubrospinal neurons. The background excitability of the rostrally located rubrobulbar neurons is probably regulated by the distal cortical input while the more proximally located dentate terminals probably exert a stronger discrete influence over their activity.
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