An electron microscopic study of the mode of termination of cortico-thalamic fibres within the sensory relay nuclei of the thalamus

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Abstract
The existence of corticofugal fibres passing from the main sensory areas of the cerebral cortex to the thalamic relay nuclei has been confirmed by an electron microscopic study of the ventroposterior, and medial and lateral geniculate nuclei 3 and 4 days after lesions in the appropriate cortical area. The degenerating cortico-thalamic fibres belong to the smallest group of myelinated axons found in the nuclei. Four days after a cortical lesion many degenerating axon terminals are found in all three nuclei, often in small clusters. These usually terminate outside the synaptic glomeruli of the nucleus on stem dendrites and on the long parent profiles of the pale axon terminals of the glomeruli. In many cases, these dendrites and long parent processes enter a glomerulus close to or at some distance from the degenerating synapse. In a few cases, a degenerating terminal ends in closer relation to a glomerulus, usually on one of the more peripherally placed pale axon terminals. These findings indicate that the terminals of cortico-thalamic fibres belong to the small dark group of axon terminals described in the normal nuclei, but the persistence of many small dark terminals after total hemidecortication suggests that others have a subcortical origin. Four days after a cortical lesion, degeneration of the cortico-thalamic terminals is at a very advanced stage, whereas at a survival period of three days, only a few terminals show unequivocal degeneration. The degenerative change thus appears to be sudden and rapid.