The synaptic organization in the medial geniculate body of afferent fibres ascending from the inferior colliculus

Summary The ventral nucleus of the medial geniculate body has been examined electron microscopically 2–5 days after destruction of the inferior colliculus. In both the ipsi- and contralateral ventral nuclei, degenerating collicular afferents are of medium diameter (1–5 ) and their degenerating terminals are distributed mainly to synaptic aggregations (glomeruli) in which they end axo-axonically and axo-dendritically. Their distribution and mode of termination indicates that these terminals belong to a class which in normal material is large, contains round synaptic vesicles and ends by means of asymmetrical synaptic complexes upon dendrites and upon the second (pale) type of glomerular terminal. It also ends by means of adhesion plaques on the same dendrites.As the terminals of corticothalamic afferents to the nucleus are already known, only the origin of two types remains to be determined: the pale terminals, which arise from structures resembling dendrites and which end only axo-dendritically, and a small, less common terminal which ends axo-axonically, axo-dendritically and axo-somatically. Both types contain flattened synaptic vesicles and end by means of symmetrical synaptic complexes.Correlative Nauta and Golgi studies suggest that the collicular afferents have a very specific spatial distribution within the cellular laminae composing the ventral nucleus.The terminal degeneration commences as a neurofilamentous hyperplasia and quickly passes to one of increased electron density. There is evidence for early removal of degenerating terminals from the postsynaptic membrane.This work was supported by a grant from the Bank of New Zealand Medical Research Fund.We are indebted to Professor J. A. R. Miles for use of the electron microscope.