The fine structure of the inferior colliculus in the cat. II. Synaptic organization.

The synaptic organization of the central nucleus of the inferior colliculus (ICc) of the cat has been investigated by means of electron microscopy. On the basis of the following criteria: the size and the shape of the synaptic vesicles, the distribution and density of the vesicular population, the size and the shape of the synaptic boutons, their origin, and the characteristics of the active synaptic zones, several types of synaptic boutons in the ICc have been discriminated: LR1, LR2, SR, SSB, F1, F2, P, DCV-terminals, and "d"-profiles. The LR1, LR2, SR and SSB bouton types contain clear, round or slightly oval synaptic vesicles and form asymmetrical synapses mainly with middle sized and small dendrites and dendritic spines. LR2-terminals not rarely contact also the neuronal perikarya, whilst the SR-boutons form exclusively axodendritic and axospinous synapses. The P, F1 and F2-boutons contain a pleomorphic vesicular population (P-boutons), with an increased degree of vesicle flattening (F1 and F2-boutons) and form symmetrical axosomatic, axodendritic and axospinous contacts. Especially often the F1-boutons form axosomatic synapses, whilst the F2-terminals end mainly on dendrites. The DCV-boutons contain a mixed population of clear round synaptic vesicles and large dense core vesicles. The DCV-boutons terminate mainly on spines and small distal dendrites by means of asymmetrical synaptic specializations. The "d"-profiles originate from dendrites, and are identical to the thalamic "d"-profiles but are far more rarely observed in the ICc. The "d"-profiles are postsynaptic mainly to the LR-types, and are presynaptic to conventional dendrites, thus participating in synaptic triads. The axonal hillocks and the initial axonal segments of the larger perikarya in the ICc are substantially innervated mainly by LR and P-boutons. Glomerulus-like formations are fairly often, especially around the LR1-terminals, contacting several small postsynaptic targets. True synaptic glomeruli are only rarely observed. Branching myelinated axons are found mainly within the fibrodendritic laminae, whilst unmyelinated collaterals, emitted by myelinated axons are especially often encountered outside the laminae. Various types of myelinated axons form nodal synapses.