Observations on the fine structure of the central nucleus of the inferior colliculus of the cat.

Abstract

AbstractThe central nucleus of the inferior colliculus has been examined electron microscopically in the adult cat, particular attention being paid to its ventrolateral division in which the majority of lateral lemniscal afferents end.Two cell types can be recognized: one, which is the equivalent of the spiny principal cell of light microscopy, is distinguished by having its soma and large proximal dendrites covered by axon terminals. More distally situated dendrites have fewer terminals ending mainly on dendritic spines. These spines resemble those of the cerebral cortex and have a modified spine apparatus. The second type of cell is the equivalent of the small multipolar cell of light microscopy. It has spineless dendrites and these and the soma receive relatively fewer axon terminals. It has a very thin axon.Several types of axon terminal can be recognized. Type I terminals are large, contain spherical synaptic vesicles and end by means of asymmetrical synaptic contacts, on the somata and proximal dendrites of principal cells and on the smaller dendrites of multipolar cells. The extracellular space on one or both sides of the synaptic complex is often characteristically widened. Correlative experimental degeneration studies show that these are the terminals of lateral lemniscal fibers. Various forms of smaller terminal also make asymmetrical contacts and have spherical vesicles. One type has a high concentration of large, densecored vesicles as well.Two types of axon terminal end in symmetrical synaptic contacts. One contains large, homogeneously flattened synaptic vesicles; the other, which is more common, contains smaller, irregularly flattened or pleomorphic synaptic vesicles.In the dorsomedial division of the central nucleus, the terminals of corticofugal fiber have spherical synaptic vesicles and end in asymmetrical synaptic contacts upon large dendritic spines.