Quantitative analyses of axonal endings in the central nucleus of the inferior colliculus and distribution of 3H‐labeling after injections in the dorsal cochlear nucleus
Authors: Oliver Douglas L. 
Published: January 2005

Quantitative analyses of electron microscopic (EM) autoradiographs were used to identify the afferents from the dorsal cochlear nucleus in the central nucleus of the inferior colliculus (IC) in the cat. In order to localize the sources of radioactivity, material from axonal transport experiments was analyzed by means of a hypothetical grain procedure which takes the cross‐scatter of beta particles into account. Measurements of the synaptic vesicles in axonal endings and a cluster analysis were used to identify different groups of endings. In order to determine which types of endings arise in the dorsal cochlear nucleus, axonal endings labeled after axonal transport and unlabeled endings were characterized and compared to the groups defined by the cluster analysis.

Axonal endings with round synaptic vesicles were labeled with more than 2 grains/μm2 which was about 30% of the radioactivity in the central nucleus of the IC. This was six to seven times greater than if the radioactivity had been randomly distributed. Other tissue compartments usually had less radioactivity. Some myelinated and unmyelinated axons were labeled, but, as a group they had lower amounts of radioactivity than predicted by random labeling. In most cases, only low levels of activity were found in glial and postsynaptic structures.

Five groups of axonal endings in the medial part of the central nucleus were identified by an analysis which clustered similar types of endings. The variance of the longest axis, the mean diameter, the variance of area, and the mean area of the synaptic vesicles were the variables most useful in distinguishing these five groups, Axonal endings with round synaptic vesicles were classified as either small, or large, or very large, while endings with pleomorphic vesicles were either large or small. Using measurements of the cross‐sectional diameter of dendritic microtubules, samples of digitized axonal endings from normal and experimental cases were normalized and could be compared directly to the groups defined by the cluster analysis. Microtubules were 21.7 nm (±1.6) in average diameter. After injections of 3H‐leucine and/or proline in the dorsal cochlear nucleus, most of the labeled endings in the IC contained small, round vesicles (less than 47 nm in diameter) although a very small number of endings with large, round vesicles also were labeled. The numerous fusiform cells of the dorsal cochlear nucleus which project to the IC are the likely source of the labeled endings with small, round synaptic vesicles. Small numbers of labeled endings with large, round synaptic vesicles may originate from giant cells in the dorsal cochlear nucleus. Unlabeled endings may arise from other brain stem neurons or from intrinsic cells.

The quantitative analysis of axonal endings provides a standard set of criteria for their identification in different experiments. In combination with experimental labeling methods, it may permit synaptic vesicle morphology to be associated with a specific neuron type. This information is a prerequi‐ site for defining the neural basis of information processing in the IC.