The fine structure of the inferior colliculus in the cat. I. Neuronal perikarya in the central nucleus.

The fine structure of the neurons in the ventrolateral and the dorsomedial parts of the central nucleus of the inferior colliculus has been studied. 318 neurons, followed in serial ultrathin sections were examined. On the basis of the cell size, four major types of neuronal perikarya were identified: large (greater than 22 microns), medium-large (18-22 microns), medium-small (12-18 microns), and small (less than 12 microns). According to the shape of the perikarya, the ultrastructural features of their organelles, the occurrence and number of axosomatic synaptic contacts, the main neuronal types were subdivided in more discrete varieties. The large neurons have three varieties (oval, irregular, and fusiform). The nucleocytoplasmic ratio of these cells is usually above 1:1.8. They possess voluminous cytoplasm, that is rich in organelles. The granular endoplasmic reticulum is organized in NISSL bodies. Extremely numerous axosomatic synapses are present. These neurons represent the large stellate and disc-shaped projection (efferent) neurons, described in GOLGI studies. The medium-large neurons have two varieties. The nucleocytoplasmic ratio is 1:1.6-1:1.3. They possess a lesser amount of cytoplasm. Only the first variety possess discrete NISSL bodies. These cells are moderately afferented. The medium-small neurons display distinct differences in the number and pattern of cell organelles, and might be splitted in four varieties. The nucleocytoplasmic ratio is 1:1.4-1:1.1. Some of the medium-small neurons (especially--the first and the second variety) are probably projection cells, whilst others (especially--the third variety) might be interneurons. The small neurons display relatively large nucleus, small amount of cytoplasm (nucleocytoplasmic ratio approx. 1:1.2), general paucity of organelles, and extremely few axosomatic synapses. The both varieties of the small neurons, in all probability, represent local circuit neurons. From the total number of 318 neurons, 250 cells were followed in long rows of serial sections, that allowed distinct discrimination. From the latter, 18.4% were classified as large, 27.2%-medium-large, 44.4%-medium-small, and 10.0%-small neurons. The remaining 68 perikarya do not offer reliable electron microscopic criteria that could allow an unequivocal identification. The description of the cell somata in the central nucleus of the inferior colliculus, and its synaptic organization (to be published) will provide a basis for future studies on the connectivity, synaptic events, and neurotransmitter interactions in the relay nuclei of the auditory system.