Descending inputs to the cat dorsal cochlear nucleus: an electron microscopic study

This experimental study provides identification of several types of terminals in the fusiform cell layer (FCL) of the cat dorsal cochlear nucleus (DCN). Eight types of synaptic terminal were defined in control animals. 22 experimental cats were then subjected to unilateral lesions of the superior olivary complex or SOC (Groups I and II), the inferior colliculus or IC (Groups III and IV), or the IC plus the dorsal nucleus of the lateral lemniscus or DNLL (Groups V and VI). After one to 14 days, animals were killed by perfusion-fixation and brain stems prepared for light and electron microscopic study of degeneration. After SOC lesions, the ipsilateral FCL showed degeneration of small (type 2a) endings on small dendritic shafts; the contralateral FCL showed degeneration of larger (type 5) endings mostly on fusiform cell somata and on some primary dendrites. Both sides showed mild degeneration of large, glomerular (type 1a) endings. After small IC lesions, the ipsilateral FCL demonstrated degenerating type 1a glomerular endings but the contralateral FCL showed severe degeneration of type 5a terminals (axodendritic to fusiform cells) and mild degeneration of both glomerular (type 1a) and axosomatic (type 5) endings. Combined IC-DNLL lesions caused extensive degeneration in a pattern similar to that following IC lesions. In addition retrograde degeneration of fusiform cells occurred bilaterally. These studies showed that: (1) type 2a endings originate in the ipsilateral SOC; (2) type 5 endings on fusiform cell bodies originate from the contralateral SOC; (3) type 5a endings on fusiform cell dendrites originate from the contralateral IC; and (4) type 1a glomerular endings probably arise from several sources, including the IC, the SOC and (possibly) the DNLL of both sides.