Descending inputs to the caudal cochlear nucleus of the cat: degeneration and autoradiographic studies.

Abstract

AbstractDescending paths from the inferior colliculus (IC) and dorsal (DNLL) and ventral (VNLL) nuclei of the lateral lemniscus and their terminations in the caudal cochlear nucleus in cats were studied both in silver degeneration material following central lesions and in autoradiographs following central injections of tritiated leucine (3H‐leu). Eight adult cats received electrolytic lesions of the IC, the IC and DNLL, or the IC, DNLL and VNLL. Control cats were subjected either to surgery alone or to a lesion in a non‐auditory nucleus. After survival periods of 1 to 16 days, all cats were killed and their brain stems prepared by modified Nauta methods. Twenty‐two other cats received injections of 3H‐leu into the dorsal IC, the ventral IC and DNLL, or the DNLL and VNLL. Their brains were prepared for LM autoradiography. After lesions of the IC, Nauta material contained moderate degeneration of fine fibers that coursed predominantly in the lateral lemniscus, and partially decussated in the caudal trapezoid body to enter the cochlear nuclei of both sides. Sparse, mainly peridendritic degeneration of fine fibers occurred in the PVCN, while moderate, peridendritic and perisomatic degeneration occurred in the deep and innermost fusiform cell layers of the dorsal cochlear nucleus (DCN). Lesions of both the ventral IC and the DNLL showed additional bilateral degeneration of medium‐caliber axons in the descending tracts as well as preterminal degeneration of these larger axons sparsely distributed in the deep layers of the DCN. The density of medium‐caliber axonal degeneration (particularly in the deep DCN) increased when lesions also included the VNLL. After 3H‐leu injections into the tectum and/or upper pons, autoradiographs confirmed the presence of sparse bilateral inputs from both the IC and from the DNLL and VNLL to ventral cochlear nucleus neurons, including some octopus cell dendrites. The autoradiographic studies also clearly revealed a predominant input to the outer layer of the DCN from the dorsal IC, a predominant input to the middle layer from the ventral IC, and a predominant input to the deeper DCN from the lateral lemniscal nuclei. Limited evidence of a topographic, perhaps tonotopic, order within the descending connections was best seen in Nauta material. The differences shown here in fiber types, trajectories, and target sites from the IC, the DNLL, and the VNLL are discussed in terms of possible functional differences among the descending influences on neurons of the caudal cochlear nucleus.