Lemniscal Nuclei Research Articles

Projections of nucleus angularis and nucleus laminaris to the lateral lemniscal nuclear complex of the barn owl

Interaural phase and intensity are cues by which the barn owl determines, respectively, the azimuth and elevation of a sound source. Physiological studies indicate that phase and intensity are processed independently in the auditory brainstem of the barn owl. The phases of spectral components of a sound are encoded in nucleus magnocellularis (NM), one of the two cochlear nuclei. NM projects solely and bilaterally to nucleus laminaris (NL), wherein interaural phase difference is computed. The other cochlear nucleus, nucleus angularis (NA), encodes the amplitudes of spectral components of sounds. We report here the projections of NA and NL to the lateral lemniscal nuclei of the barn owl. The lateral lemniscal complex comprises nucleus olivaris superior (SO); nucleus lemnisci lateralis, pars ventralis (LLv); and nucleus ventralis lemnisci lateralis (VLV). At caudal levels, VLV may be divided into a posterior (VLVp) and an anterior (VLVa) subdivision on cytoarchitectonic grounds. At rostral levels, the cytoarchitectural differences diminish and the boundaries between the two subdivisions become obscured. Likewise, our data from anterograde tracing studies suggest that at caudal levels the terminal fields of NA and NL remain confined to VLVp and VLVa, respectively. They merge, however, at rostral levels. The data also suggest that NL projects to the medial portion of the ipsilateral SO and that NA projects bilaterally to all parts of SO and LLv. Studies with the retrograde transport of horseradish peroxidase confirm these projections.

Nuclei of the lateral lemniscus project directly to the thalamic auditory nuclei in the pigeon

Non-mesencephalic origins of ascending afferents to the thalamic auditory nuclei, nuclei ovoidalis (Ov) and semilunaris parovoidalis (SPO), were identified in the pigeon in retrograde tracing experiments. These origins comprise dorsal and ventral lateral lemniscal nuclei. Injections of anterograde tracers into these nuclei produced terminal labelling in SPO in particular. These experiments show for the first time that the inferior colliculus is not an obligatory relay to the auditory thalamus.

The human auditory brain stem as a generator of auditory evoked potentials

Data on the size, location, and orientation of human brain stem auditory nuclei are discussed here from the point of view of the potential role of these structures in generation of the brain stem auditory evoked response. Due to reduction in size of several nuclei in the human brain stem, the structures most likely to be generators of far-field potentials are the cochlear nuclei, medial olivary nuclei, dorsal lemniscal nuclei, inferior colliculi, and their axonal projections. Consideration of the anatomy of the human auditory brain stem pathway forms the basis for a model of generation of auditory evoked potentials.

Origin of ascending auditory projections to the nucleus mesencephalicus lateralis pars dorsalis in the chicken

Ascending auditory projections to the nucleus mesencephalicus lateralis pars dorsalis (MLd) were studied in white Leghorn chickens by means of unilateral injections of horseradish peroxidase into the MLd and by injections of tritiated leucine into nucleus angularis or the combined nucleus magnocellularis and nucleus laminaris. The experiments showed that nucleus angularis sends an extensive projection to the contralateral MLd and a smaller projection to the rostral pole of the ipsilateral MLd; the lagenar region contributes to these bilateral connections. Nucleus angularis also projects bilaterally to the superior olive and nucleus ventralis lemnisci lateralis and to the contralateral nucleus lemnisci lateralis pars ventralis and dorsal nucleus of the lateral lemniscus. Projections from nucleus laminaris were demonstrated to the ipsilateral superior olive, to the contralateral lemniscal nuclei and a small medial region in MLd bilaterally; the contralateral projection is much denser than the ipsilateral one. Other nuclei having ascending connections with MLd include the contralateral superior olive, the ipsilateral nucleus lemnisci lateralis pars ventralis, the contralateral nucleus ventralis lemnisci lateralis and the contralateral MLd. The ipsilateral superior olive and nucleus ventralis lemnisci lateralis also project to MLd but much more sparsely than in their contralateral projection. Although several of these findings correspond with auditory connections previously shown in the pigeon brainstem, they differ fundamentally in that we find both nucleus angularis and nucleus laminaris projecting to different areas of the MLd on both sides of the brain. In particular, our observation that the cochlear nucleus has bilateral connections with MLd demonstrates an important avian similarity with the brainstem auditory pathways of other terrestrial vertebrates.

Ascending lateral line pathways to the midbrain of the clearnose skate, Raja eglanteria.

Efferent projections of the electroreceptive dorsal and the mechanoreceptive intermediate octavolateralis nuclei were revealed, in the brain of the clearnose skate Raja eglanteria, by means of silver degeneration and autoradiographic methods. Efferents from each nucleus give rise to ipsilateral and contralateral lemnisci and commissural components. Commissural fibers terminate within their respective nuclei of the opposite side. The lemnisci from each nucleus parallel each other as they course from medullary to mesencephalic levels; those from the dorsal nucleus assume a lateral position and terminate within the lateral part of the nucleus of the lateral line lemniscus and the lateral nucleus of the midbrain; those within the medial part of the lateral line lemniscal nucleus and the dorsomedial mesencephalic nucleus. A substantial number of fibers of each class course through the mesencephalic nuclei and terminate bilaterally within the central tectal zone. The segregation of electroreceptive and mechanoreceptive information is maintained from medullary to mesencephalic levels although there probably is convergence within the central tectal zone and the magnocellular nucleus, the only octaval center to receive terminals from secondary lateral line fibers.

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

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.