Nucleus Of Medial Geniculate Body Research Articles

Stereological analysis of vascular network of subcortical auditory centers

<p><strong>Introduction. </strong>Subcortical auditory centers have several sources of blood supply. Cochlear nuclei are vascularized by the anterior inferior cerebellar artery. Superior cerebellar and posterior cerebral arteries supply the inferior colliculus nucleus, while the medial geniculate body nucleus is supplied by several posterior cerebral artery branches. The aim of the research was to quantify the vascular network of subcortical auditory centers. </p><p><strong>Methods.</strong> Twelve adult brains, both sexes, aged 21 to 78 years, without signs of neurological diseases were analyzed in this study. Samples for histological sections, stained by the Mallory method, were obtained from strata cut at levels: the junction of the middle and rostral thirds of the oliva, the inferior colliculi, and the thalamic pulvinar. Volume, surface, and length density of the vascular network of subcortical auditory centers were analyzed stereological parameters. </p><p><strong>Results.</strong> All parameters had the highest values in the medial geniculate body nucleus. Statistically significant difference was found in the volume, surface, and length density between vascular networks of the cochlear nuclei and medial geniculate body nucleus, and between inferior colliculus nuclei and medial geniculate body. Vessels in cochlear nuclei and inferior colliculus nuclei differed only in length density. </p><p><strong>Conclusion.</strong> Cochlear nuclei and inferior colliculi nuclei blood vessels have a similar density and diameter, but vessels in inferior colliculi nuclei are more tortuous. In the medial geniculate body nucleus vessels are larger, denser and more tortuous compared to the other two subcortical auditory centers</p>

Exploring functions for the non-lemniscal auditory thalamus.

The functions of the medial geniculate body (MGB) in normal hearing still remain somewhat enigmatic, in part due to the relatively unexplored properties of the non-lemniscal MGB nuclei. Indeed, the canonical view of the thalamus as a simple relay for transmitting ascending information to the cortex belies a role in higher-order forebrain processes. However, recent anatomical and physiological findings now suggest important information and affective processing roles for the non-primary auditory thalamic nuclei. The non-lemniscal nuclei send and receive feedforward and feedback projections among a wide constellation of midbrain, cortical, and limbic-related sites, which support potential conduits for auditory information flow to higher auditory cortical areas, mediators for transitioning among arousal states, and synchronizers of activity across expansive cortical territories. Considered here is a perspective on the putative and unresolved functional roles of the non-lemniscal nuclei of the MGB.

Connections of cat auditory cortex: I. Thalamocortical system

Despite the functional importance of the medial geniculate body (MGB) in normal hearing, many aspects of its projections to auditory cortex are unknown. We analyzed the MGB projections to 13 auditory areas in the cat using two retrograde tracers to investigate thalamocortical nuclear origins, topography, convergence, and divergence. MGB divisions and auditory cortex areas were defined independently of the connectional results using architectonic, histochemical, and immunocytochemical criteria. Each auditory cortex area received a unique pattern of input from several MGB nuclei, and these patterns of input identify four groups of cortical areas distinguished by their putative functional affiliations: tonotopic, nontonotopic, multisensory, and limbic. Each family of areas received projections from a functionally related set of MGB nuclei; some nuclei project to only a few areas (e.g., the MGB ventral division to tonotopic areas), and others project to all areas (e.g., the medial division input to every auditory cortical area and to other regions). Projections to tonotopic areas had fewer nuclear origins than those to multisensory or limbic-affiliated fields. All projections were organized topographically, even those from nontonotopic nuclei. The few divergent neurons (mean: 2%) are consistent with a model of multiple segregated streams ascending to auditory cortex. The expanded cortical representation of MGB auditory, multisensory, and limbic affiliated streams appears to be a primary facet of forebrain auditory function. The emergence of several auditory cortex representations of characteristic frequency may be a functional multiplication of the more limited maps in the MGB. This expansion suggests emergent cortical roles consistent with the divergence of thalamocortical connections.

Connections of the posterior thalamic region with the auditory ectosylvian cortex in the dog.

The purpose of the present study was to define auditory cortical areas in the dog on the basis of thalamocortical connectivity patterns. Connections between the posterior thalamic region and auditory ectosylvian cortex were studied using axonally transported tracers: fluorochromes and biotinylated dextran amine. Cyto- and chemoarchitecture provided grounds for the division of the posterior thalamic region into three complexes, medial geniculate body (MGB), posterior nuclei (Po), and lateromedial and suprageniculate nuclei (LM-Sg). Distinctive cytoarchitectonic features and the distribution of dominant thalamocortical connections (determined quantitatively) allowed us to define four ectosylvian areas: middle (EM), anterior (EA), posterior (EP), and composite (CE). We found that each area was a place of convergence for projections from five to eleven nuclei of the three thalamic complexes, with dominant projections derived from one or two nuclei. Dominant topographical projections from the ventral nucleus to area EM confirmed physiological reports that it may be considered a primary auditory area (AI). We found the anterior part of the EM to be distinct in having unique strong connections with the deep dorsal MGB nucleus. Area EA, which receives dominant projections from the lateral Po (Pol) and medial MGB nuclei, as well as area EP, which receives dominant connections from the dorsal caudal MGB nucleus, compose two parasensory areas. Area CE receives dominant projections from the extrageniculate nuclei, anterior region of the LM-Sg, and Pol, supplemented with an input from the somatosensory VP complex, and may be considered a polymodal association area.

Postnatal cytoarchitecture of the rat medial geniculate body.

The medial geniculate body (MGB) is a thalamic structure that provides vital information flow to the forebrain for complex acoustic processing. The development of cytoarchitectural features of the MGB was examined in rat to identify age-related patterns of growth in major geniculate compartments that have been described previously (Clerici and Coleman [1990] J. Comp. Neurol. 297:14-31; Clerici et al. [1990] J. Comp. Neurol. 297:32-54): the ventral (MGv), dorsal (MGd), and medial (MGm) divisions. Results show that, on the day of parturition, all major nuclei of each division are characterized, including the ovoid (OV) and ventral (LV) nuclei of MGv; the dorsal, deep dorsal (DD), caudodorsal, limitans, and suprageniculate nuclei of MGd; and the MGm. The MGv and MGd, which display comparable areas at birth, show rapid growth to postnatal day 7 (PND7), which then slows until PND11, around the time of ear canal opening; subsequently, MGv accelerates growth to reach larger adult size. From PND11 to PND16, thionin facilitates parcellation by extensive staining of dendritic processes of MGd, MGm, and lateral posterior nucleus neurons but not neurons of the MGv or the dorsal lateral geniculate nucleus. Golgi stains after birth reveal restricted dendritic arborizations in MGv cells and dichotomous branching patterns of MGd neurons. Somal size in MGB increases dramatically subsequent to afferent innervation and again following onset of auditory function. Somal growth occurs between all postnatal age groups tested for OV, LV, and DD nuclei, although LV segments related to high and low frequencies do not differ. Cell packing density predicts the expanse of major MGB divisions better than somal size. These results demonstrate the integrity and growth patterns of MGB nuclei and divisions from nascence and provide a substrate for subsequent study of anatomical and physiological development of the MGB.

Tonotopic organization in ventral nucleus of medial geniculate body in the cat.

Responses of single units and clusters of units to tone burst stimulation were recorded at 100-micron intervals along vertical electrode penetrations through the medial geniculate bodies of eight barbiturate-anesthetized cats. Marking lesions were placed at two or three locations along most penetrations to aid in histological reconstruction of electrode tracks. Best frequencies and suprathreshold-response latencies were studied at each location along a penetration. The ventral nucleus is physiologically characterized as a region containing narrowly tuned, short-latency (less than or equal to 40 ms) responses and an orderly tonotopic organization. Best frequencies were plotted as a function of depth along single electrode penetrations, and the sequences from different locations in the ventral nucleus were compared. Two-dimensional best-frequency maps were obtained at different rostrocaudal levels. Each map was constructed from best frequencies encountered along several electrode penetrations in the same transverse plane in one brain. We divided the ventral nucleus into seven different rostrocaudal levels, each characterized by a different pattern of tonotopy. Caudolaterally, isofrequency contours parallel the ventrolateral border of the medial geniculate body. At middle levels, low- and mid-frequency contours course ventromedially from the dorsal border of the ventral nucleus toward its medial border, then turn sharply and continue ventrolaterally. Higher-frequency contours parallel this course, but consist of discontinuous dorsal and ventral segments. Rostrally, isofrequency contours are vertically oriented. A model of the three-dimensional tonotopic organization of the ventral nucleus is described that is consistent with the two-dimensional best-frequency maps obtained at different rostrocaudal levels and with locations of ventral nucleus neurons labeled by horseradish peroxidase injections into low-, mid-, and high-frequency representations in auditory cortex. The model includes a planar component and a concentric component. Within the planar component, low frequencies are represented laterally and high frequencies are represented rostromedially. Within the concentric component, low frequencies are represented in a central column that extends mediolaterally through a hole in the mid-frequency representation. The mid-frequency representation in turn is partially surrounded by the high-frequency representation. There is a continuous representation of a "single" frequency in both the planar and concentric components of the model.(ABSTRACT TRUNCATED AT 400 WORDS)