Ipsilateral Inferior Colliculus Research Articles

ラット脳幹破壊によるＡＢＲ陽性緩徐波の変化について

Both components of the ABR, sequential fast wavelets and positive slow wave, originate from different anatomical structures with different electrophysiological mechanisms. Origins of the positive slow wave were studied using some destructive methods in rats. After the mechanical destruction of the right cochlea, lesions were made in several parts of the auditory brain stem structures. The wave forms of the positive slow wave and the fast wavelets were observed before and after the destructions. The lesions were examined by serial sections for microscopic examination.Considering the results, the following conclusions were obtained:1) The positive slow wave might have its generators in the several parts of the brain stem auditory pathway, that is, ipsilateral cochlear uncleus, contralateral superior olivary complex, contralateral nucli of the lateral lemniscus and contralateral inferior colliculus.2) On the contrary, cochlea, ipsilateral lateral lemniscus and ipsilateral inferior colliculus are not essential to generate the positive slow wave.It is presumed that the positive slow wave is likely to be generated by the gray matter in the brain stem structures and that the sequential fast wavelets are generated primarily by the white matter.

Origin of afferents to physiologically defined regions of the medial geniculate body of the cat: ventral and dorsal divisions.

To study the origin of afferents to the medial geniculate body (MGB), single unit recordings were first conducted to define physiologically a given region in this auditory thalamic nucleus. Horseradish peroxidase (HRP) was then injected extracellularly, in order to label retrogradely the neurons whose axon terminals end in this region. The principal inputs to the MGB are coming from various nuclei of the brain stem, the auditory cortex and the reticular complex of the thalamus. The ventral division receives its cortical inputs principally from the primary (AI) and the posterior (PAF) auditory cortical fields, with a quantitatively smaller contribution of the secondary (AII) and anterior (AAF) cortical fields. On the other hand, the dorsal division receives a majority of its cortical inputs from AII, with a less important contribution of AI and PAF. The auditory cortex sends roughly as many axons to these two divisions as does the brain stem, mainly the inferior colliculus (IC). The analysis of ascending inputs to the same regions of the MGB reveals that, on the average, 88%, 7% and 5% of them are coming from the ipsilateral IC, the contralateral IC and other nuclei of the brain stem, respectively.

Topographical organization of the inferior collicular projection and other connections of the ventral nucleus of the lateral lemniscus in the cat.

The topographic distribution of projections from the ventral nucleus of the lateral lemniscus (VNLL) in the cat was investigated with the autoradiographic tracing method. The origin of minor projections was verified by retrograde tracing methods. Small injections of tritiated leucine were placed in restricted zones of VNLL. A major afferent fiber system to the inferior colliculus was labeled in all cases. From the injection site labeled fibers coursed through and around the nuclei of the lateral lemniscus to enter the ipsilateral inferior colliculus. Regardless of the position or small size of the injection, labeled fibers distributed widely in the inferior colliculus. Fibers ended in the central nucleus and deeper layers of the dorsal cortex in most cases. There was also labeling in the ventrolateral nucleus, but very few fibers ended as lateral as the lateral nucleus. A small number of labeled fibers passed from the inferior colliculus into the nucleus of the brachium of the inferior colliculus and adjacent tegmental areas. Some labeled fibers entered the commissure of the inferior colliculus where they were traced into the dorsal cortex and rostral pole of the inferior colliculus on the side contralateral to the injection site. Though the projections labeled in individual cases were similar in their divergent pattern within the central nucleus of the inferior colliculus, specific variations in the pattern were found. The dorsal zone of VNLL projected more heavily to the deeper layers of the dorsal cortex and an adjacent field in the central nucleus than the other zones. Dorsal injections in the middle zone of VNLL, on the other hand, labeled the medial part of the central nucleus more heavily, whereas ventral injections in the middle zone resulted in heavier lateral labeling. The ventral zone of VNLL projected heavily to a central field in the central nucleus. In addition to this major afferent system of VNLL to the inferior colliculus, a smaller descending projection was found. The descending projection ended mainly in the dorsomedial periolivary region and ventral nucleus of the trapezoid body. However, in some cases a few fibers were traced to the cochlear nuclei. Finally, we observed projections to the medial geniculate body from the dorsal and ventral zones of VNLL that ended diffusely in the medial division of the medial geniculate body. Possibly some fibers from the dorsal zone contribute to a broader projection of the lateral tegmentum to the dorsal division of the medial geniculate body.

Inhibitory contours in the inferior colliculus as revealed by the 2-deoxyglucose method.

2-Deoxyglucose experiments were carried out during which tone bursts were presented to one ear of cats, while white noise bursts were presented to the other ear. Autoradiography revealed that the tones had produced clear bands of reduced labelling in the ipsilateral inferior colliculus, in addition to the expected band of increased labelling in the contralateral inferior colliculus. The positions of the bands or contours appeared to be organized tonotopically into iso-frequency inhibitory contours.

Some physiological consequences of neonatal cochlear destruction in the inferior colliculus of the gerbil, Meriones unguiculatus

Physiological consequences of neonatal cochlear ablation were assessed in the inferior colliculus of adult gerbils subjected at 2 days of age to ablation of one cochlea. Responses to tonal stimulation of the unoperated ear were evaluated at 100 μm intervals in the ipsilateral inferior colliculus. Compared with excitatory responses to ipsilateral stimulation in the inferior colliculus of unoperated animals, responses recorded in experimental animals are more widespread, stronger and occur at lower thresholds. The ubiquity of excitatory responses to ipsilateral stimulation in neonatally ablated animals indicates that responses to ipsilateral stimulation in normal gerbils are partly dependent upon interactions during development between pathways conveying information from each ear.

Anatomy of the auditory pathways, with emphasis on the brain stem.

The principal auditory leading to the cerebral cortex and therewith to conscious perception passes from the cochlea, via the cochlear nuclei, the inferior colliculus and the medial geniculate body to the contralateral auditory cortex in the temporal lobe. All components of this pathway are cochleotopically organized. The fibre stream leading from the cochlear nuclei to the contralateral inferior colliculus consists of both direct and multisynaptic components. The latter are interrupted one or more times in the superior olivary complex and/or the nuclei of the lateral lemniscus, cell assemblies that lie embedded into the main ascending fibre stream. The auditory cortex receives not only impulses from the contralateral cochlea, but also from the ipsilateral cochlea. The ipsilateral projection reaches the ipsilateral inferior colliculus after a synaptic interruption in the ipsilateral superior olivary complex. Fibres conveying impulses from the contralateral side back to the ipsilateral side are probably also involved. The auditory system of the brain stem is characterized by the presence of several well-developed commissures. Most of these contain, in addition to decussating components, true commissural fibres. The various auditory centres in the brain stem are not only way stations in the ascending auditory pathways, but also serve as relays in descending auditory projections. Most important among the latter is the cortico-cochlear projection which is synaptically interrupted in the inferior colliculus and the periolivary nuclei. The final link in this descending projection is formed by the bundle of Rasmussen, the fibres of which leave the central nervous system and innervate the inner and outer hair cells in the organ of Corti. The pathways for the auditory startle response, the auditory orientation reflex and the stapedius and tensor tympani reflexes are briefly described.

The nucleus paragigantocellularis lateralis in the rat. Demonstration of afferents by the retrograde transport of horseradish peroxidase.

Injections of horseradish peroxidase (HRP) were placed in the middle or caudal portion of the nucleus paragigantocellularis lateralis (PGCL) and 24 h later the entire spinal cord and brain were processed and examined for labeled neurons. Spinal afferents arise from all levels of the cord. Rexed's lamination scheme was adapted to the spinal cord of the rat and labelled neurons were localized to laminae IV, V, VII, VIII and X mainly on the side contralateral to the injection. At cervical levels, labeled neurons were consistently found bilaterally. The medial reticular nuclei of the medulla and pons contained HRP-labelled perikarya, which were concentrated most heavily in the nuclei reticularis medullae oblongatae ventralis, gigantocellularis, and pontis caudalis predominantly ipsilateral to the injection. The medial vestibular nucleus was consistently labeled. HRP-labeled perikarya were found bilaterally within the commissural portion and in the medial part of the nucleus of the solitary tract on the side of the injection. The rostral portion of the PGCL receives afferents from some secondary auditory nuclei: the ipsilateral inferior colliculus and the posterior ventral cochlear nucleus bilaterally. Thus, the rostral PGCL may be involved in auditory feedback loops. The caudal raphe nuclei are a major source of afferents to the caudal PGCL. The lateral hypothalamic area, paraventricular nucleus, and zona incerta also contain labeled neurons when injections are centered in the caudal portion of the nucleus.

Second order auditory pathways in the chimpanzee.

Substantial portions of the dorsal, and almost the entire posteroventral and anteroventral (Av) cochlear nuclei were aspirated unilaterally in a chimpanzee. Axonal degeneration was studied by the Fink-Heimer method. The greatest amount of degeneration was followed medially from the region of Av into the lateral part of the trapezoid body. Degeneration also coursed around the superior surface of the restiform body and was traced into the dorsal and intermediate acoustic striae. Within the superior olivary complex, degeneration was distributed to: the ipsilateral lateral superior olive; laterally and medially oriented dendrites of the ipsilateral and contralateral medial superior olivary nuclei respectively (some perisomatic degeneration also was present bilaterally); the contralateral medial trapezoid nucleus; retro-olivary and preolivary cell groups bilaterally. Abundant degeneration passed into the contralateral lateral lemniscus and was distributed largely to its ventral nucleus. The contralateral central nucleus of the inferior colliculus was a major site of termination of ascending second order auditory fibers. The caudal tip of the ipsilateral ventral nucleus of the lateral lemniscus received abundant degeneration, but this diminished rostrally. The ipsilateral inferior colliculus contained a moderate amount of degeneration. A fair number of degenerated second order auditory fibers ascended in the contralateral brachium of the inferior colliculus and were distributed both to the principle and magnocellular divisions of the medial geniculate body. This pathway appears to represent a phylogenetic advance in the brain of the great ape.

Efferent projections of the insular and temporal neocortex of the cat

Anterograde degeneration resulting from small lesions placed in either the insular or temporal cortex were traced with the Fink-Heiner reduced silver procedure. In neocortical regions ipsilateral to the lesion axonal degeneration was present in auditory subdivisions AI, AII, Ep, I, T, in the second somatosensory are(SII), in the anterior and middle suprasylvian gyrus, in the posteromedial suprasylvian and posterior lateral gyri, in the posterior splenial gyrus, in the anterior two-thirds of the cingulate gyrus and in the orbitofrontal regions. With respect interhemispheric connections, evidence was obtained for a dual pattern of projection. In addition to significant amounts of axonal and terminal degeneration in the corresponding insular or temporal fields, axonal degeneration was also present in posterior AII. In the thalamus degeneration was found in the medial dorsal, suprageniculate, and lateral posterior-pulvinar nuclei. In the posterior nuclear group (Po) and the principal division of the medial geniculate (GMp) evidence was obtained for a topographic pattern of projection; significantly more degeneration occurred in caudal Po following insular lesions whereas with temporal lesions more degeneration occurred in caudal GMp. Degeneration was also found in the dorsal cortex of the ipsilateral inferior colliculus, bilaterally in the deep layers of the superior colliculus and the periventricular central gray region, ipsilaterally in the ventromedial aspects of the head and body of the caudate nucleus, and in the lateral and central nuclei of the amygdala. These findings are discussed in terms of their significance for a possible role for the insular and temporal neocortex (I-T) in both multimodal sensory discrimination and sensory-visceral integrative functions.

Cortical projections to the inferior colliculus of the cat

Cortical projections to the inferior colliculus were studied in the cat by the Nauta and the Fink and Heimer degeneration techniques. The auditory cortex projects to all parts of the ipsilateral inferior colliculus, with the the terminations especially heavy in the external nucleus and the dorsomedial part of the central nucleus. Projections from the somesthetic, visual, and motor areas of the cortex terminate in various parts of the external nucleus and the medial part of the central nucleus. These connections are discussed as a possible inhibitory function to the inferior colliculus and thus influencing the ascending auditory system. The subdivisions of the inferior colliculus are also discussed.