Biomedical subjects

D K Morest

Publications and source records attributed to D K Morest.

At least 73 records · Page 4Linked to original sources

PubMed1984

Fine structure of cochlear innervation in the cat.

Examination of adult and juvenile cat cochleas by electron microscopy and semi-serial sections permitted identification of the cytological features characteristic of the afferent and efferent nerve fiber populations identified in Golgi impregnations of the cochlea. This study demonstrated the distribution of synaptic contacts made by these fiber populations. As in the Golgi findings, radial and outer spiral afferent fibers were identified in well separated zones of the inner spiral bundle. The trunks of the outer spiral fibers, containing many microtubules and few neurofilaments, at first coursed spirally below the inner hair cells on the proximal face of the inner pillar, turned abruptly between adjacent pillar cells and entered the tunnel without branching. Radial afferents, containing many neurofilaments and a few microtubules, coursed through the inner spiral bundle, maintaining a radial or oblique orientation and proceeded directly toward the inner hair cells. Efferent fibers in the region of the inner spiral bundle were distinguishable by size, by orientation, and, to a lesser extent, by cytology. Small (1 micron) efferent fibers, containing few neurofilaments, an occasional microtubule, and mitochondria, occurred in the inner and tunnel spiral bundles and formed large varicosities, which contacted radial afferents. A separate population of much thicker efferents, containing many neurofilaments, mitochondria and dense-cored vesicles, but no microtubules, did not enter the inner spiral bundle but coursed directly to the level of the tunnel spiral bundle on the proximal face of the inner pillar cells. These fibers crossed the tunnel at the level of the tunnel spiral bundle and, upon reaching the outer hair cells, formed large synaptic contacts on outer hair cells and on outer spiral fibers as well. Some of these efferent fibers also synapse on afferent fibers while crossing the tunnel. The findings agree with previous observations with the Golgi method showing that entirely separate populations of spiral ganglion cells innervate the inner and outer hair cells. Likewise, there are efferent fibers innervating only inner or outer hair cells, but the probability of efferent fibers to both inner and outer hair cells cannot be ruled out.

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PubMed1984

Transganglionic transport of D-aspartate from cochlear nucleus to cochlea--a quantitative autoradiographic study.

This study concerns the connections of the inner and outer hair cells and the different types of ganglion cells of the mammalian cochlea with the central nervous system by making use of their putative neurotransmitters. D-[3H]Aspartate (D-ASP), a putative marker for glutamatergic neurons, was injected into the cochlear nucleus of cats and guinea pigs and the cochleas prepared for light microscopic autoradiography after varying survival times. A quantitative, statistical autoradiographic method is described. Grain counts per unit area were made for each of 14 tissue compartments in the cochlea and normalized to permit comparisons between cases. An operationally defined background labeling level was computed for each case and a statistical test for significance applied to the neuron-containing tissue compartments. With increasing survival times, significant labeling appeared successively in the cochlear nerve root, in each type of spiral ganglion cell, and in the neuron-containing tissue compartments of the organ of Corti. The findings are consistent with uptake of D-ASP and retrograde transport by cochlear nerve axons from the cochlear nucleus to the perikarya and peripheral processes of the spiral ganglion. We conclude that axons of all spiral ganglion cells project to the cochlear nucleus and that this nucleus is directly connected with both the inner and outer hair cells. Transganglionic transport of D-ASP from the cochlear nucleus is consistent with the hypothesis that the cochlear nerve axons use glutamate or aspartate as a neurotransmitter.

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PubMed1983

The neuronal architecture of the dorsal division of the medial geniculate body of the cat. A study with the rapid Golgi method.

The neurons in the nuclei of the dorsal division of the medial geniculate body were studied with the rapid Golgi method in kittens and young adult cats. The dorsal nuclei contain two principal cell types: a large stellate neuron with radiate dendrites and a bushy neuron with tufted dendrites, each with extensive dendritic fields. Two sizes of cells with locally arborizing axons (local circuit neurons) were found. The small and commonly observed one is stellate in shape, has a limited dendritic domain, and an axon with multiple, often profuse collaterals ending in the vicinity of the cell. A second, and much less common variety (large local circuit neuron) is somewhat larger and has fewer axon collaterals. Subtle, but distinct variations in these cell types distinguish the deep and superficial dorsal nuclei and also the anterior tier of nuclei (deep dorsal and superficial). These functional differences may correlate with the relative morphological homogeneity of the ventral nucleus compared to the extremely heterogeneous medial division. The dorsal division should be regarded as part of the pulvinar-lateralis posterior complex both structurally and functionally. In the suprageniculate nucleus, the principal neurons are stellate cells with large perikarya and numerous and extensive dendrites covered with appendages. The large axon is devoid of collaterals. A small local circuit cell with several axon collaterals, and sparse, restricted dendrites has also been observed. In the adjacent posterior limitans nucleus, the principal neuron has a medium-sized, piriform or somewhat elongated perikaryon, a few very long radiating dendrites, which may span the depth of the nucleus, and a long, poorly branched axon. Small neurons are also seen here. A comparison of the structure, connections, and function of the medial geniculate body suggests that the dorsal division is predominantly, but probably not exclusively auditory, while the ventral nucleus is entirely auditory and relatively homogeneous, and the medial division, polymodal and heterogeneous with respect to input.

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PubMed1983

Noise-induced degeneration in the brain and representation of inner and outer hair cells.

Noise that destroys receptor cells in the chinchilla cochlea also results in degeneration of axonal endings in the brain from the cochlear nerve fibers and the auditory pathways ascending to the superior olivary complex and inferior colliculus. The patterns of degeneration provide experimental evidence for differential representation of inner and outer hair cells and their functions.

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PubMed1983

A study of cochlear innervation in the young cat with the Golgi method.

Individual afferent and efferent nerve fibers were identified and traced in Golgi-impregnated cochleas of cats from newborn to one month old. Afferent radial fibers project radially without varicosities to terminate at the base of one or two inner hair cells. Outer spiral fibers have both radial and spiral orientations within the organ of Corti, do not form varicosities while crossing the base of the tunnel, and spiral for long distances in the outer spiral bundles. They contact many outer hair cells of more than one row both en passant and by small terminal branchlets. Two separate groups of efferent fibers are identifiable. Thin efferent fibers with many large varicosities spiral for long distances in the inner and tunnel spiral bundles; varicosities in the inner spiral bundle may contact radial afferent fibers or hair cells, depending on age. Thick radial efferent fibers course radially through the tunnel spiral bundle and across the upper part of the tunnel, often in fascicles. They contact a few outer hair cell bases by large terminals. The spiral expanse of the terminals is limited. These fibers are most common in the more basal turns of the organ. The present results confirm the anatomical separation of radial and spiral afferent fiber systems and identify two separate efferent populations beyond the neonatal period in the cat. The major features of afferent innervation discernible in Golgi-impregnated cochleas are present at birth, although some simplification of afferent fibers probably occurs during the first postnatal week. In contrast, the efferent fiber pattern undergoes important changes during the first few weeks after birth. In mature animals, the fine spiral efferents probably contact only afferent fibers, whereas the thick radial efferents may contact both outer hair cells and spiral afferent fibers. The possibility that some individual efferents branch to both inner and outer hair cell regions in the older cats cannot be ruled out.

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PubMed1983

Selective labeling of spiral ganglion and granule cells with D-aspartate in the auditory system of cat and guinea pig.

The present study sought to locate putative glutamatergic or aspartatergic pathways in the auditory system of cats and guinea pigs. We injected 0.06 to 3 mM D-[3H] aspartate (D-Asp) in the cochlear nucleus before preparation for light microscopic autoradiography. At short survival times (15 and 40 min) there was heavy labeling of astrocytic somata. Labeling patterns typical of cochlear nerve endings decorated neurons in the cochlear nucleus, e.g., cell bodies and dendritic trunks of octopus cells. Labeling patterns consistent with retrograde axonal transport by the parallel fibers of granule cells appeared in the molecular layer of the dorsal cochlear nucleus and in the external granular layer. Retrograde labeling of the cochlear nerve root fibers also occurred. Consistent with these results are companion biochemical findings on the rapidly dissected cochlear nuclei of guinea pigs. The dorsal, anteroventral, and posteroventral cochlear nuclei, each, evinced uptake of D-Asp. Subsequently, electrical stimulation of each nucleus released a portion of the accumulated amino acid. Most of this release probably came from synaptic endings. Another group of experiments compared autoradiographic localization of 0.06 to 3 mM D-Asp to that of horseradish peroxidase (HRP) 6 hr to 2 d after injections in the cochlear nucleus. Astroglial cell bodies were no longer labeled by D-Asp, but spiral ganglion cell bodies in the cochlea and granule cell bodies in the cochlear nucleus were. Perikarya of the periolivary and ventral cochlear nuclei projecting to the dorsal cochlear nucleus were labeled by HRP and not by D-Asp. Thus, comparisons with the HRP findings indicate that D-Asp labeling resulted from a selective retrograde transport. There was no evidence for a selective anterograde axonal transport. The present observations support the hypothesis that cochlear nerve fibers and granule cells may use L-glutamate and/or L-aspartate as a transmitter in the cochlear nucleus.

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PubMed1983

The medial division of the medial geniculate body of the cat: implications for thalamic organization.

The structure of neurons and axons was studied in the medial division of the medial geniculate body of the cat with the Golgi methods. The results show that the medial division consists of morphologically heterogeneous neurons. The main types, in descending order of frequency, are medium-sized neurons with (1) radiate, (2) tufted, or (3) elongate dendrites; (4) small stellate or radiate neurons, including Golgi type II cells with a locally arborizing, sparsely branching axon collateral system; (5) large neurons, which are weakly tufted. A variety of afferent axons impose a reticulate appearance on the fiber architecture of the medial division. The dominant element in the neuropil consists of axons terminating in the medial division as well as a collateral system of fibers traveling to the adjacent ventral and dorsal divisions of the medial geniculate body. Four types of extrinsic axons are described, including two kinds of thin axons with collateral systems, thick fibers with restricted branches, and large axons with elaborate, serpentine collaterals. Compared to the dorsal and ventral divisions of the medial geniculate body, where, respectively, radiate and tufted neurons are more frequent, the medial division is intermediate in a sense-not that the degree of radiate or tufted dendritic branching is less well developed, but neither type of cell predominates. Moreover, all of the cell types are overlapping in distribution, although the large ones tend to be more common rostrally in the medial division. Likewise, there is no clear-cut regional segregation of axonal types. The ascending projections to the medial division originate in many different structures, involving purely auditory tracts from the inferior colliculus, spinal inputs, and pathways from polysensory regions, such as the midbrain tegmentum and deep layers of the superior colliculus. The spinothalamic tract projects most heavily to the rostromedial region of the medial division bordering the ventrobasal complex. The auditory input from the central nucleus of the inferior colliculus tends to project more heavily to the caudolateral region next to the ventral nucleus of the medial geniculate body. However, these inputs overlap in the medial division. Moreover, the pathways from the tegmentum and the superior colliculus have a broad distribution in the medial division. Still other inputs are known, not to mention those from the cerebral cortex and a widespread and complicated pattern of thalamocortical projections.(ABSTRACT TRUNCATED AT 400 WORDS)

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PubMed1982

Neuronal architecture in nucleus magnocellularis of the chicken auditory system with observations on nucleus laminaris: a light and electron microscope study.

This report presents the major structural features of neurons and their afferent input in nucleus magnocellularis, the avian homologue of the mammalian anteroventral cochlear nucleus. Results of light-microscope observations, as seen in Golgi, Nissl, and normal fiber preparations, as well as ultrastructural morphology are reported. In addition, cells and axons in nucleus laminaris, the presumed homologue of the mammalian medial superior olivary nucleus, are also described. In Golgi-impregnated material, the mature principal cell in nucleus magnocellularis has an ovoid soma encrusted with somatic spines. A dendrite, when present, emerges from the cell soma, travels for a short distance and breaks into a tuft of stubby terminal branches. Foremost among the afferents to nucleus magnocellularis are auditory nerve axons that terminate in large, axosomatic endings, or end-bulbs, covering a large portion of the somatic surface. Other afferents, which also end in relation to the perikaryon, are of undetermined and perhaps multiple origins. The neurons resemble the bushy cells of the mammalian anteroventral cochlear nucleus. Evidence is presented that individual axons from the nucleus magnocellularis bifurcate and send branches to the nucleus laminaris bilaterally, thus placing constraints on the binaural interactions possibly involved in lateralization functions. In electron micrographs, the end-bulbs appear as large, elongate structures which can cover a third of the cell soma. Multiple sites of synaptic specialization occur along these terminals. The synaptic membrane complexes may form directly on the cell body or on the sides or crests of somatic spines. These complexes are characterized by asymmetric membrane densities with a cluster of clear, spherical vesicles on the axonal side. Other small terminal profiles are also present on the somata receiving the end-bulbs. Dendritic profiles are scarce, in agreement with observations in Golgi impregnations. The structural findings indicate that the medial part of the nucleus magnocellularis is homologous to the anterior part of the mammalian anteroventral cochlear nucleus in that the neurons of nucleus magnocellularis are homologous to the bushy cells of the cat. On this basis, the cells in nucleus magnocellularis could faithfully preserve the acoustic response patterns generated in the auditory nerve. This should, in turn, allow a secure relay of bilateral latency differences essential for binaural interactions in the nucleus laminaris.

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PubMed1982

Sequential alterations of neuronal architecture in nucleus magnocellularis of the developing chicken: a Golgi study.

Nucleus magnocellularis in the chicken consists predominantly of a population of medium-sized cells which receive large, axosomatic endings from the auditory nerve. The morphological development of these cells and their auditory input were studied with the Golgi methods. At 7 1/2-9 days of incubation (embryonic days 7 1/2-9, staged according to the Hamburger-Hamilton series), cells in nucleus magnocellularis have several long, branched dendrites, which often end in bulbous swellings. By embryonic day 10, efferent axons have already grown out from the cells and characteristic terminal plexuses of these axons are seen in nucleus laminaris bilaterally. The dendrites of cells in nucleus magnocellularis have been replaced by a multitude of long somatic processes, giving the cell body a shaggy appearance. This arrangement is maintained up to embryonic day 15, when a remarkable second transformation occurs. The cells lose their somatic processes and present bald, round profiles. Around embryonic days 17-18 a primitive-looking process with a tip like a growth cone emerges from the cell body and somatic spines are evident. By days 19-20, one or two thin, frail dendritic processes can be seen. Correlated with this dramatic series of changes in the cells is a fixed sequence of transformations of the incoming axons. Around embryonic day 10, primary sensory axons in nucleus magnocellularis end in swellings resembling growth cones. Between days 11 and 13, following the explosive growth of somatic processes there is a corresponding expansion and ramification of the auditory nerve endings. On embryonic day 14, there is a condensation of the terminal axon branches, which now form a compact, highly branched plexus. Between days 16 and 17, the plexus coalesces into a calycine structure, now approaching its final form, the end-bulb of Held, which is achieved by embryonic days 19-20. The transformation of the plexus to the calycine form occurs around the same that the cell loses its somatic processes. The parallel sequence in the morphogenetic stages of the assembly of the end-bulbs and their target cells evinces a correlation, if not a causal relationship between the sensory axons and the developing neurons. The arrangement of the somatic processes and axonal branches during the early, multipolar stage would provide an opportunity for optimum interactions between the synaptogenetic processes of the afferent axons and the target cells. The later morphological transformations could orchestrate the specific, cell-to-cell interactions which accompany, or even depend on the activity of the definitive end-bulb synapse.

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PubMed1982

Sequential alterations of neuronal architecture in nucleus magnocellularis of the developing chicken: an electron microscope study.

The development of the auditory nerve endings and their target cells in nucleus magnocellularis was studied by electron microscopy of perfusion-fixed brains from embryonic day 12 to hatching. Embryonic days 12-13: somatic processes extend from the perikaryon. The cytoplasm of the soma and processes contains free ribosomes, mitochondria, lysosomes, rough endoplasmic reticulum, Golgi apparatus and an eccentric, heterochromatic nucleus. Small profiles of auditory nerve fibers containing round, clear vesicles make specialized contacts, including some synapses, on distal somatic processes but rarely on proximal somatic processes or on the soma. The postsynaptic zones contain a flocculent matrix. Days 15-17: somatic processes disappear and occasional attachment plaques are seen between cell bodies. The nucleus appears euchromatic. Cytoplasmic organelles form a dense matrix indicative of intense metabolic activity. Somatic spines are evident. The afferent axons form large, vesiculated profiles located, increasingly, on the cell body and somatic spines, with many points of synaptic contact. Opposite each ending a band of amorphous, flocculent material fills the postsynaptic cytoplasm. Embryonic day 18-hatching: the somatic cytoplasm becomes less dense; stacks of rough endoplasmic reticulum start to condense. Afferent axon terminals mature, especially the synaptic membrane complex and associated densities. The postsynaptic flocculent material diminishes in extent until it is found associated only with somatic spines. The ultrastructural observations on the maturation of nucleus magnocellularis closely corroborate and extend previous results with the Golgi methods. Developing auditory nerve fibers initially synapse on the distal parts of the somatic processes of the immature cells. As the somatic processes disappear or retract, axonal endings move to the soma and develop into large axosomatic end-bulbs. Possibly, the somatic processes as they retract drag the auditory nerve endings to the cell body. The findings also suggest a role of the transiently appearing, flocculent material of the postsynaptic regions in the formation of synapses.

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PubMed1982

The neuronal architecture of the anteroventral cochlear nucleus of the cat in the region of the cochlear nerve root: Golgi and Nissl methods.

This report characterizes the cells and fibers in one part of the cochlear nucleus, the posterior division of the anteroventral cochlear nucleus. This includes the region where the cochlear nerve root enters the brain and begins to form endings. Nissl stains reveal the somata of globular cells with dispersed Nissl substance and those of multipolar cells with coarse, clumped Nissl bodies. Both parts of the posterior division contain cells with each Nissl pattern, but in different relative numbers and locations. Golgi impregnations demonstrate two types of neurons: bushy cells, with short bush-like dendrites, and stellate and elongate cells, with long tapered dendrites. Several varieties of bushy cells, differing in the morphology of the cell body and in the size and extent of the dendritic field, can be distinguished. Comparison of the distributions of these cell types, as well as cellular morphology, suggest that the globular cells recognized in Nissl stains correspond to bushy neurons, while the multipolar cells correspond to stellate and elongate neurons. Golgi impregnations reveal large end-bulbs and smaller boutons from cochlear nerve fibers, as well as boutons from other, unidentified sources, ending in this region. The particular arrangements of the dendritic fields of the different cell types and the axonal endings associated with them indicate that these neurons must have different physiological properties, since they define different domains with respect to the cochlear and non-cochlear inputs.

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PubMed1982

The neuronal architecture of the anteroventral cochlear nucleus of the cat in the region of the cochlear nerve root: horseradish peroxidase labelling of identified cell types.

Golgi impregnations of the posterior part of the cat's anteroventral cochlear nucleus have revealed two types of neurons, bushy cells with short bush-like dendrites and stellate cells with long, tapered processes; Nissl stains have revealed globular and multipolar cell bodies with dispersed and clumped ribosomal patterns, respectively. In the present study, we injected horseradish peroxidase into the trapezoid body. Ipsilaterally, retrograde, diffuse labelling of neurons, presumably through damaged fibers, yielded Golgi-like profiles of numerous bushy cells with typical dendrites and with thick axons projecting toward the trapezoid body. Stellate cells were almost never labelled in this way. Anterograde diffuse labelling of thick axons demonstrated calyx endings in the contralateral medial nucleus of the trapezoid body. In the electron-microscope, the perikarya of diffusely-filled bushy neurons were found to have the dispersed ribosomal pattern and the kinds of synaptic endings typical of globular cells, including large profiles of end-bulbs from cochlear nerve axons. After injections restricted to the medial trapezoid nucleus, granularly-labelled cells in the cochlear nucleus were almost completely confined to the contralateral side; Nissl counterstaining showed them to be globular cells in the posterior part of the anteroventral cochlear nucleus. After larger injections, involving surrounding regions of the superior olivary complex, granular labelling occurred throughout the ventral cochlear nucleus on both sides. There is also evidence that stellate cells in Golgi impregnations correspond to multipolar cell bodies in Nissl stains. We conclude that bushy cells typically correspond to globular cells, which receive end-bulbs from the cochlea and send thick axons to the contralateral medial trapezoid nucleus, where they form calyces on principal cells. Principal cells, in turn, are known to project to the lateral superior olive and to one of the nuclei of origin of the crossed olivo-cochlear bundle, which feeds back to the cochlea. In this circuit, correlations between synaptic patterns and particular physiological signal transfer characteristics can be suggested. These could be related to binaural intensity interactions in the lateral superior olive and to a regulatory loop involving the olivo-cochlear bundles.

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PubMed1982

The neuronal architecture of the anteroventral cochlear nucleus of the cat in the region of the cochlear nerve root: electron microscopy.

We have studied the posterior division of the anteroventral cochlear nucleus, where the cochlear nerve root enters the brain, in the cat. In Nissl preparations, this region contains two types of neuronal cell bodies: globular and multipolar. The two types can be identified in the electron-microscope by comparing Nissl substance and rough endoplasmic reticulum. Globular cell bodies receive many synaptic terminals, which cover 85% of the surface. In contrast, multipolar cell bodies are almost entirely wrapped by thin glial sheets--synaptic terminals contact less than 15% of the surface and tend to cluster at the bases of dendrites. Synaptic terminals are of three kinds, types 1, 2, and 3, which contain large round, small round-to-oval, and small flattened synaptic vesicles, respectively. Terminals of all three kinds synapse on both types of cell bodies. However, only globular cell bodies receive the largest type 1 terminals, which correspond to end-bulbs, seen in Golgi impregnations to arise from cochlear nerve axons. Cochlear ablation leads to degeneration of type 1, but not type 2 or 3 terminals. We conclude that neurons with globular cell bodies receive heavy somatic input from the cochlear nerve, as well as from other sources. Neurons with multipolar cell bodies receive very little input to their perikarya--giving their dendrites a more important role in determining their response properties. We suggest a morphological basis for correlating individual kinds of neurons with certain electrophysiological response types.

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PubMed1982

Synapse formation is related to the onset of neuron-specific enolase immunoreactivity in the avian auditory and vestibular systems.

Neuron-specific enolase (NSE) is a unique form of the glycolytic enzyme, enolase, which, in the brain, is strictly localized in differentiated neurons and neuroendocrine cells. In this report evidence is presented that NSE is first seen in the cell bodies of developing chick auditory and vestibular neurons shortly after the time when they first form immature synapses. Subsequently, immunoreactive NSE increases and spreads into transiently appearing structures which participate in the morphogenesis of mature synaptic arrangements. These data correlate synaptogenesis with the differentiation of a specific neuronal enzyme.

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PubMed1981