Ultrastructural organization of endings of the efferent fibers of zones A1 and A4 of the auditory cortex of the cat in the superior olivary complex

Distribution of synapses formed by straight corticofugal fibers running from the zone A1 and A4 of the acoustic cortex in the main basilar nuclei of the cat superior olive has been studied electron microscopically. These synapses are presented as axodendritic structures situating on processes of small and middle diameters, predominantly at the side of the operation (both in the medial and lateral complexes). Problems on functional influence of the acoustic cortex projections on subcortical acoustic centers depending on the form and amount of the synaptic terminals are discussed.     

Morphology of the cat auditory cortex

The ultrastructural features of the primary auditory cortex of the cats and the character of the endings of geniculo-cortical afferent fibers in the early stages of experimental degeneration evoked by destruction of the medial geniculate body were studied. In all layers of the cortex asymmetrical synapses with round synaptic vesicles on dendritic spines and on thin dendritic branches of pyramidal and nonpyramidal neurons are predominant. Symmetrical synapses with flattened or polymorphic vesicles are distributed chiefly on the bodies of the neurons and their large dendrites. Because there are few symmetrical synapses which could be regarded as inhibitory it is postulated that inhibitory influences may also be transmitted through asymmetrical synapses with round vesicles. Other types of contacts between the bodies of neurons, dendrites, and glial processes also were found in the auditory cortex. Degenerating terminals of geniculo-cortical fibers were shown to terminate chiefly in layer IV of the cortex on pyramidal and nonpyramidal neurons. Degeneration was of the dark type in asymmetrical synapses with round vesicles. The results are dicussed in connection with electrophysiological investigations of the auditory cortex.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 5, No. 5, pp. 519–524, September–October, 1973.     

Distribution and ultrastructure of thalamic afferents in the cat auditory cortex

Ultrastructural features of thalamic afferent fibers were studied in the cat auditory cortex in the early stages (on the 4th day) of experimental degeneration produced by destruction of the medial geniculate body. A coordinate grid was used in conjunction with an electron micro-scope to study the topography of the degenerating elements over wide areas of sections, so that the density of degeneration could be determined quantitatively in different layers of the cortex. Degenerating axons were found in all layers. Most of the large (5–7 µ) degenerating axons are located in layer VI; their diameters were smaller in the upper layers of the cortex. Degenerative changes affecting synaptic terminals of thalamo-cortical afferents were of the "dark" type. Fibers of the geniculo-cortical tract were shown to terminate mainly in cortical layer IV. A few degenerating synapses were found in the molecular layer. Terminals with sperical synaptic vesicles are found mainly on the spines of dendrites where they form "asymmetrical" contacts. A few degenerating axo-somatic synapses were observed on stellate neurons in layer IV. The results are discussed in connection with electrophysiological investigations of the cat auditory cortex during stimulation of specific afferent fibers.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 4, No. 6, pp. 612–620, November–December, 1972.     

Role of the geniculate body in performing conditioned reflexes to amplitude-modulated stimuli in the rat

In 13 laboratory rats with bilateral auditory cortex ablation, the border frequency of amplitude-modulation still allowing differentiation between tonal and amplitude-modulated stimuli, did not change after bilateral section of the brachii of the posterior colliculi. Bilateral auditory cortex ablation and section of the brachii drastically disturbed this differentiation when the modulation frequencies were higher than 27-31 Hz. The data suggest that the completion of coding of amplitude-modulated stimuli does not take place at the level of the medial geniculate body, and that border frequencies defined after auditory cortex ablation are linked with the frontier posterior colliculi--thalamo-cortical system.     

Electron microscope study of sources in the cat primary auditory cortex (AI) projecting to the medial geniculate body

An electron microscope study of retrogradely labeled pyramidal neurons in layer VI of the primary auditory cortex (AI) after injecting horseradish peroxidase (HP) into the medial geniculate body was carried out in cats. Not less than 57.8±1.9% on average of the perimeter of perikaryon profiles of corticogeniculate neurons labeled with HP were found to be covered with astroglia processes. Between three and eight synapses occupying an average of 10.8±1.0% of the perimeter length were found on the perikaryon profiles of these neurons. Nearly all synapses (a total of 98.7%) at the soma of corticogeniculate neurons had symmetrical active zones, being made up of axonal terminals with flattened synaptic vesicles. Anterogradely HP-labeled axonal terminals of geniculocortical fibers were also found in the neuropil of layer VI in area AI, in addition to retrogradely labeled neurons. They contained large round synaptic vesicles and formed asymmetrical synapses. The potential role of axosomatic synapses in the shaping of corticogeniculate neuronal activity is discussed.A. A. Bogomolets Institute, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 22, No. 2, pp. 171–178, March–April, 1990.     

Morphological characteristics of the neuronal population in the feline auditory cortex projecting into the medial geniculate body

The location and morphological profile of auditory cortex neurons projecting to the medial geniculate body were investigated in adult cats using horseradish peroxidase retrograde axonal transport techniques. Sources of descending projections to the medial geniculate body from auditory cortex areas I and II were found to be neurons belonging to deep-lying layers (layer VI and layer V to a lesser extent). By far the majority of corticogeniculate neurons in the auditory cortex were pyramidal cells. In layer VI of the primary auditory area (A1), the number of corticogeniculate neurons reaches 60% of all cells belonging to that layer. The average area (M±m) of the profile of perikarya of corticogeniculate neurons in layer VI, area Al equaled 139.3±2.5 µm² and 219.5±7.0 µm² in layer V neurons; average size of long diameter: 15.0±0.19 and 18.3±0.4 µm respectively. The lower regions of layers III and IV in area Al were found to be the termination point of the greater mass of anterogradely-labeled geniculocortical fibers (terminals of relay neuron axons belonging to the medial geniculate body).A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 21, No. 4, July–August, pp. 513–521, 1989.     

Electrophysiologic and behavioral changes in cats following section of the brachia of the inferior colliculi]

Evoked potentials to acoustic stimuli were recorded in the temporal cortical area, the medial geniculate body and the posterior lateral thalamic nucleus in acute experiments on anaesthetized cats. Section of the brachia of the inferior colliculi in an acute experiment resulted in the disappearance of potentials in the examined structures. A distinct correlation has been revealed between the recovery of evoked potentials in the cortico-thalamic auditory structures (in four to six weeks) and the possible elaboration of conditioned reactions within this time period after lesion of the inferior colliculi brachia. The involvement of the temporal area in the general brain activity appears to be one of the major conditions for the formation of new conditioned connections. Possible ways of restoration of afferent input to the temporal cortical area after lesion of the inferior colliculi brachia are discussed.     

Evoked potentials of the auditory cortex on paired stimuli

Changes in the responsiveness of the auditory cortex to an acoustic click and to direct stimulation of the medial geniculate body were studied by the method of evoked potentials in an extended experiment on cats with implanted electrodes. It is shown that the minimum interval between two stimuli for which a second click produces an EP in the auditory cortex is from 30 to 50 msec. The relative refractory period consists of two parts. The first (50–100 msec) is characterized by a rapid recovery, and the second (about 500 msec) by a slow recovery. In contrast with a click, direct stimulation of the geniculate body does not produce a refractory condition but one of facilitation. The effects of Nembutal and chloralose anesthesia and the state of alertness on the recovery of auditory cortex responsiveness were investigated. The reason for the absence and the reduction of an EP from the auditory cortex to a testing click during absolute and relative refractory periods is not a passive decrease of excitability of the usual refractory kind, but an active interplay of excitatory and inhibitory processes in the cerebral cortex, geniculate bodies, and reticular formation of the brain stem.A. A. Bogomolets' Institute of Physiology, Academy of Sciences, Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 1, No. 1, pp. 54–64, July–August, 1969.     

Visual deprivation modifies oscillatory activity in visual and auditory centers

Loss of vision may enhance the capabilities of auditory perception, but the mechanisms mediating these changes remain elusive. Here, visual deprivation in rats resulted in altered oscillatory activities, which appeared to be the result of a common mechanism underlying neuronal assembly formation in visual and auditory centers. The power of high-frequency β and γ oscillations in V1 (the primary visual cortex) and β oscillations in the LGN (lateral geniculate nucleus) was increased after one week of visual deprivation. Meanwhile, the power of β oscillations in A1 (the primary auditory cortex) and the power of β and γ oscillations in the MGB (medial geniculate body) were also enhanced in the absence of visual input. Furthermore, nerve tracing revealed a bidirectional nerve fiber connection between V1 and A1 cortices, which might be involved in transmitting auditory information to the visual cortex, contributing to enhanced auditory perception after visual deprivation. These results may facilitate the better understanding of multisensory cross-modal plasticity.     

Connections between the parietal cortex with the lateral suprasylvian gyrus (Clare-Bishop area) and auditory cortex in cats

Projections between areas 5 and 7 and the lateral suprasylvian gyrus (Clare-Bishop area) were investigated using anterograde degeneration techniques. This showed a topographic organization of projections from areas 5 and 7 to the lateral suprasylvian gyrus. Area 5 association fibers terminate mainly in the anterior portion of the lateral suprasylvian gyrus; this corresponds to the intermediate zone and anterior section of the posterior suprasylvian region. Area 7 efferents are located more caudally, terminating in the posterior section of the intermediate zone and in the posterior region, excluding the outer posterior limits. Fields 5 and 7 give rise to single efferent fibers terminating in the auditory cortex. Fibers from area 5 terminate in the medial ectosylvian and medial, sylvian gyri, i.e., in zones Al and AII or areas 22 and 50. A projection from area 7 terminates at the superior border of the medial ectosylvian gyrus, corresponding to the upper limit of zone A1 or areas 22 and 50.L. A. Orbeli Institute of Physiology, Academy of Sciences of the Armenian SSR, Erevan. Translated from Neirofiziologiya, Vol. 22, No. 6, pp. 739–745, November–December, 1990.     

Linear dimensions of the lamina of the tectum mesencephali during human ontogenesis]

The dynamics of the establishment of linear dimensions of the lamina of tectum mesencephali in ontogenesis was studied in 145 preparations of human brain. The investigation was performed with by the M. M. Sauliak-Savitskaja apparatus (1953). The obtained data were processed by the method of mathematical analysis. The relief of the superior colliculi was found to be formed earlier than that of the inferior colliculi. The linear dimensions of both colliculi are practically equal to those of adult men during the third year of life. No correlative relationship between the sex of the objects under study and the linear dimensions of the lamina of tectum mesencephali was found as well as between thr right and left parts of the lamina tecti. The linear dimensions of it in all age groups are very variable.     

Altered Neuronal Intrinsic Properties and Reduced Synaptic Transmission of the Rat's Medial Geniculate Body in Salicylate-Induced Tinnitus

Sodium salicylate (NaSal), an aspirin metabolite, can cause tinnitus in animals and human subjects. To explore neural mechanisms underlying salicylate-induced tinnitus, we examined effects of NaSal on neural activities of the medial geniculate body (MGB), an auditory thalamic nucleus that provides the primary and immediate inputs to the auditory cortex, by using the whole-cell patch-clamp recording technique in MGB slices. Rats treated with NaSal (350 mg/kg) showed tinnitus-like behavior as revealed by the gap prepulse inhibition of acoustic startle (GPIAS) paradigm. NaSal (1.4 mM) decreased the membrane input resistance, hyperpolarized the resting membrane potential, suppressed current-evoked firing, changed the action potential, and depressed rebound depolarization in MGB neurons. NaSal also reduced the excitatory and inhibitory postsynaptic response in the MGB evoked by stimulating the brachium of the inferior colliculus. Our results demonstrate that NaSal alters neuronal intrinsic properties and reduces the synaptic transmission of the MGB, which may cause abnormal thalamic outputs to the auditory cortex and contribute to NaSal-induced tinnitus.     

Neuronal populations in the posterior group of thalamic nuclei projecting to the amygdala and auditory cortex in cats

Neuronal populations which are sources of fiber tracts to the amygdala and auditory cortexin the posterior group of thalamic nuclei and adjacent structures of the cat mesencephalon were studied by the retrograde axonal transport of horseradish peroxidase method. It was shown that the peripeduncular, suprageniculate, and subparafascicular nuclei form numerous projections to the amygdala. In all parts of the posterior group of thalamic nuclei, common zones of localization of sources of ascending pathways into the amygdala and auditory cortex were demonstrated. A powerful source of projections to the amygdala from the caudal part of the medial geniculate body was discovered.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 16, No. 2, pp. 213–224, March–April, 1984.     

Response characteristics of neurons in the medial geniculate body of the little brown bat to simple and temporally-patterned sounds

We examined the auditory response properties of neurons in the medial geniculate body of unanesthetized little brown bats (Myotis lucifugus). The units' selectivities to stimulus frequency, amplitude and duration were not significantly different from those of neurons in the inferior colliculus (Condon et al. 1994), which provides the primary excitatory input to the medial geniculate body, or in the auditory cortex (Condon et al. 1997) which receives primary input from the medial geniculate body. However, in response to trains of unmodulated tone pulses, the upper cutoff frequency for time-locked discharges (64 ± 46.9 pulses per second or pps) and the mean number of spikes per pulse (19.2 ± 12.2 pps), were intermediate to those for the inferior colliculus and auditory cortex. Further, in response to amplitude-modulated pulse trains, medial geniculate body units displayed a degree of response facilitation that was intermediate to that of the inferior colliculus and auditory cortex inferior colliculus: 1.32 ± 0.33; medial geniculate body: 1.75 ± 0.26; auditory cortex: 2.52 ± 0.96, P < 0.01). These data suggest that the representation of isolated tone pulses is not significantly altered along the colliculo-thalamo-cortical axis, but that the fidelity of representation of temporally patterned signals progressively degrades along this axis. The degradation in response fidelity allows the system to better extract the salient feature in complex amplitude-modulated signals. Accepted: 9 January 1999     

An immunocytochemical mapping of somatostatin in the cat auditory cortex

Using an indirect immunoperoxidase technique, the localization of somatostatin-28 (1-12)-like immunoreactive fibers and cell bodies in the auditory cortex of the cat (anterior, primary, secondary, temporal, ventral, ventroposterior, posterior and dorsoposterior auditory fields) was studied. In general, the distribution of SOM-ir structures is widespread in the auditory cortex of the feline. A high density of immunoreactive fibers as well as a low density of cell bodies containing somatostatin were observed in all the layers of the eight above-mentioned auditory fields. These data indicate that somatostatin-28 (1-12) could act as a neurotransmitter and/or a neuromodulator in the auditory cortex of the cat. The origin of the SOM-ir fibers in the auditory cortex of the cat, as well as the issue of whether the cell bodies containing somatostatin-28 (1-12) are local or projecting neurons is discussed.     

Effect of destruction of the auditory centers of the brain on the range of echolocation by bats Rhinolophus ferrum-equinum (the large horseshoe-nose bat)

Effect of the auditory cortex, internal geniculate bodies and inferior colliculus removal on echo-ranging detection of immovable wire obstacle by bats has been studied in the behavioural procedure. It is shown that destruction of the auditory cortex results in an irreversible decrease (to 74.6% of control values) in limiting detection range of obstacle by animals. Extensive destructions of internal geniculate bodies decrease limiting detection range only in the first postoperative days. In the case of the inferior colliculi + removal the limiting detection range irreversibly decreases to values equaling 38.5% of control ones. A suggestion that the obtained effects are a consequence of the auditory system sensitivity deterioration in bats is substantiated.     

Responses of medial geniculate body neurons to auditory cortical stimulation

Extracellular and intracellular unit responses of thepars principalis of the medial geniculate body to stimulation of the first (AI), second (AII), and third (AIII) auditory cortical areas were studied in cats immobilized with D-tubocurarine. In response to auditory cortical stimulation both antidromic (45–50%) and orthodromic (50–55%) responses occurred in the geniculate neurons. The latent period of the antidromic responses was 0.3–2.5 msec and of the orthodromic 2.0–18.0 msec. Late responses had a latent period of 30–200 msec. Of all neurons responding antidromically to stimulation of AII, 63% responded antidromically to stimulation of AI also, confirming the hypothesis that many of the same neurons of the medial geniculate body have projections into both auditory areas. Orthodromic responses of geniculate neurons consisted either of 1 or 2 spikes or of volleys of 8–12 spikes with a frequency of 300–600/sec. It is suggested that the volleys of spikes were discharges of inhibitory neurons. Intracellular responses were recorded in the form of antidromic spikes, EPSPs, EPSP-spike, EPSP-spike-IPSP, EPSP-IPSP, and primary IPSP. Over 50% of primary IPSP had a latent period of 2.0–4.0 msec. It is suggested that they arose through the participation of inhibitory interneurons located in the medial geniculate body.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 8, No. 1, pp. 5–12, January–February, 1976.     

Functional organization of auditory cortical fields in the mongolian gerbil (Meriones unguiculatus): Binaural 2-deoxyglucose patterns

Summary Unilaterally deafened (cochlear destruction) gerbils were exposed to white noise after injection of 14-C-2-deoxyglucose. The labelling patterns were compared to those of unstimulated operated animals, noise stimulated control animals and bilaterally ear plugged animals.Serial transverse, horizontal and tangential autoradiographs through the cortex were analysed. In lesioned animals, labelling was strongly reduced on the side contralateral to the lesion in the high frequency regions of A1 and the anterior auditory field (AAF). We assume that these regions correspond to the high frequency EI cell areas. Fine banding could be seen superimposed on this pattern in transverse and tangential sections. We suggest that this may be due to alternating strips of EI and EE cells orthogonal to iso-frequency contours. In the low frequency regions of A1 and AAF, labelling asymmetries were also present, but were less pronounced. We assume that these effects are due to low frequency EE cells.In sub-cortical structures, labelling was reduced in the inferior colliculus and ventral part of the medial geniculate body contralateral to the lesioned ear, but no labelling pattern was visible. We presume that the spatial separation of EE and EI inputs to these structures is not marked enough to allow labelling patterns to be seen. In the superior olivary complex, labelling was reduced on the side contralateral to the lesioned ear in the medial dendritic field of the medial superior olivary nucleus and in the nucleus of the trapezoid body. Ipsilateral to the lesioned ear, labelling was reduced in the lateral dendritic field of the medial superior olive.     

Thalamic activation modulates the responses of neurons in rat primary auditory cortex: an in vivo intracellular recording study

Auditory cortical plasticity can be induced through various approaches. The medial geniculate body (MGB) of the auditory thalamus gates the ascending auditory inputs to the cortex. The thalamocortical system has been proposed to play a critical role in the responses of the auditory cortex (AC). In the present study, we investigated the cellular mechanism of the cortical activity, adopting an in vivo intracellular recording technique, recording from the primary auditory cortex (AI) while presenting an acoustic stimulus to the rat and electrically stimulating its MGB. We found that low-frequency stimuli enhanced the amplitudes of sound-evoked excitatory postsynaptic potentials (EPSPs) in AI neurons, whereas high-frequency stimuli depressed these auditory responses. The degree of this modulation depended on the intensities of the train stimuli as well as the intervals between the electrical stimulations and their paired sound stimulations. These findings may have implications regarding the basic mechanisms of MGB activation of auditory cortical plasticity and cortical signal processing.     

Corticalization of two divisions of the visual system during vertebrate evolution

Data on the evolution of the visual system in vertebrate phylogeny are described. Visual projections are demonstrated in the telencephalon of cyclostomata (lampreys). The existence of a retino-thalamo-telencephalic pathway is demonstrated in elasmobranchs (skates). Two visual pathways are present in amphibians (frogs) and reptiles (turtles): retino-thalamo-telencephalic and retino-tecto-thalamo-telencephalic, and these overlap partly at the thalamic level in the lateral geniculate nucleus and completely in the telencephalon. In turtles the earliest visual and tectal impulses relay on their way to the telencephalon in the lateral geniculate body, and later impulses relay in the nucleus rotundus. In mammals (rats) visual tecto-cortical connections are seen; judging from the latent period of potentials arising in the visual cortex in response to stimulation of the superior colliculi these connections have one synaptic relay in the thalamus. The much shorter latent periods of visual evoked potentials recorded in the tectum of the monkey than in turtles (under identical chronic experimental conditions) confirm the views of morphologists on the progressive development of the tectal division of the visual system in vertebrate phylogeny. It is concluded that corticalization of both divisions of the visual system, i.e., the existence of telencephalic representation, appears in the early stages of vertebrate evolution.