Incorporation of labelled amino acids into the acoustic cortex of the cat

Labelled amino acids, 14C-leucine, 3H-glycine, 14C-GABA, 14C-glutamic acid and 14C-aspartic acid were applied to the surface of the cat's auditory cortex in dried filter paper strips for 40 min. The distribution of the amino acids taken up by the cortex was examined by autoradiography.The right acoustic area was stimulated by 2 cps acoustic clicks, through an ear-phone. Leucine showed cellular localization which was considerably dispersed by cortical excitation. Glycine was concentrated in nerve cells, mainly in layers I-II, but excitation intensified and extended its incorporation. GABA was accumulated rather diffusely in layers I-II with scattered cellular labelling which was depressed by stimulation. Glutamic and aspartic acids did not show any characteristic distribution pattern. The authors emphasize that the incorporation of glycine seems to be a good indicator of neurons being in excitation. The localizations of GABA uptake shows close correlation with the site of its physiological action.     

Application of the glycine labelling method to the cerebellum, hippocampus and spinal cord

3H-glycine was applied to the cat cerebellar cortex under resting conditions and during inferior olive stimulation which activated the climbing fiber system on a restricted area. Electric recording was made. The autoradiograms showed, that under resting condition labelled glycine was incorporated mainly in granule, Golgi and basket cells and only a few Purkinje and stellate cells were active. Also cerebellar glomeruli remained without labelling. On climbing fiber stimulation Purkinje cells became activated singly and grouped, also Golgi and stellate cells increased in number. Granule cells were totally inhibited. 3H-glycine, when applied to the rat hippocampus, the dentate gyrus, CA1 and CA4 fields showed labelling at low frequency stimulation. When 400 Hz high frequency stimulation periods were interposed, long-term potentiation ensued. The overall labelling of each hippocampal region was intensified significantly, indicating that glycine incorporation may be a sign not only of excitation but also of long-term potentiation. 3H-glycine was applied to frog spinal cord during rest and dorsal root stimulation. Interneurons and motor neurons excited by the afferent fibers showed intensive glycine uptake. It is concluded that the glycine labelling method is suitable for detecting neural excitation in the structures dealt with in this paper.     

Response of parietal association cortex neurons to acoustic stimulation before and after auditory cortex blockage in the cat

The effect of auditory cortex blockade on response patterns of parietal association cortex neurons responding to different frequency tones was investigated in the cat. Blockade was produced by two methods: bilateral isolation and application of a 6% Nembutal solution to the auditory cortex surface. Frequency threshold curves were plotted for all test neurons. The majority of test neurons (84%) displayed one or two characteristic frequencies before blockade, as against only 63% of all neurons responding following blockade. Changes also affect the range of frequencies at which the cells could respond. Virtually all test neurons responded to application of a broad spectrum of frequencies under normal conditions. After blockade of the auditory cortex 69% of neurons no longer responded to tones above 8–10 kHz. This would suggest that mainly information on high frequency tones is transmitted via the auditory cortex. The question of where acoustic information for parietal association cortex neurons mostly originates is also discussed; association thalamic nuclei are thought to be the main source.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 18, No. 3, pp. 354–360, May–June, 1986.     

Auditory cortex basal activity modulates cochlear responses in chinchillas

Background

The auditory efferent system has unique neuroanatomical pathways that connect the cerebral cortex with sensory receptor cells. Pyramidal neurons located in layers V and VI of the primary auditory cortex constitute descending projections to the thalamus, inferior colliculus, and even directly to the superior olivary complex and to the cochlear nucleus. Efferent pathways are connected to the cochlear receptor by the olivocochlear system, which innervates outer hair cells and auditory nerve fibers. The functional role of the cortico-olivocochlear efferent system remains debated. We hypothesized that auditory cortex basal activity modulates cochlear and auditory-nerve afferent responses through the efferent system.

Methodology/Principal Findings

Cochlear microphonics (CM), auditory-nerve compound action potentials (CAP) and auditory cortex evoked potentials (ACEP) were recorded in twenty anesthetized chinchillas, before, during and after auditory cortex deactivation by two methods: lidocaine microinjections or cortical cooling with cryoloops. Auditory cortex deactivation induced a transient reduction in ACEP amplitudes in fifteen animals (deactivation experiments) and a permanent reduction in five chinchillas (lesion experiments). We found significant changes in the amplitude of CM in both types of experiments, being the most common effect a CM decrease found in fifteen animals. Concomitantly to CM amplitude changes, we found CAP increases in seven chinchillas and CAP reductions in thirteen animals. Although ACEP amplitudes were completely recovered after ninety minutes in deactivation experiments, only partial recovery was observed in the magnitudes of cochlear responses.

Conclusions/Significance

These results show that blocking ongoing auditory cortex activity modulates CM and CAP responses, demonstrating that cortico-olivocochlear circuits regulate auditory nerve and cochlear responses through a basal efferent tone. The diversity of the obtained effects suggests that there are at least two functional pathways from the auditory cortex to the cochlea.     

Area 39 and 41 neurons sensitive to acoustic stimulation in the rat cortex: Polysensory properties

A comparative analysis of the polysensory properties of 102 neurons in areas 39 and 41 (the associative and auditory cortices, respectively) was performed in acute experiments on rats under chloralose-nembutal anesthesia. In the auditory cortex, the evoked potentials (EP) recorded from the surface of the above area in response to acoustic tonal, electrical cutaneous, and light stimulations almost always were distinguished by their shorter (4–5 msec) latency and higher amplitude. We studied neurons in both areas; their responses to the pure tones of various frequencies and to the stimulations of other modalities were compared. Bi- and polysensory neurons constituted 56.4% in area 39, and only 23% in area 41. The depth distribution of the responding neurons in areas 39 and 41 was different. Neurons with selective sensitivity to different frequencies of tonal signals were found in both areas. Usually monomodal neurons demonstrated selective properties in the auditory cortex, and 70% of them had a characteristic frequency. Over one-half of polymodal cells were frequency-selective in the associative cortex.Neirofiziologiya/Neurophysiology, Vol. 26, No. 3, pp. 223–229, May–June, 1994.     

Receptive fields of auditory cortical neurons in the cat

Receptive fields of auditory cortical neurons were studied by electrical stimulation of nerve fibers in different parts of the cochlea in cats anesthetized with pentobarbital. The dimensions of the receptive fields were shown to depend on the topographic arrangement of the neuron in the auditory cortex. The more caudad the neuron on the cortical projection of the cochlea in the primary auditory cortex, the more extensive its receptive field. The receptive fields were narrowest in the basal turn of the cochlea and were symmetrical with respect to their center. It is suggested that the region of finest discrimination of acoustic stimuli in cats is located in the basal region of the cochlea, i.e., in that part of its receptor system which has the narrowest receptive field and is represented by significantly more (than the middle and apical regions of the cochlea) nerve cells in the primary auditory cortex [1].A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 13, No. 5, pp. 467–473, September–October, 1981.     

Distribution of labeled amino acids and delta-sleep inducing peptide in the body after instillation into the conjunctiva of the rabbit eye

Dynamics of 3H-valine, 3H-glycine and 3H-DSIP distribution in various brain structures, tissues and liquids of an organism due to administration of these substances in eye conjunctive were studied in rabbits with scintillation spectrometry method. Marked amino acids and DSIP were observed in all substrates in 10 min after administration. Maximal activity was found in 2 h in the brain visual cortex and in 30 min in cardiac tissue, spleen and optical chiasma.     

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.     

An Overrepresentation of High Frequencies in the Mouse Inferior Colliculus Supports the Processing of Ultrasonic Vocalizations

Mice are of paramount importance in biomedical research and their vocalizations are a subject of interest for researchers across a wide range of health-related disciplines due to their increasingly important value as a phenotyping tool in models of neural, speech and language disorders. However, the mechanisms underlying the auditory processing of vocalizations in mice are not well understood. The mouse audiogram shows a peak in sensitivity at frequencies between 15-25 kHz, but weaker sensitivity for the higher ultrasonic frequencies at which they typically vocalize. To investigate the auditory processing of vocalizations in mice, we measured evoked potential, single-unit, and multi-unit responses to tones and vocalizations at three different stages along the auditory pathway: the auditory nerve and the cochlear nucleus in the periphery, and the inferior colliculus in the midbrain. Auditory brainstem response measurements suggested stronger responses in the midbrain relative to the periphery for frequencies higher than 32 kHz. This result was confirmed by single- and multi-unit recordings showing that high ultrasonic frequency tones and vocalizations elicited responses from only a small fraction of cells in the periphery, while a much larger fraction of cells responded in the inferior colliculus. These results suggest that the processing of communication calls in mice is supported by a specialization of the auditory system for high frequencies that emerges at central stations of the auditory pathway.     

Prolonged maturation of cochlear function in the barn owl after hatching

Cochlear microphonics (CMs), which represent the electrical activity of hair cells, and compound action potentials (CAPs), which represent the activity of the auditory nerve, were recorded from the round window of the inner ear, in owlets aged between 5 and 97 days posthatching, i.e., from soon after hatching to beyond fledgling. At the earliest ages examined, animals showed very insensitive CM and virtually no CAP responses. Thus, hearing in barn owls develops entirely posthatching and the birds appear to be profoundly deaf well into the second week. Thresholds improved gradually after that and CMs reached their adult sensitivity at 5 weeks posthatching at all frequencies. Compound action potential responses appeared progressively later with increasing frequency. Adult neural sensitivity was achieved about 1 week later than for the CM responses at most frequencies, but took until 9–10 weeks posthatching at the highest frequencies (8–10 kHz). This indicates an apex-to-base maturation sequence of neural sensitivity within the cochlea, with a disproportionately long period to maturity for the most basal regions. Compound action potential amplitudes matured even later, at about 3 months posthatching, at all frequencies. This suggests a prolonged immaturity in the temporal synchrony of spiking in the auditory nerve.     

Does the magnocellular octaval nucleus process auditory information in the toadfish, Opsanus tau?

Previous work on auditory processing in Opsanus tau has focused on the descending octaval nucleus; however, the magnocellular octaval nucleus receives similar inputs from the otolithic endorgans. The purpose of this study was to assess whether cells in any of the three subdivisions of the magnocellular nucleus respond to auditory frequencies and encode sound source direction. Extracellular recording sites were chosen based on anatomical landmarks, and neurobiotin injections confirmed the location of auditory sites in subdivisions of the magnocellular nucleus. In general, the auditory cells in M2 and M3 responded best to frequencies at or below 100 Hz. Most auditory cells responded well to directional stimuli presented along axes in the horizontal plane. Cells in M3 (not M2) also responded to lateral line stimulation, consistent with otolithic endorgan and lateral line inputs to M3. The convergence of auditory and lateral line inputs in M3, the lack of Mauthner cells in this species, and previous evidence that the magnocellular nucleus does not contribute to ascending auditory pathways suggest to us that the large cells of M3 may play a role in rapid behavioral responses to particle motion stimuli in oyster toadfish.     

The effects of neck flexion on cerebral potentials evoked by visual,auditory and somatosensory stimuli and focal brain blood flow in related sensory cortices

Background

A flexed neck posture leads to non-specific activation of the brain. Sensory evoked cerebral potentials and focal brain blood flow have been used to evaluate the activation of the sensory cortex. We investigated the effects of a flexed neck posture on the cerebral potentials evoked by visual, auditory and somatosensory stimuli and focal brain blood flow in the related sensory cortices.

Methods

Twelve healthy young adults received right visual hemi-field, binaural auditory and left median nerve stimuli while sitting with the neck in a resting and flexed (20° flexion) position. Sensory evoked potentials were recorded from the right occipital region, Cz in accordance with the international 10–20 system, and 2 cm posterior from C4, during visual, auditory and somatosensory stimulations. The oxidative-hemoglobin concentration was measured in the respective sensory cortex using near-infrared spectroscopy.

Results

Latencies of the late component of all sensory evoked potentials significantly shortened, and the amplitude of auditory evoked potentials increased when the neck was in a flexed position. Oxidative-hemoglobin concentrations in the left and right visual cortices were higher during visual stimulation in the flexed neck position. The left visual cortex is responsible for receiving the visual information. In addition, oxidative-hemoglobin concentrations in the bilateral auditory cortex during auditory stimulation, and in the right somatosensory cortex during somatosensory stimulation, were higher in the flexed neck position.

Conclusions

Visual, auditory and somatosensory pathways were activated by neck flexion. The sensory cortices were selectively activated, reflecting the modalities in sensory projection to the cerebral cortex and inter-hemispheric connections.     

Regeneration of the eighth nerve fibers after transection in the red-eared turtle,Chrysemys scripta elegans

The present study examined the time sequence of degeneration and regeneration after transection of the eighth nerve in the red-eared turtle as well as the chromatolytic reaction of the turtle auditory ganglion cells. Horseradish peroxidase (HRP) transport between auditory ganglion cells and the medulla identified eighth nerve connections. The course of eighth nerve degeneration was followed with Fink and Heimer degeneration stain and HRP reaction. Cresyl-violet-stained sections through auditory ganglion cells were observed for chromatolysis. Degeneration by-product was intense in the eighth nerve and primary auditory nuclei in turtles surviving 25 and 32 days after eighth nerve transection. Turtles surviving 45 days or less after eighth nerve transection showed HRP reaction product in the eighth nerve to the point of its dorsolateral penetration into the medulla following cochlear duct injections. Acoustic tubercle injections in 50-day survivors showed HRP filling in eighth nerve and auditory ganglion cells. Cochlear duct injections in 67-day survivors demonstrated HRP filling in the eighth nerve and acoustic tubercle. Sections stained for degeneration in 67-day survivors showed little or no degeneration by-product and 80- and 90-day survivors showed none. The proportion of chromatolytic auditory ganglion cells was greatest in the 50-day postoperative turtles when compared to control turtles and other survival stages. Animals which survived longer than 50 days had reduced numbers of chromatolytic cells. Results suggest that the eighth nerve fibers are regenerated to primary brainstem auditory nuclei in experimental turtles surviving 50 days or more. Regeneration occurs between the 45th and 50th day following transection.     

Functional correlates of characteristic frequency in single cochlear nerve fibers of the Mongolian gerbil

Summary Single-unit recordings obtained from the auditory nerve of the Mongolian gerbil, Meriones unguiculatus, revealed functional differences in the response properties of neurons tuned to low and high frequencies. The distribution of neural thresholds displayed a distinct rise for auditory nerve fibers with characteristic frequencies] (CFs) between 3–5 kHz. This frequency band also marked abrupt changes in both the distribution of spontaneous discharge rates and the shape of the neural tuning curve. For neurons of all CFs, spontaneous firing rates were inversely related to neural threshold but unrelated to sharpness of neural tuning. The range of CF thresholds encountered, even when data from many animals were combined, rarely exceeded 20 dB, suggesting that cochlear nerve responses obtained from this species display little inter-animal variability. These results are compared with similar data from other species and discussed in terms of recent studies on sound communication and cochlear anatomy in gerbils.Abbreviations CF characteristic frequency - SR spontaneous discharge rate     

Functional organization of the human auditory cortex

We investigated the functional organization of the human auditory cortex using a novel electrophysiological recording technique combined with an advanced brain magnetic resonance imaging (MRI) technique. Tonotopic mapping data were obtained during single unit recording along the Heschl’s gyrus. Most of the units studied (73%) demonstrated sharply tuned excitatory responses. A tonotopic pattern was observed with the best frequencies systematically increasing as more medial-caudal recording sites were sampled. Additionally, a new auditory field along the posterior aspect of the superior temporal gyrus has been identified using a high spatial resolution direct recording technique. Results obtained during electrical stimulation demonstrate functional connectivity between the primary auditory cortex and the auditory field in the posterior superior temporal gyrus.     

Morphological and physiological differences of the auditory system in three related bushcrickets (Orthoptera: Phaneropteridae, Poecilimon)

Abstract. The auditory system of three closely related bushcrickets was investigated with respect to morphological and physiological differences. The size of the acoustic vesicle in the prothorax cavity and the size of the acoustic spiracle were compared to differences in auditory tuning of the tympanic nerve and differences in the directionality. The results indicate that a small auditory vesicle and auditory spiracle provide reduced sensitivity in the high frequency range (above 10—15 kHz), but increase sensitivity at low frequencies (below 10 kHz). The directionality of the hearing system deteriorates at frequencies between 10 and 25 kHz in species with a small spiracle and trachea. The evolutionary implications of these differences of the auditory systems are discussed. They are considered to be influenced more by ecological factors than bioacoustical ones.     

Bedside Evaluation of the Functional Organization of the Auditory Cortex in Patients with Disorders of Consciousness

To measure the level of residual cognitive function in patients with disorders of consciousness, the use of electrophysiological and neuroimaging protocols of increasing complexity is recommended. This work presents an EEG-based method capable of assessing at an individual level the integrity of the auditory cortex at the bedside of patients and can be seen as the first cortical stage of this hierarchical approach. The method is based on two features: first, the possibility of automatically detecting the presence of a N100 wave and second, in showing evidence of frequency processing in the auditory cortex with a machine learning based classification of the EEG signals associated with different frequencies and auditory stimulation modalities. In the control group of twelve healthy volunteers, cortical frequency processing was clearly demonstrated. EEG recordings from two patients with disorders of consciousness showed evidence of partially preserved cortical processing in the first patient and none in the second patient. From these results, it appears that the classification method presented here reliably detects signal differences in the encoding of frequencies and is a useful tool in the evaluation of the integrity of the auditory cortex. Even though the classification method presented in this work was designed for patients with disorders of consciousness, it can also be applied to other pathological populations.     

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.     

An immuno-electron-microscopic study of the localization of VIP-like immunoreactivity in the adrenal gland of the rat

Summary VIP-like immunoreactivity was revealed in a few chromaffin cells, medullary ganglion cells and a plexus of varicose nerve fibers in the superficial cortex and single varicose fibers in the juxtamedullary cortex and the medulla of the rat adrenal gland. VIP-like immunoreactive chromaffin cells were polygonal in shape without any distinct cytoplasmic processes and they appeared solitarily. Their cytoplasm contained abundant granular vesicles having a round core and the immunoreactive material was localized to the granular core. VIP-immunoreactive ganglion cells were multipolar and had large intracytoplasmic vacuoles. The immunoreactive material was localized not only in a few granular vesicles but also diffusely throughout the axoplasm. VIP-immunoreactive varicose nerve fibers in the superficial cortex were characterized by abundant small clear vesicles and some large granular vesicles, while those in the juxtamedullary cortex and medulla and the ganglionic processes were characterized by abundant large clear vesicles, as well as the same vesicular elements as contained in the nerves in the superficial cortex. The immunoreactive material was localized on the granular cores and diffusely in the axoplasm in both nerves. Based on the similarity and difference in the composition of the vesicles contained in individual nerves, it is likely that the VIP-immunoreactive nerve fibers in the medulla and the juxtamedullary cortex are derived from the medullary VIP-ganglion cells, while those in the superficial cortex are of extrinsic origin. The immunoreactive nerve fibers in both the cortex and the medulla were often in direct contact with cortical cells and chromaffin cells, where no membrane specializations were formed. The immunoreactive nerve fibers were sometimes associated with the smooth muscle cells and pericytes of small blood vessels in the superficial cortex. In addition they were often seen in close apposition to the fenestrated endothelial cells in the cortex and the medulla, only a common basal lamina intervening. Several possible mechanisms by which VIP may exert its effect in the adrenal gland are discussed.     

A quantitative comparison of the hominoid thalamus: III. A motor substrate—the ventrolateral complex

Nuclear volumes, nerve cell densities, numbers of neurons, and volumes of nerve cell perikarya of the thalamic ventrolateral complex (VL), a neural substrate for movement, were measured in specimens from two gibbons, one gorilla, one chimpanzee, and three humans, and the values were compared. The human VL had about one-and-a-half times as many neurons as did those of the great apes. The relative frequencies of the sizes of nerve cell perikarya differed slightly in the ventrolateral segment of VL; no differences were noted in the rest of VL. Compared with findings from other parts of the thalamus, the differences in the volumes of VL were greater than those found in the thalamic sensory nuclei, similar to those of rest of the thalamus, and less than those found in the whole brain. The increased number of neurons in human VL was similar to that of the somatosensory relay complex, but greater than those of the auditory and visual nuclei and less than those of the limbic and association nuclei. In human evolution, the numbers of neurons in the VL appeared to increase at a faster rate than did neurons of the pyramidal tract, whereas the motor cortex apparently increased at a rate greater than VL.