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PubMed1977

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.

Animals↗
PubMed2001

Age-related tonotopic map plasticity in the central auditory pathways.

Inner hair cell lesions to the basal turn of the cochlea effectively result in a partial deafferentation of the auditory system. At the level of the midbrain (central nucleus of inferior colliculus) cochleotopic maps, based on single unit response characteristic frequency, are changed after such deafferentation. When a cochlear lesion is induced in a neonatal animal (chinchilla), the reorganization of the frequency map is more extensive than that resulting from similar deafferentation in the adult subject. Neonatal cochlear lesions result in an over-representation of sound frequencies corresponding to the border of the cochlear lesion, while similar lesions in the adult do not. The results suggest that significant plasticity exists in the auditory midbrain during early post natal development (even in a precocious species, such as chinchilla); however, this plasticity is largely lost in the mature animal. A conceptual model for the frequency map re-wiring is presented.

Age Factors↗
PubMed1985

The persistence of somatosensory and auditory pathway evoked potentials in severe hypoglycemia in the cat.

In a previous study it was shown that during severe insulin-induced hypoglycemia in rats and cats (0.38 mmol/l, i.e., 6.8 mg% and 0.8 mmol/l, i.e., 14 mg% respectively) with isoelectric EEG, the latency and amplitude of the auditory nerve-brain-stem evoked responses were not affected. In the present study on cats, the above evoked responses were complemented by recording in addition the cortical auditory evoked potential and the peripheral, brain-stem and cortical components of the somatosensory evoked potentials. Each of these evoked potentials remained in the presence of 0.75 mmol/l glucose in plasma. The persistence of the somatosensory cortical evoked potential was unexpected since two other groups have reported the disappearance of this potential during hypoglycemia. The types of neuronal activity which can still be recorded in severe hypoglycemia are probably generated by neuronal structures with lower metabolic demands such as axons and oligosynaptic pathways, surviving on the consumption of endogenous substrates with compensatory elevation of local cerebral blood flow.

Animals↗
PubMed1988

Organization of a sensory neuropile in the auditory pathway of two groups of Orthoptera.

The anterior intermediate sensory neuropile (aISN) is a prominent neuropile in the ventral nerve cord of locusts and bushcrickets. Previous studies have shown that it receives its main sensory input from auditory receptors. In this paper we examine the structural and physiological relationship between tympanal receptor terminations and the dendrites of sound-sensitive interneurones in the homologous neuropile of locusts and bushcrickets. Each individual receptor fibre of the bushcricket terminates in a somewhat different target area of the neuropile. The ordering is with respect to the characteristic frequency of the fibres (tonotopic) in the anterior-posterior and dorsoventral axis. In the locust, representatives of the four tympanal receptor groups branch in different areas of the aISN. Most of the dorsal neuropilar region, and the anterior ventral region, do not receive input from tympanal receptors. The dendrites of identified sound-sensitive interneurones were examined in the context of this afferent projection. Local interneurones as well as intersegmental interneurones in bushcrickets have dendritic branches in the whole aISN or part of it and thus overlap with at least some receptors. By recording intracellularly from their main neurites, short-latency synaptic potentials were found in response to receptor spikes indicating monosynaptic input. The tuning of these neurones could be predicted by their dendritic morphology. In contrast, in the locust only local and bisegmental neurones are monosynaptically connected with tympanal receptors, but not the studied intersegmental neurones. This is consistent with the finding that most or all branches of intersegmental neurones lie in the dorsal area of neuropile where no receptors terminate. Anatomical and physiological evidence is presented for identified local neurones providing the excitatory and inhibitory synaptic input for such intersegmental neurones. The difference in the basic wiring diagram in the homologous neuropile of the two orthopteran groups is discussed with respect to the possible different roles that sound plays in their behaviour.

Animals↗
PubMed1981

[Medium-latency acoustically evoked brain potentials used for examination of the auditory pathway (author's transl)].

Potentials of the 10-15 ms latency range evoked by acoustic clicks and Gauss-shaped tone bursts habe been investigated in normal hearing adults, 20 patients with cochlear damages, and 6 cases of temporal lobe processes. Methods and results are compared to those of brain stem audiometry. Mean and standard latency ranges are calculated for the different peaks (Fig. 1). Amplitudes should be used only for side difference evaluation in the same patient, because of their big interindividual variation. In cases of profound high frequency hearing loss (Fig. 2) medium-latency potentials yield true threshold values in the low frequency range, where brainstem potentials are failing. The medium latency potentials show a substantial decrease of amplitude for stimuli contralateral to the damaged side (Fig. 3). So this method can be a functional hearing test to detect or at least suspect temporal damages.

Audiometry↗
PubMed1977

[Postmortem changes of fine structure in the auditory pathways (organ of Corti and nucleus cochlearis of the guinea pig (author's transl)].

Autolytic changes in the guinea pig cochlear nucleus were studied by means of light- and transmission electron microscopy from 5 min. up to 5 h post mortem. Hyperchromasy and shrinking of neurons was observed already after 5 min. in areas, where the neurons have a high number of primary afferent nerve endings (Kane 1973). A second mode of post mortem degeneration was found. It was characterized by a swelling of the cell, which showed a diminished stain-ability. This lytic swelling seemed to be less rapid and probably less harmful in the beginning. The cochleae were examined by scanning electron microscopy. In contrary to known damaging agents (like noise or antibiotics) the outer haircells showed hardly any changes in the surface structures up to 1 hour in the basal coil, while lytic protrusions were observed in outer and inner haircells already after 5 min. in the apical parts. Post mortem changes in the cochlea and in the nucleus cochlearis show two different modes of autolytic degeneration.

Animals↗
PubMed1991

Parallel processing of afferent input by identified interneurones in the auditory pathway of the noctuid moth Noctua pronuba (L.).

1. Interneurones 501 and 504 are identified sound-sensitive interneurones in the pterothoracic ganglion of the noctuid moth Noctua pronuba. Both neurones receive monosynaptic input from the A1 afferent and experiments with current injection suggest that the synapse is chemical. The EPSPs evoked in either IN 501 or 504 by the A1 afferent do not facilitate. 2. Temporal integration in INs 501 and 504 was compared by presenting the moth with tones at repetition rates found in the search, approach and terminal phases of the echolocating call of a hunting bat. INs 501 and 504 differ in their capacity to resolve stimulus repetition rates because the mean decay times of their compound EPSPs differ by a factor of three, although both interneurones receive monosynaptic input from the A1 afferent. 3. The features extracted from the authentic, prerecorded, call of an echolocating bat at the level of the pterothoracic ganglion were examined by recording sequentially from a range of interneurones in the same preparation. The capacity of INs 501 and 504 to encode the various phases of the call was examined in the light of their measured mean decay times and related to the avoidance behaviour of the insect.

Acoustic Stimulation↗
PubMed1991

[Localization of substance P in middle ear mucosa and peripheral auditory pathways in guinea pigs].

The distribution and intracellular localization of substance P (SP) in middle ear mucosa (MEM), cochlea and spiral ganglion (SG) were studied by immunohistochemical technique and immunoelectron microscopy. There was a widespread distribution of SP positive nerve fibers (NF) along the median and small vessels of MEM. SP-immunoreactivity (SP-IR) positive cells could be seen in the MEM near the promontorium tympani. In the Corti's organ, SP-IR positive products were located at the base of inner hair cells. The majority of positive NF emerged like strings of beads and were radially distributed from osseous spiral laminal to the Corti's organ. About 50% of the SG cells were SP-IR positive. Two types of SP-IR positive NF were found in the VIII cranial nerve by light microscopy. Small clear vesicles with a diameter of 50-70nm were localized in the cytoplasm of the type-I SG cells by immunoelectron microscopy. In the outer membrane and inside the mitochondria, SP-IR positive substances could be distinguished as an electron dense matter. The possibility of SP as an afferent neurotransmitter or modulator in cochlea and the significance of its presence in the MEM were discussed.

Animals↗