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PubMed1988

Glycine-like immunoreactivity in the rat auditory pathway.

From neurophysiological and biochemical studies it has been suggested that glycine can function as a major inhibitory neurotransmitter in the central nervous system of mammals. In the present study, anti-glycine antiserum was obtained from rabbits immunized with glycine conjugated to rabbit serum albumin via glutaraldehyde and purified by affinity chromatography. The antibody thus obtained was found specific for glycine as determined by an enzyme immunoassay system. The immunocytochemical distribution of glycine in the auditory tract and internal ear was investigated with the antibody. In the central auditory pathway, glycine-like immunoreactivity was mainly located in the ventral and dorsal cochlear nuclei, trapezoid body, lateral lemniscus and inferior colliculus. In the labyrinth, immunoreactivity was detected in the vestibular ganglion and the supporting cells of the crista ampullaris and the organ of Corti, but not in the spiral ganglion. These findings suggest an important role of glycine in the auditory and vestibular pathways.

Animals↗
PubMed2000

KCNQ4, a K+ channel mutated in a form of dominant deafness, is expressed in the inner ear and the central auditory pathway.

Mutations in the potassium channel gene KCNQ4 underlie DFNA2, an autosomal dominant form of progressive hearing loss in humans. In the mouse cochlea, the transcript has been found exclusively in the outer hair cells. By using specific antibodies, we now show that KCNQ4 is situated at the basal membrane of these sensory cells. In the vestibular organs, KCNQ4 is restricted to the type I hair cells and the afferent calyx-like nerve endings ensheathing these sensory cells. Several lines of evidence suggest that KCNQ4 underlies the I(K,n) and g(K,L) currents that have been described in the outer and type I hair cells, respectively, and that are already open at resting potentials. KCNQ4 is also expressed in neurons of many, but not all, nuclei of the central auditory pathway, and is absent from most other brain regions. It is present, e.g., in the cochlear nuclei, the nuclei of the lateral lemniscus, and the inferior colliculus. This is the first ion channel shown to be specifically expressed in a sensory pathway. Moreover, the expression pattern of KCNQ4 in the mouse auditory system raises the possibility of a central component in the DFNA2 hearing loss.

Amino Acid Sequence↗
PubMed1993

The binaural auditory pathway: membrane currents limiting multiple action potential generation in the rat medial nucleus of the trapezoid body.

In this paper we describe the membrane currents of neurons in the medial nucleus of the trapezoid body (MNTB), which serves as an inverting relay in the binaural auditory pathway. In the following paper (Forsythe & Barnes-Davies (Proc. R. Soc. Lond. B 251, 151 (1993))) we describe the synaptic inputs to the MNTB and discuss the significance of these results for transmission through this nucleus, where the fidelity of information transfer will depend on the integration of synaptic responses with the intrinsic postsynaptic membrane properties. Whole-cell patch clamp recordings were made from MNTB neurons using a thin-slice preparation of the rat brain stem. Resting potentials were -70 mV with a neuronal input resistance of 250 M omega and a membrane time constant of 14 ms. Voltage-clamp studies showed that MNTB neurons possess an inward sodium current, an outward current similar to a delayed rectifier and an inward rectifier. In addition, a novel transient outward current exhibiting rapid kinetics and a sustained current are present, which are both blocked by micromolar concentrations of 4-aminopyridine (4AP). Current-clamp recording showed that MNTB neurons respond to depolarization with a single overshooting action potential (AP); 4AP blocked a fast after-hyperpolarization, increased AP duration, and converted the single AP response on depolarization to a train of action potentials.

4-Aminopyridine↗
PubMed1975

Averaged evoked potentials in cats with lesions of auditory pathway.

Averaged evoked activity was recorded from needle electrodes placed at the vertex of the calvaria and adjacent to each bulla in anesthetized cats in response to click stimuli. The portion of the response from 0 to 10 msec was analyzed. Activity during the first 3 msec was greatly reduced on the side ipsilateral to a lesion involving destruction of the cochlea or section of the eighth nerve and its blood vessels. Activity after 4 msec was greatly reduced on the side ipsilateral to destruction of the cochlear nuclei. No effect was found with destruction of both inferior colliculi. The bulla-vertex evoked responses were also compared to those recorded from the round window. The results support the premise that change in the wave-form of the early evoked potential can be used to determine site of loss of acoustic information along the auditory pathway.

Acoustic Stimulation↗
PubMed1987

Electrophysiologic assessment of auditory pathways in high risk infants.

This study evaluated auditory processing in a group of 59 infants at risk for subsequent hearing and language disorders due to low birthweight and/or perinatal asphyxia. Auditory system integrity was evaluated electrophysiologically by recording the auditory brainstem response (ABR), middle latency response (MLR) and the cortical auditory evoked potential (CAEP). 63% of the babies had normal peripheral function or slight unilateral impairment; 84% had normal brainstem auditory system function; 82% showed normal MLRs; and 81% showed normal CAEPs. Fifty-three percent of the babies were normal on all tests and only 3% were deviant on all tests. The remaining infants showed diverse patterns of peripheral, brainstem and cortical abnormalities.

Auditory Cortex↗
PubMed1999

Projections of cochlear root neurons, sentinels of the rat auditory pathway.

In certain rodents, the root of the cochlear nerve contains a population of large neurons, known as cochlear root neurons (CRNs), an essential element of the primary acoustic startle pathway. To characterize the projections of the CRNs, we made stereotaxically guided, iontophoretic injections of biotinylated tracers into the cochlear nerve root of albino rats. CRN axons, which are remarkably thick, enter the trapezoid body, cross the midline, and ascend in the rostral aspect of the lateral lemniscus to reach the upper levels of the midbrain. As a group, CRN axons produce a characteristic pattern of profusely ramified collaterals that innervate specific brainstem regions. The main target of CRN axons is the contralateral pontine reticular formation, where collaterals terminate in the caudal pontine reticular nucleus (PnC) and, to a lesser degree, in the ventrolateral tegmental area, the oral pontine reticular nucleus, and the rostral and medial paralemniscal regions. Other targets of CRN axons include the lateral paragigantocellular nucleus of both sides, the ipsilateral facial motor nucleus and PnC, and the contralateral intercollicular tegmentum and superior colliculus. Notably, CRNs apparently do not innervate any of the nuclei of the auditory brainstem, as usually defined, even though their axons pass through or in close proximity to them. The fact that CRNs innervate several reticular and tectal structures that mediate auditory alerting and escape behaviors suggests that they are "early warning neurons," i.e., true sentinels of the auditory pathway.

Animals↗
PubMed1975

[Clinical diagnosis of the auditory pathway using different acoustical evoked responses (author's transl)].

Evoked responses originating from cochlea, brain stem, and cortex are clinically used for differential diagnosis of hearing losses. The diagnostic range and the reliability of the different ERA methods are discussed. In our clinic brain stem potentials recorded by a nonsurgical method have been used as a routine audiometric test in more than 900 cases. The procedure has proved to be easier than electrocochleography and gives nearly the same information about cochlear and middle ear function if there is no VIIIth nerve or lower brain stem damage. For topical diagnosis of the lower auditory pathway additional recording of ECoG is necessary. In the examination of a cortical deafness the recording of brain stem potentials yields the same result as the electrocochleography according to Aran (Fig. 2). In a case of an Apallic syndrome (Fig. 3) brain stem potentials are found only for high intensity clicks, and latencies are abnormally increased. The cochlear potentials simultaneously recorded from the promontory are quite normal. So damage of the brain stem is confirmed.

Adult↗
PubMed2000

Assessment of the functioning of peripheral auditory pathways after cochlear re-implantation in young children using intra-operative objective measures.

The intra-operative electrical auditory brainstem response (EABR), electrical stapedius reflex threshold (ESRT) and the early post-operative behavioural threshold level (T-level) were recorded in five children undergoing cochlear re-implantation. The aim of the study was to assess objectively the effect of re-implantation on intra-operative objective measures and to investigate neuronal function. The children were aged between 2.06 years and 4.5 years at first implantation. Following failure of the first device, re-implantation was carried out 1.42-5.52 years later. Characteristics of the EABR and ESRT across the electrode array were typical of the expected pattern of responses on both occasions. In particular, the slopes of the amplitude input/output (I/O) functions for wave eV of the intraoperative EABR were similar for both the first and second implants even though absolute thresholds were generally elevated after re-implantation. This elevation in intra-operative threshold was more pronounced than the change in early post-operative behavioural threshold level for electrical stimulation (T-level). Our findings confirm a high level of neuronal survival after re-implantation. Threshold of the intra-operative EABR at the time of re-implantation greatly underestimates the sensitivity of the subsequent early post-operative T-levels.

Auditory Pathways↗
PubMedDate not supplied

Comparison of hearing threshold determined by auditory pathway electric responses and by behavioural responses.

In order to evaluate their reliability for determing the hearing threshold, the cochlear microphonic potentials, the auditory nerve and brain stem neural evoked responses as well as the cortical evoked responses were compared with the behavioural hearing thresholds of the same subjects in the same session. The threshold for recording the cochlear microphonic potnetial was found to be appreciably higher than the behavioral threshold. The threshold for recording the auditory nerve and brain stem responses was within a few decibels of the behavioural threshold. The thresold of the cortical evoked response was several decibels higher. It is concluded that (1) the auditory nerve and brain stem neural evoked responses are the best indicators of hearing threshold; (2) the cortical evoked responses are usually comparable, and (3) all types of evoked responses are indispensable aids in the evaluation of hearing and the determination of site of lesion in the auditory system.

Audiometry↗
PubMed1990

Neurogenesis in a marsupial: the brush-tailed possum (Trichosurus vulpecula). I. Visual and auditory pathways.

The times of origin of neurons in the visual and auditory systems were studied in a marsupial, the brush-tailed possum, using tritiated thymidine autoradiography. Within the subcortical visual pathways, most neurons are generated between postnatal days 5 and 21, and the neurons of the primary visual cortex up to postnatal day 68. In the subcortical auditory pathways, most neurons are generated between postnatal days 5 and 28, and all auditory cortex neurons have appeared by postnatal day 46. Neurons in a single layer of cerebral cortex are generated during a period of about 2 weeks. Thus cortical neurogenesis in marsupials extends over a period similar to that seen in primates.

Animals↗