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At least 955 records · Page 53Linked to original sources
PubMed1989

Thresholds of cat cochlear nucleus neurons to microwave pulses.

Action potentials of neurons in cat dorsal and posteroventral cochlear nuclei were recorded extracellularly with glass microelectrodes while the head of the cat was exposed to microwave pulses at 915 MHz using a diathermy applicator. Response thresholds to acoustic tones, acoustic clicks, and microwave pulses were determined for auditory units with characteristic frequencies (CFs) from 278 Hz to 39.2 kHz. Tests with pulsatile stimuli were performed for durations of 20-700 mus, principally 20, 70, and 200 mus. Brainstem midline specific absorption rate (SAR) threshold was as small as 11.1 mW/g per pulse, and specific absorption (SA) threshold was a small as 0.6 muJ/g per pulse. Microwave thresholds were generally lower for CF less than 9 kHz, as were most acoustic thresholds. However, microwave threshold was only weakly related to click threshold and CF-tone threshold of each unit.

Animals↗
PubMed1986

Inhibitory circuitry in the ventral cochlear nucleus is probably mediated by glycine.

Intracellular recordings from brain slice preparations of the ventral cochlear nuclei (VCN) of mice have shown that both the major cell types, stellate and bushy cells, distinguishable by their responses to intracellularly injected current (Oertel, 1983; Wu and Oertel, 1984), receive late inhibitory as well as early excitatory inputs when the auditory nerve is stimulated electrically. When the extracellular Cl- concentration was lowered or when the intracellular Cl- was raised, the reversal potential of IPSPs became more positive; the reversal potentials were independent of the extracellular K+ concentration. Therefore, IPSPs result from increases in Cl- permeability. To determine whether either or both GABA or glycine might mediate the inhibition, the sensitivity of cells to bath-applied putative neurotransmitters was tested. All cells responded to applications of 0.1-10 mM GABA and glycine with large drops in input resistance; these drops were Cl- dependent. To determine which of these 2 substances was more likely to mediate the IPSPs, antagonists specific to GABA and glycine were tested for their ability to block the IPSPs. All IPSPs were eliminated by 1 microM strychnine, a blocker of glycine-mediated inhibition; they were not consistently blocked by 100 microM bicuculline or by 100 microM picrotoxin, blockers of GABA-mediated inhibition. These results indicate that the inhibition is likely to be mediated by glycine. A simple interpretation of the finding that IPSPs have latencies (1.2-4 msec) at least 2X as long as EPSPs (0.6-0.9 msec) is that cells in the VCN are excited monosynaptically by auditory nerve fibers, and that they are inhibited disynaptically through interneurons within the VCN. To test physiologically whether EPSPs and IPSPs are, respectively, monosynaptic and polysynaptic, 500-700 microM sodium pentobarbital was applied to the preparation. Pentobarbital raised the thresholds of all impaled cells and their synaptic inputs. EPSPs could be evoked in the presence of pentobarbital by raising the stimulus strength, as expected when thresholds are raised in a monosynaptic circuit; even if the thresholds of IPSPs were lower than those of EPSPs in normal saline, they were raised above those of EPSPs in the presence of pentobarbital. The finding that the thresholds of IPSPs are raised more than those of EPSPs supports the interpretation that IPSPs are mediated through a polysynaptic pathway, and this may explain why inhibition has been detected inconsistently in vivo.

Animals↗
PubMed1994

[Implantation on the cochlear nucleus. Apropos of a patient with bilateral neuroma].

A new auditory brainstem implant (ABI), manufactured by Cochlear, was implanted in a young patient suffering from a large bilateral acoustic neuroma, associated with multiple other tumors due to a tuberor sclerosis of Bourneville. The operating procedure and the first results are described. A short historical summary, the anatomical basis and the difficulties due to side effects of stimulation are exposed.

Adult↗
PubMed1992

Aberrant projection induced by otocyst removal maintains normal tonotopic organization in the chick cochlear nucleus.

Nucleus magnocellularis (NM), a second-order nucleus in the chick auditory system, is topographically and tonotopically organized. The basilar papilla (cochlea) projects onto the ipsilateral NM via the auditory nerve. The anteromedial region of NM is innervated by the proximal end of the basilar papilla and responds to high-frequency sounds; more posterolateral regions receive input from more distal locations along the papilla and respond to progressively lower frequencies. NM projects exclusively to the third-order neurons of nucleus laminaris (NL). Otocyst removal prevents the formation of the ipsilateral cochlea and cochlear nerve and results in the development of an aberrant functional projection from the contralateral NM to the "deafferented" NM on the operated side of the brain (Jackson and Parks, 1988). In the present experiment, the otocyst was removed unilaterally and the tonotopic organization of the deafferented NM was physiologically mapped at 17-18 d of embryonic age (E17-E18). Quantitative analyses revealed that the frequency organization of the deafferented NM is almost identical to that in normal embryos. Progressively higher characteristic frequencies were recorded at successively more anterior and more medial locations in the nucleus, and the orientation of the tonotopic axis was indistinguishable from normal. Furthermore, the correlation between characteristic frequency and anatomical location is comparable in the deafferented (r = 0.91) and normal (r = 0.87) NM. The only noticeable discrepancy is that characteristic frequencies in NM on both sides of the brain of operated embryos are higher than the frequencies observed previously at comparable regions of the nucleus in unoperated controls.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗