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  • 1975-1979  (3)
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  • 1
    ISSN: 1432-1106
    Keywords: Electric fish ; Electrolocation ; Single unit recording ; Mesencephalon
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary 1. Extra- and intracellular recordings from single units in the magnocellular mesencephalic nucleus (MMN) of the torus semicircularis, related to the fast electrosensory system are reported for the weakly electric fish Gymnotus carapo (Gymnotidae). 2. The non-spontaneously active units responded with single action potentials to the electric organ discharge (EOD) and to artificial electrical pulses with a very short latency of 0.8–1.5 ms. This strongly suggests, in agreement with morphological data, that transmission takes place through electrical synapses. 3. The dynamic range (probability and latency of the single action potential) of the response is extremely narrow and about the same as found in the relevant electrosensory fibres. Intracellular stimulation gives the same response characteristics and dynamic range. 4. The recovery of the response was studied in detail using different stimulus combinations of double pulses at varying delays. Under all conditions, the recovery period to evoke a test response after a conditioning stimulus and response increased in length with the strength of the conditioning stimulus. Inversely, the conditioning stimulus to prevent the unit from firing again had to be stronger as the delay between the two stimuli was increased. 5. Since there is no evidence of neural inhibition causing the long lasting and graded recovery characteristics for MMN units, an attempt was made to explain the findings by classical neurophysiological considerations adapted for electrical synaptic transmission (“current sink” theory). 6. This neural mechanism means that, if at all, the relatively weaker stimulus is not responded to, which protects the fish from being jammed by external pulses of physiological amplitude. In contrast, very strong foreign pulses can completely abolish responses to own EODs especially when timed appropriately. Both effects are discussed in view of their significance for the fish's electrosensory system and communication.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1432-1351
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Medicine
    Notes: Summary 1. Recordings were made from single inferior colliculus neurons of two closely related bat species,Molossus ater andMolossus molossus, both emitting short CF-FM echolocation calls which differ only in frequency range. Employing pure tone stimuli, minimum thresholds, tuning curves, response patterns and spike count functions were measured and compared between the two species. 2. The audiograms (evoked potential measurements and distribution of single neuron thresholds) of both species are rather broadly tuned, but maximum auditory sensitivity is reached at different frequency ranges according to the different spectral content of the orientation calls. 3. Single unit data concerning tuning curves, Q10dB-values, response patterns and spike count functions are very similar in the samples obtained from the two molossid species and closely resemble data from bats using FM-orientation calls. 4. The inferior colliculus of molossids is tonotopically organized. Asymmetrical and symmetrical tuning curves were found. Q10dB-values rarely exceeded 20, and so are values known as characteristic for other mammals. The dominant response pattern class is the “phasic-on” type with no or low spontaneous activity. Spike count functions of the non-monotonic type prevail. 5. Data are compared with results from “long CF-FM-bats”, revealing striking species differences in frequency selectivity of single neurons and organization of the ascending auditory pathway. This suggests different strategies in information processing which are discussed as adaptations to the species specific orientation calls.
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 1432-1351
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Medicine
    Notes: Summary 1. Recordings were made from single inferior colliculus neurons ofMolossus ater andMolossus molossus (Molossidae) to pure tone pulses and FM-pulses with different modulation heights, durations (20 ms, 2 ms) and directions (upward, downward sweeps). 2. Minimum thresholds to pure tone pulses at best frequency and FM-pulses centered at best frequency of equal duration were compared. Using stimuli of 20 ms duration, 58% of the neurons inM. ater (49% inM. molossus) had equal threshold values within ±5 dB, 28% inM. ater (37% inM. molossus) responded with more than 5 dB higher thresholds to FM-stimuli. With stimulus durations of 2 ms, the distribution changed: 27% of the neurons inM. ater (41% inM. molossus) responded 5 to 60 dB more sensitively to the FM-stimulus that mimics the FM-component of the orientation call, only 17% inM. ater (26% inM. molossus) responded more sensitively to pure tone stimuli. Comparison of thresholds to FM-stimuli of different sweep rates also revealed a population of neurons more sensitive to the fast sweep rate (22%M. ater, 29%M. molossus). 3. Specialized neurons were found that did not respond to a particular stimulus configuration (e.g. neurons not responding to upward sweeping FM-stimuli, neurons not responding to slowly sweeping FM-stimuli, etc.). 4. Response patterns of single neurons were found to depend on the frequency-time-course of the stimulus. Especially with slowly sweeping FM-signals, neuronal response activity (measured in number of spikes per stimulus) could be greater than to any other stimulus configuration employed. A stabilization of initial latency was frequently observed with fast sweeping FM-stimuli. 5. Threshold Signal/Noise-ratios were measured by masking the neuron's response to the signal (pure tone or FM) by bandpassed pseudorandom noise. The threshold Signal/Noise-ratios where either equal or lower for FM-stimuli than for pure tone stimuli of equal duration, i.e. more noise intensity is needed to mask the response to FM.
    Type of Medium: Electronic Resource
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