Measuring the action potential transmission delay (ATD). A: extracellular recordings of responses to acoustic stimulations of principal cells of the medial nucleus of the trapezoid body (MNTB) and their presynaptic input, the calyces of Held in the Mongolian gerbil. Due to the size of the calyx, extracellular recordings resemble complex waveforms that consist of the discharge of the calyx (prepotential) and the action potential (AP) of the principal cell. ATD was measured as the distance between the positive peak amplitudes of prepotential and postsynaptic AP. B: ATD measured under silent condition depends on the spontaneous discharge rate of a given MNTB unit. Significantly higher ATD spont are found in units with higher spontaneous rates. C: extracellular recording (top) of a unit's response to a sequence of pure tones (bottom, duration: 100 ms, interstimulus interval: 100 ms, stimulus amplitudes in decreasing order, not to scale). D: mean complex waveforms of the same unit as in C for 5 different ATDs (380 – 460 s, black to red) aligned to the prepotential (top) and to the postsynaptic AP (bottom). Individual complex waveforms of ATDs in a 10 s bin were pooled into each mean complex waveform. E and F: changes in ATD and firing rate correlate (same unit as in C). ATD (E) changes as a function of intensity and frequency of the acoustic stimulus and the firing rate (F) show a very similar profile.  

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... coordinates of the MNTB were determined by on-line analysis of acoustically evoked multi-unit activity using low-imped- ance glass micropipettes (1 M, filled with 3 M KCl, GB150TF-10, Science Products). Single-unit responses, identified by their com- pound waveform (CW, Fig. 1D), were recorded using high-impedance glass micropipettes (5-15 M). The voltage signal was preamplified (Neuroprobe 1600, A-M Systems), band-pass filtered (0.3-7 kHz) and further amplified (PC1, TDT). Voltage traces were digitized at a sampling rate of 97.7 kHz (RP2.1, TDT) and stored for subsequent ...

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... waveforms were generated at a sampling rate of 97.7 kHz using custom written MATLAB software (Version 7.3, The Math- works), transferred to a RP2.1 real-time processor, D/A converted and further sent to a custom-made earphone (acoustic transducer: DT 770 pro, Beyerdynamic) for calibrated (i.e., convolved with the inverse impulse response of the speaker) near-field stimulation. Responses to three stimulation paradigms were recorded: 1) pure tones (100 ms duration, 5 ms cosine rise/fall time, 100 ms interstimulus interval) of predefined frequency/intensity combinations (20 frequencies on a logarithmic scale, 10 intensity levels, 3-5 repetitions) resulting in the unit's frequency response area ( Fig. 1 F), 2) a sequence of 60 pure tones [50 ms duration, 25 repetitions, 10 ms cosine rise, 4 ms cosine fall time, 80 dB SPL, unit's characteristic frequency (CF)] with variable interstimu- lus interval (random 1-500 ms on a quadratic scale) for determining the recovery time of the action potential transmission delay, and 3) two natural sounds, one dominated by low frequencies (gerbil gnawing on cage, 9 s duration, 25 repetitions, band-pass filtered between 500 and 2,000 Hz, Fig. 5A1) and one dominated by high frequencies (gerbil's call when barging into a group of individuals, 5.5 s duration, 25 repetitions, Fig. 5A2). ...

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... characteristic frequency (CF)] with variable interstimu- lus interval (random 1-500 ms on a quadratic scale) for determining the recovery time of the action potential transmission delay, and 3) two natural sounds, one dominated by low frequencies (gerbil gnawing on cage, 9 s duration, 25 repetitions, band-pass filtered between 500 and 2,000 Hz, Fig. 5A1) and one dominated by high frequencies (gerbil's call when barging into a group of individuals, 5.5 s duration, 25 repetitions, Fig. ...

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... analyzed the ATD at the calyx of Held and its activity- dependent changes for 122 principal cells of the MNTB in the anesthetized Mongolian gerbil. ATD was measured as the time between the positive peak amplitudes of the extracellularly recorded CWs, which correspond to the pre-and postsynaptic action potential, respectively (Fig. 1A, see DISCUSSION for more ...

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... evidence for a modulation of ATD was obtained from a sequence of acoustic stimulations which induced different firing rates (FRs, Fig. 1C). ATDs collected from the CWs of these responses differed on the order of 100 s (Fig. 1D). Comparing the ATD for CWs elicited at different FRs indi- cated that the source of ATD changes has a systematic, activ- ity-dependent component. Further support for this hypothesis came from a comparison of the tuning characteristics of FR and ...

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... evidence for a modulation of ATD was obtained from a sequence of acoustic stimulations which induced different firing rates (FRs, Fig. 1C). ATDs collected from the CWs of these responses differed on the order of 100 s (Fig. 1D). Comparing the ATD for CWs elicited at different FRs indi- cated that the source of ATD changes has a systematic, activ- ity-dependent component. Further support for this hypothesis came from a comparison of the tuning characteristics of FR and ATD. Tunings based either on ATD (Fig. 1E) or on FR (Fig. 1F) exhibited a remarkably ...

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... the CWs of these responses differed on the order of 100 s (Fig. 1D). Comparing the ATD for CWs elicited at different FRs indi- cated that the source of ATD changes has a systematic, activ- ity-dependent component. Further support for this hypothesis came from a comparison of the tuning characteristics of FR and ATD. Tunings based either on ATD (Fig. 1E) or on FR (Fig. 1F) exhibited a remarkably similar dependence on the stimu- lus' intensity and frequency. Finally, differences in ATD ex- isted already during spontaneous activity (ATD spont ). For spon- taneous rates between 0 and 113 Hz (median: 10 [2, 29] Hz, n 122), the ATD spont varied between 294 and 748 s (mean: 443 63 s, n 122) ...

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... responses differed on the order of 100 s (Fig. 1D). Comparing the ATD for CWs elicited at different FRs indi- cated that the source of ATD changes has a systematic, activ- ity-dependent component. Further support for this hypothesis came from a comparison of the tuning characteristics of FR and ATD. Tunings based either on ATD (Fig. 1E) or on FR (Fig. 1F) exhibited a remarkably similar dependence on the stimu- lus' intensity and frequency. Finally, differences in ATD ex- isted already during spontaneous activity (ATD spont ). For spon- taneous rates between 0 and 113 Hz (median: 10 [2, 29] Hz, n 122), the ATD spont varied between 294 and 748 s (mean: 443 63 s, n 122) among units. ATD ...

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... and frequency. Finally, differences in ATD ex- isted already during spontaneous activity (ATD spont ). For spon- taneous rates between 0 and 113 Hz (median: 10 [2, 29] Hz, n 122), the ATD spont varied between 294 and 748 s (mean: 443 63 s, n 122) among units. ATD spont tended to be larger in units with high spontaneous activity (r 0.41, P 0.001, Fig. ...

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... was 82 38 s, but increases could be as high as 218 s (Fig. 2E). The average increase of ATD was 0.26 0.11 s/Hz FR. The maximum ATD changes corresponded to an average relative increase of 19 9% and depended strongly on the maximum FR reached in single units (r 0.62, P 0.001, Fig. 2C). Although ATD spont depended on the sponta- neous FR of a unit (Fig. 1B), spontaneous FR allowed no prediction about the activity-dependent increases of ATD (r 0.02, P 0.83). However, in spontaneously active units the spontaneous FRs are often far higher than the minimum FRs because certain stimuli can suppress the firing rate below the spontaneous FR. To check whether the ATD is already ele- vated by ...

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... check if activity-dependent changes of ATD are present in responses elicited by naturally occurring sounds, we used ambient sounds (Fig. 5A1) and gerbil calls (A2) as acoustic stimuli. The first stimulus had mostly low-frequency content and was used to drive low-CF units; the second stimulus was dominated by high frequencies and was used to drive high-CF units. Units responded to the stimuli with changes in their FRs according to the frequency content and the sound ...