| Temporal and frequency features in acoustic signals are important for sound recognition. Most information contained in signals for communication, such as speech, is embodied in the temporal fluctuations of both the amplitude and spectra. Therefore, temporal features are important substrate in converting information. However, the mechanisms of temporal processing and amplitude features in the central auditory system are far less understood than those for frequency processing. This study aimed to explore the possible mechanism of temporal processing and amplitude features in the central auditory system by analyzing the two response parameters of auditory neurons:spike counts and spike timing.Our study included three parts:In the part I, we recorded extracellularly from single units located in the inferior colliculus (IC) of barbiturate-anesthetized BALB/c mice. The stimuli were pure tones at characteristic frequency (CF) with different durations and amplitudes. In the partâ…¡, we used the whole-cell patch clamp recording technique to investigate the intrinsic membrane properties of IC neurons. We examined the effects of amplitude and duration of membrane depolarization and hyperpolarization of different waveforms on the spike response of IC neurons. In the part III, we used the whole-cell patch clamp recording technique to investigate the intrinsic membrane properties of the MNTB pricinpal cells. We also examined the effects of amplitude and duration of membrane depolarization and hyperpolarization of different waveforms on the spike response.Partâ… The previous studies about the acoustic stimulus duration response of auditory neurons were focused on duration tuning or selectivity, which were based on the spike counts or spike probabilities of auditory neurons. However, the study of using the response parameter-spike timing is very scarce. And the only one or two study just used the special acoustic amplitude. Duration tuning has been well documented in central auditory system of several species of bats. Recently, duration tuning has also been found in non-echolocation mammals in different stations along the auditory pathway. However, the numbers of duration tuning neurons of non-echolocation mammals were found smaller than in bats in these studies. Moreover, the stability of duration tuning has been disputable. In some studies they have suggested that the properties of duration tuning will shift with other stimulus parameters, such as the stimulus amplitude, stimulus repetition rate, et al.In the first part of the paper, we recorded extracellularly from fifty single units located in the IC of barbiturate-anesthetized BALB/c mice. The stimuli were pure tones at characteristic frequency (CF) with different durations and amplitudes. These neurons were analyzed by spike count (SC) and first spike latency (FSL) respectively to detect both their duration and amplitude functions. Their CFs and MTs ranged from 7 to 42 kHz and from 10 to 80 dB SPL. Most of neurons (84%) showed onset pattern response. And about 42% of IC neurons showed some duration selectivity.36% of duration tuning neurons are long-pass tuned. Most of the neurons (56%) in this group were the onset firing pattern. Thus, the number of duration tuning neurons in mice is comparable to that in bats. However, some of the duration tuning neurons were dependent of acoustic stimulus amplitudes. And some studies considered that these unstable tuning neurons were not the truly duration tuning neurons. Here, we regarded these duration tuning neurons as the diversity of the intrinsic properties of auditory neurons to deal with the diversity of auditory information. After adding the amplitude parameter, the duration tuning abilities of these neurons were more powerful. On the other hand, the response time of these duration tuning neurons did not exist acoustic amplitude dependency. Therefore, the duration information of acoustic stimulus is not coded by the spike timing of auditory neurons but by the spike counts or spike probability.In this part of study, we also explored the amplitude response of IC neurons at different stimulus durations. We observed that the amplitude functions showed both diversity (monotonic pattern, non-monotonic pattern, saturated pattern) and unstableness when using spike count; meanwhile using FSL, the amplitude functions showed surprisingly oneness of characters. Therefore, FSL is a better parameter than SC in evaluation of the response of a neuron to acoustic stimulus amplitude in mouse inferior colliculus.Part IIPrevious studies have shown that the interplay between the excitation and inhibition in the IC contributes to auditory processing, selectivity and tuning of many parameters of the acoustic signal. In the studies of bats, the role of inhibitory inputs in duration tuning was clearly demonstrated. To the best of our knowledge, however, the role of inhibition in duration tuning has not yet been verified in non-echolocating mammals. Many in vivo electrophysiological studies have indicated that synaptic inhibition plays an important role in determining the output of auditory neurons. Inhibition regulates the firing rate, shapes the tuning curves, rate level functions and firing patterns of IC neurons in response not only to pure tone bursts but also to complex sounds, communication sounds or binaural signals. Timing of inhibition is also very important for encoding specific attributes of sounds. For example, an excitatory rebound that emerges after an inhibition can summate with a late excitatory response to enhance the neuron's membrane excitation and increase the possibility of its firing. The interaction of excitation and inhibition can be a neuronal mechanism for encoding of specific characteristics of sounds. In response to hyperpolarizing current injection, a striking feature is rebound depolarization (RD), Often accompanied by one or two anode break action potentials (APs), following membrane hyperpolarization. RD has been observed in many neurons in other regions of the brain, including cerebellum, hippocampus, thalamus, subthalmus and hypothalamus. Most of the neurons exhibiting the RD have rhythmic spontaneous activity in vivo. The RD is known to be critical in controlling firing pattern and regulating the intrinsic status of the neuron. Postinhibitory RD in IC neurons has been proposed as an important component to encode particular features of a sound, eg., duration, direction of frequency-modulated sweep, rate of periodic frequency amplitude modulations and sound gaps.In the second part of the paper, we used the whole-cell patch clamp recording technique to investigate the intrinsic membrane properties of IC neurons We examined the effects of amplitude and duration of membrane depolarization and hyperpolarization on the spike response of IC neurons. Whole-cell patch clamp recordings were obtained from 20 neurons for the present study. All neurons were triggered to fire by depolarizing current injection. There were seven of twenty neurons responded to injections of hyperpolarizing currents. The result showed that for the depolarizing current injection, the response latency decresed as the current level increased but independent of current duration; for the hyperpolarizing current injection, the response latency were related to both the current level and the current duration.Part IIIThe synaptic connecions in central nervous system were very complicated. Most neurons receive multi-synaptic inputs. Simultaneous whole-cell recordings are difficult to make sure the relationship of the input and output. Fortunately, there is one special synapse, the calyces of Held in the auditory pathway. The calyx of Held is thought to arise from globular bushy cells in the anterior ventral cochlear nucleus (aVCN), which project onto principal neurons of the contralateral the medial nucleus of the trapezoid body (MNTB). The MNTB principal cells provide inhibitory glycinergic projections to neighbouring nuclei in the superior olivary complex, including the lateral superior olive (LSO) and the medial superior olive (MSO). The LSO and MSO are the first nuclei in which binaural information converges, with the calyx of Held/MNTB synapse forming a fast "inverting" relay, at which excitation originating from the contralateral cochlea is converted into inhibition to the ipsilateral auditory brainstem. The large size of the calyx of Held allows it to harbour hundreds of active zones and thus a single presynaptic action potential (AP) releases hundreds of quanta, generating a large EPSC that rapidly depolarises the MNTB neuron to threshold. Hence, the large size of the presynaptic terminal guarantees rapid signalling, preserving the timing information of the acoustic signal for processing by the binaural circuits underpinning sound localisation. A single MNTB principal neuron receives input from only one calyx-type synapse, although multiple calyceal inputs are occasionally observed in-5% of principal neuron recordings in mice and-20% of the afferent fibres give rise to two calyces on separate MNTB principal neurons. In addition to the calyceal input, principal cells receive conventional excitatory synapses and inhibitory inputs. The calyx of Held has also become a model of studying synapse. During the last decade, this preparation has been increasingly employed to investigate basic mechanisms of synapse in the central nervous system. In the part three of the paper, we used the whole-cell patch clamp recording technique to investigate the intrinsic membrane properties of principle cells of MNTB. We examined the effects of amplitude and duration of membrane depolarization and hyperpolarization on the spike response of principle cells. Whole-cell patch clamp recordings were obtained from 40 neurons. All principle cells were triggered to fire by the depolarizing current injection. There were thirteen of forty principle cells responded to the injections of hyperpolarizing current. Also we analyzed the effect of the current waveform on the first spike latency-amplitude functions and first spike latency-duration functions. The result showed that for the depolarizing current injection, the response latency depended on the level of the onset of the depolarizing current injection. And the spike counts were related to the subsequent amplitude and duration. For the hyperpolarizing current injection, the response latencies were related to the amplitude and duration of the hyperpolarizing current injection. Higher levels of hyperpolarizing current induced a larger RD and anode break APs, and shorter first spike latency; longer durations of hyperpolarizing current induced a shorter time to back to resting membrane potential and shorter first spike latency.SummaryBased on the three part of our study, we can speculate the encoding way of in the central auditory system to stimulus information as follows:For the excitation afferent information, the central auditory neuron can encode the information by spike timing and spike counts. The first spike latency can present the stimulus amplitude at the onset of stimulus duration. The larger amplitude at the onset stimulus duration, the shorter the first spike latency. And the spike counts can encode the amplitude of the subsequent stimulus duration, thus can present the stimulus duration. The longer the stimulus duration and the larger the amplitude of the subsequent stimulus duration, the more spikes fired.The inhibition afferent information could mdodified the membrane excitability of the neurons. And the timing of inhibition is also very important for encoding specific attributes of sounds. For example, an excitatory rebound that emerges after an inhibition can summate with a late excitatory response to enhance the neuron's membrane excitation and increase the likelihood of its firing. The interaction of excitation and inhibition can be a neuronal mechanism for encoding of specific characteristics of sounds. The central auditory neuron can encode the inhibition information by spike timing. When the amplitude of-inhibitory information was at the low level, first spike latency can encode the stimulus amplitude. As the stimulus amplitude increased, the spike of neuron emerged earlier and earlier. But when the amplitude of inhibitory information was at the high level, first spike latency can encode the stimulus duration. As the stimulus duration lengthened, the spike of neuron emerged earlier and earlier.
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