| The sound is the media for animals to know the changes of the outside world and communicate with congener. The auditory sensation as a special feeling of sensing the sound supplies a sonic communication basic. The auditory system of mammals has many unbelievable abilities. Its high sensitivity and high accuracy can discriminate and encode the specific sound from the complicate sound environment selectively. As human, because of the development and application of language, the information getting from the auditory system is an important, fast and convenient, and irreplaceable way from some kind of sense. The nuclei of each auditory center system has its own function. The inferior colliculus (IC) neurons receive inputs from the ascending and descending pathway. IC is very important in integrating the auditory information, and was considered to be the most important nuclei in the auditory center system. In the processing of sound discrimination, is there any interrelationship of the IC neurons to encode the sound between tone and click? This is the objective of our paper.The ear is a peripheral sensory organ. It consists of external ear, middle ear and the cochlea of inner ear. The air sparse and density wave caused by the sound source oscillation cause inner ear lymph’s and the basilar membrane’s oscillation, and transform the sound wave mechanical energy to the nerve impulse of the auditory fibers through the phono sensitive transduction of the organ of Corti, and transmit to the auditory cortex to produce hearing. IC is an important nuclei in the auditory pathway, not only receiving ascending projections from the lower brainstem, but also accepting the descending inputs from higher auditory center. The projection form complex neural network of the IC. Existing studies showed that the projection can be divided into inhibitory input and excitatory input. The main ascending input to the inferior colliculus is from the cochlear nucleus, superior olivary complex, lateral lemniscus nucleus, which formed monaural and binaural input pathways. Descending projection to the inferior colliculus is from the auditory cortex(AC), medial geniculate body(MGB) and the superior colliculus(SC). The classification of IC neurons is dependent on the monaural inputs and the binaural integrate information properties, including contralateral neurons, binaural excitation neurons, contralateral excitation and ipsilateral inhibition neurons and other types. IC neurons receive direct or indirect, monaural or binaural ear, ipsilateral or contralateral, inhibitory or excitatory inputs from many nucleus, which provide the anatomical and physiological basis for the inferior colliculus in binaural information integration. IC receives the ascending binaural inputs from the lateral superior olive(LSO) and the medial superior olive(MSO). The projection from the MSO is excited, and the projection from the ipsilateral LSO is inhibited. And the projections from contralateral LSO are inhibited or excited. In addition, the dorsal nucleus of lateral lemniscus(DNLL) provided binaural IC and contralateral DNLL inhibited inputs. The projections from LSO, MSO and DNLL axon periphery interlap, which indicate that it is complicate to integrate the auditory information in the IC nucleus.The electrophysiology and histochemistry studies indicated that, IC had its own tonotopic organization, and the isofrequency area in IC was very regular. The low frequency area lied in the dorsal lateral of IC, and the high frequency area lied in the ventral of IC. When encoding the sound frequency, the different IC neurons represent different frequency tuning curve(FTC). In the superficial layer of dorsal IC and lateral nucleus of IC, the neurons’tuning curves were broad and irregular, and the central of IC(ICC) and the frequency tuning curve of the deep layer of dorsal cortex of IC were narrow. The FTC of ICC can divide into three types:①V-shape tuning curve:As the intensity of sound increased, the FTC expand to low frequency and high frequency area.②U-shape tuning curve:Also called intensity tolerance tuning curve. The width changes little as the sound intensity increased.③closed-shape tuning curing:Also called high threshold tuning curve. The neurons do not response to high intensity with best frequency. Because there is a threshold in low and high intensity respectively, they shape a closed tuning curve. ICC neurons’ frequency selectivity are different as the sound direction are different. In the free sound field, when the sound source turns from contralateral to ipsilateral, most neurons frequency tuning curves become narrow from broad, representation as the Q values become high from low. In the sound direction dependent frequency selectivity formation, binaural inhibition is very important.Click is the sound produced by52-200μs square concurrent pulse exporting to the ear phone or loudspeaker. From the time domain property of click from the ear phone or loudspeaker, the shapes of the acoustics information and electrical information are quite different, because the frequency response area from the ear phone or loudspeaker is very limit, like a bandpass filter which filter the very low and very high frequency from sound information. The duration of click can shorten to milliseconds, and its frequency spectrum extent is very broad. The properties of click are dependent on the transient response characteristics of the ear phone, not the width of the electrode pulse which determine the impact energy with pulse amplitude and affect the intensity of click. Click was applied in clinic generally, i.e., the auditory brainstem response(ABR) evoked by click can use to judge the hearing level and cochlear pathology of the subjects.In this study, we used open sound field to give tone or click contralaterally. The loudspeaker was located10cm away from the right ear, and the left IC area was exposed and covered with liquid paraffin to prevent the surface of the tissue from dry. Single-unit recordings in vivo were made with micropipettes, and data were acquired and processed with patch clamp system HEKA EPC800amplifier and Tucker-Davis Technologies(TDT) system3. Firstly, pure tone with50ms duration and5ms rising/fall time were played through BrainWare software contralaterally, and record the receptive field of IC neuron to tone. Secondly, giving click with0.1ms duration to test whether this neuron response to click or not. If so, the same pure tone as the first step were exposed to the mouse again to record this neuron’s receptive field after giving click. If not, the neuron was passed. After experiment, we analyzed the relative parameters of the IC neurons receptive field to approach the relationship of IC neurons when encoding click and tone. IC neurons receive binaural inputs, and the projection from the contralateral superior olive complex to IC were excitable, and the projections from ipsilateral LSO to IC were inhibitive. The projections from the high central cortex to IC were excitable mainly. Except for the projections from the ascending pathway to IC, IC neurons also accepted the projections from the contralateral IC and inner IC. The projections from one side to the other side were excitable or inhibitive. In our studies, because we used open sound field to exposed stimuli, in order to reduce the ipsilateral nucleus projections to IC, we plugged the ipsilateral ear of the mouse with agar to record the excitable inputs from the contralateral nucleus. In this way, we can analyze the relationship of click and tone in IC. Totally,28inferior colliculus neurons were recorded in mouse, which responded to both click and tone. In the recording processing, we found that not all neurons responded to click, and such neurons had a good receptive field to tone. With regard to these neurons, we did not make any experiment at all. We used the characteristic frequency(CF), the minimum threshold(MT), bandwidth(BW) and spike counts(SC) of neurons response RF as indexes to compare the changes of IC neurons responses to tone after giving click. We found that some neurons’responses to tone after giving click had changed, their CFs or shifted from low frequency to high frequency or from high frequency to low frequency, and did not recover any more. And their RF shapes had changed too. The MTs of these neurons also higher than those before giving click, that is to say, after giving click, higher intensity needed to induce the IC neurons discharge. The BWs of the IC neurons’receptive field or became larger, or became smaller. The spike counts had changed too. After statistics analysis, these changes had statistics significance. In other words, click can affect the processes of IC neurons to tone, and the changes were irreversible.In the auditory research, there is a phenomenon called auditory masking. Masking can define as the interference of one sound to another sound generally. When the two different sound stimuli present to the human ear, the two sound will interfere, which will make one sound be tested uneasy. This phenomenon is called masking effectiveness. In the masking experiment, one sound called target or signal sound, the other sound called masking sound. According to the time relationship between masking sound and test signal, masking can divide into simultaneous masking and non-simultaneous masking. The latter one can divide into forward masking that the masking the sound presented before test signal, and backward masking that the masking sound presented after test signal. As the intensity of masking sound increased, or the intensity of test signal decreased, we will experience a continuous excess from hearing to non-hearing, which indicated the part masking happened. The masking effectiveness had two characteristics:First, when the masking sound was close to the test signal, the masking effectiveness would be obvious. When the masking sound was far away from test signal, the masking effectiveness would be weakened as follow. When the distance of the masking sound and the test signal was far enough, the masking effectiveness would disappear. Second, the time course of forward masking and backward masking was different. The time course of forward masking was100-200ms, which was much longer than that of backward masking(10-20ms).We can conclude that:Click can change the response properties of IC neurons to tone, which demonstrate that when encoding different sound in the auditory system, the responses properties of neurons themselves to a specific tone can be changed. |
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