| When we blocked an ear,we found that the sound clarity will decline a lot and the position of sound is not easy to judge,because the binaural information integration mechanism of auditory system play an important role.When the source is not located in the center of the people or animals space,it will reach the ears has interaural intensity difference(IID)and interaural time difference(ITD),binaural information integration nuclei encode these differences,which would be helpful to the positioning of the sound source,moreover binaural information integration nuclei through audio information processing of binaural ear,thus to identify the sound.In auditory nuclei of interior colliculus(IC)is a very important nuclei,binaural information integration is the central nuclei in the auditory system,not only receiving ascending projections from the lower brainstem but also accepting the descending inputs from the higher auditory center.These projections form the complex neural networks of the IC.Previously extracellular recording indicate that IC neurons receiving binaural ears,or single ear,directly or indirectly,inhibitory or excitatory project,and provide the anatomical and physiological basis for binaural information integration of the IC.In the IC binaural neurons,some binaural properties of neurons come from the lower direct input of binaural neurons,relative to a single input source,mainly comes from the super olive complex(SOC);Some binaural properties of neurons is formed within the IC,the input source is more complex,are still not entirely clear.Previously extracellular recording indicate that binaural response is determined by contralateral ear response,and ipsilateral ear to basically be gain control function,the gain function is usually characterized by inhibition.So inferior colliculus in the ears in the process of information integration,ipsilateral ear to the inhibition of via?What kind of synaptic circuits to ipsilateral IC is still not clear.This experiment through on the different experimental animal models,using in vivo single cell and in vivo whole-cell recording method to research the input loop of synapses,binaural information integration is analyzed on the loop.First,under the condition of closed acoustic field,records a total of 46 ICC neurons,ipsilateral ear,contralateral ear,and binaural,at the same time for different frequency and intensity of pure tone,found that the ICC neurons binaural sound response is determined by the reactions to contralateral ear,and ipsilateral also can record to the reaction,but binaural response significantly weaker than contralateral ear response,ipsilateral play inhibition effect,and the size of the inhibition has nothing to do with ipsilateral sound response,the response threshold value is usually higher than contralateral response threshold of 30 dB.Where the ipsilateral ear sound response from?To solve this problem,we adopt under the condition of open sound field,block the mouse ears of experimental animal model of single-unit records,ears closed models by using cotton ball,soft paraffin and plasticine encloses the mice ears on both sides,then records the ICC neurons,under the condition of ears closed records the ICC neuron's receptive field(RF),and then open the record with ear plugs,record the neuron's RF again,and then open the record to give ear plugs the neuron's RF,respond to 11 sound recorded neurons analysis found that ears closed under the condition of the ICC,noting that the generally high acoustic response threshold of neurons are in more than 50 dB,after open the ipsilateral ear plugs neuron response threshold and RF has no obvious change,but open the contralateral ear neurons reaction that have an obvious changes,threshold is lower 30 dB,the RF also significantly larger.in addition we also through the auditory nerve anesthesia side single-unit records.the result indicate that lidocaine anesthesia(local anesthetics)on one side of the auditory nerve,the contralateral are records of the ICC neurons action potential disappear entirely,action potential recovery after local anaesthetic effect vanished.Blocked binaural model and the results showed that the closed of sound field on one side the auditory nerve anesthesia with ear,under the condition of reaction may be caused by bone conduction effect.In order to better study the loop of inhibition and eliminate the effects of bone conduction effect.we finally adopted unilateral cochlear damaged mouse model,on the basis of the model of bilateral IC single-unit records,found on the same side of the cochlea damage(normal contralateral ear)neurons recorded sound response.On the seven cochlear damage model,at the same side of the cochlea damage of ICC 105 cells were sealing and in which the acoustic response of cell number is 70,and no reaction cell digital 33,with completely different is in the contralateral cochlea damage.In the cochlea damage on the side of the ICC,70 cells were sealing and to respond to the sound of the cell number is zero.Single ear damage results show that in the cochlea damaged the contralateral ICC don't record any reaction neurons,ipsilateral can't make it on the same side the ICC neurons action potential.This with our previous ipsilateral ear to sound alone under the closed sound field conditions can cause the ipsilateral acoustic response ICC results are different.The result shows that bone conduction effect,as with the inhibitory effect of ear,and ipsilateral ear to the reaction itself has nothing to do,so ipsilateral ear to the inhibition of what through the loops?At the single-unit level,we cannot record sound reaction neurons in contralateral ICC,when we damage on one site cochlea.so we need whole-cell recording at the synapses level research.By in vivo whole-cell recording found in cochlea damaged the contralateral ICC can record the excitatory postsynaptic currents(EPSC)and inhibitory postsynaptic currents(IPSC),but in the single-unie recording level,we can't record sound reaction response neurons.Inhibitory input sources,there may be three loops:1.The inhibitory input from low nuclei directly ascending.2.Through the contralateral inhibition of ICC to input.3.Auditory cortex(AC)on the same side of ICC inhibitory regulation.For 1 directly ascending projection in the nuclei of low,mainly from the ipsilateral LSO and bilateral DNLL previous research has very clear.For the contralateral inhibitory input loop between the ICC is still not clear.especially the synaptic loops.So we still use mouse model of unilateral cochlear damage in the cochlea damage was carried out on the side the ICC whole-cell recording at the same time.Through on the same side of the cochlea damage(normal contralateral ear)ICC muscimol injection(a kind of GABAa receptor agonist,long time silence neurons)silence the ICC.In the cochlea damage contralateral recorded four neurons,found recorded by IPSC and EPSC amplitude in varying degree is reduced,but the amplitude of the IPSC reduce EPSC amplitude to reduce the proportion of larger proportion.Between the ICC have synapses connection.Because of the contralateral input decreased but not completely disappear,there are other sources.So we choose silence side AC to study AC regulation function of the ICC.We choose on the same side of the cochlea damaged neurons recorded the ICC,with lidocaine in silence the AC side.Due on the same side of the cochlea damage(normal contralateral ear)can record to single cell response neuron,so we carried on the single record,in a single cell level,we recorded the total of eight ICC neurons,found silence ipsilateral ICC neurons after AC response before the delivery of a marked change,RF Narrows,the total number of action potential distribution to reduce,etc.,showed the AC to the ipsilateral ICC for excitatory regulation.Then we by in vivo whole-cell recording methods to research the regulation mechanism of synapses,12 ICC neurons were recorded.After AC silent ipsilateral recorded by the ICC EPSC amplitude decreased significantly,and the extent of IPSC changed little.From the point of our existing data,the AC on the same side of the ICC to excitatory regulation and mechanism of AC in ipsilateral ICC through the EPSC regulation.Above all,the ICC binaural sound response is determined by the contralateral,ipsilateral main inhibition,but itself has nothing to do with the size of the inhibition.under the conditions of closed sound field,the ipsilateral response is caused by bone conduction effect.ICC accept ipsilateral input,the inhibitory input besides input directly by the low nucleus,there is the contralateral ICC neuron's inhibitory input.It shows that the ICC is part of binaural information integration,and the excitement of lateral inhibition of the ipsilateral projection relationship which enhance the difference between the ears,helps to sound positioning and the effective extraction of sound.In addition,in the process of sound transmission exists bone conduction effect,the bone conduction transfer is due to the skull caused by the vibration,bilateral acoustic information transmission without any differences,and give ear to the inhibitory mechanism can increase the difference between the ears,help to eliminate adverse effect brought by the bone conduction.According to the above experimental results,the following conclusions:(1)Under the conditions of closed sound field,the ipsilateral response is caused by bone conduction effect.2.The inhibition of ipsilateral from low direct projection and contralateral ICC,suggests that ICC is part of binaural information integration.3.The AC on ipsilateral ICC for excitatory regulation. |
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