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BackgroundThe auditory cortex of rat which concludes primary auditory cortex and secondary auditory cortex, is divided into four areas according to the characteristics of frequency:auditory cortex area 1(A 1), the area before the auditory cortex (anterior auditory field, AAF), posterior auditory cortex (P), ventral posterior auditory cortex(VP).Primary auditory cortex include A1 and AAF, which are mainly receiving projection from MGV, and secondary auditory cortex include A 2, posterior auditory cortex and other areas outside of the primary auditory cortex, which primarily receive projection of dispersion from outside the MGV.As the other low-level projection neurons, neurons in those areas are not obvious tuning characteristics.In general, A1 and AAF have tonotopically organized, neurons to the low frequency are mainly distributed in the postero-ventral of A1, and the high frequency response of neurons are mainly distributed in the antero-dorsal of Al, response frequency of neurons from low frequency to high frequency gradually increase from caudal to antero in the Al. The response frequency appear reverse on the border of AAF, response frequency of neurons from the border area to the antero area in theAAF is diminishing according high frequency to low frequency. The primary auditory cortex not only presences tonotopically organized in the cross section, but also has certain structure distribution in depth section. A large number of research results have shown that the visual cortex, auditory cortex, somatosensory cortex, motor cortex and other joint cortex of different species has function of column structure. This column structure is the neurons of similar function in the cortex as a vertical arrangement to form a cylindrical structure, is considered to be the basic structure and function unit of the neocortex. Most of researchs use multiple unit records to record pure tone reaction of the different depth of primary auditory, which found the same CF from the surface to the deep cells and confirmed the existence of frequency processing function structure in auditory cortex.According to the construct different, primary auditory cortex can be divided into six layers:1, Layer I, a small number of neurons and developed the neuropil of is the most distinguishing feature of the layer Ⅰ.Which contains more than 90% of GABA inhibitory neurons. Their direction is limited to the layer Ⅰ, Ⅱ, and Contact the pyramidal cells which are located in deep layer (Ⅲ-Ⅴ) formed rich synapses; 2, Layer Ⅱ, which contact with the adjacent cortex, providing the connection between the auditory cortex layer I and Ⅲ. Layer Ⅱ is mainly constituted with medium size of nonpyramidal cells, but there are a little of small pyramidal cells in the deeper half layer (layer Ⅱ b).Their vertical axons can Specific to dominate deep primary cells,and involved in the formation of the funcational column; 3, Layer III, the layer is characterised by pyramidal cells. The top dendritic of small cone cells only extend to the layer Ⅱ, so which is unlikely to be affected by GABA neurons in layer Ⅰ.Large pyramidal cell dendrites is extend to layer , so it can accept a wide range of synaptic connections, including the remote control from ipsilateral and contralateral cortex; 4, Layer Ⅳ, most of the cells is GABA neurons. The cell accepts control from the thalamus, other cortex and internal cortex, and also accepts control from the contralateral cortex through traffic team, this layer is visible axon plexus from pyramidal cells of lay Ⅱ、Ⅲ, and the local axon of layer IV cells; 5, Layer V, the pyramidal cells is larger, the top rough dendrite dry is very little branch, and extend to layer II and I, where the extension and curly structure formed in the cortex parallel to the surface. On the function, the pyramidal cells of layer V are the origin of multiple outgoing control of auditory cortex, including the ipsilateral auditory cortex、the contralateral auditory cortex、MGB、IC、and CN; 6, Layer VI, which is composed with pleomorphic cells mainly dominants subcortical areas.A large number of studies have shown that auditory information in advanced transmission and the information processing of auditory center relies on two ways: one is lemniscal pathway, also known as specific pathway, neurons and organization structure in this pathway which has the tonotopically organized, will transmite information of characteristics of the audio voice from low level auditory nuclei to MGV, up to the cortex layer Ⅲ and Ⅳ, and then projectting outside the MGB ventral area and the nucleus and peripheral receptors after thought cortex integration and treatment; Another way is nonlemniscal pathway, also known as nonspecific pathway, this pathway is nontonotopically organized, and the projection is more diffuse, not limited to primary auditory cortex, might not even primarily projection to the primary auditory cortex area, is known as nonspecific projection system, which comes from the center of lateral area outside the MGB neurons. They are projectting to the auditory cortex, the layer Ⅰ and VI, and the mercies of GABA neurons of layer Ⅰ can affect the other deep pyramidal neurons.Hromadka recorded single cell in youth awake rats, founding that spontaneous and acoustically induced activity in each layer are showed the low issuance rate, and think that there were no significant differences between cortex. Most previous studies have tended to understand and explore frequency response characteristics of sound of A1, but the sound of the secondary auditory cortex response characteristic research is not much, especially the response characteristics change of different layer is less. This experiment used white noise to detect sound, and designd noise x tone and click for stimulation. The probe are recorded with a single blind method, to observe laminar distribution and the characteristics of spontaneous and neurons responses to voice signal in anesthesia rats of Al and A2 and sober mice of Al, and study the processing characteristic-s of auditory cortex in different layers, especially the structure fou-ndation and the loop connection.2. Results2.1 The number of 246 neurons which response to stimulation in AI area was 182 when detecting the anesthetic rats. From the percentage of the responsive cells in each layers of auditory cortex to voice scanned as the order of noise, tone and click, we found:1. The neurons in primary auditory cortex mostly respond to three kinds of voice, which indicating the neurons can react to a variety of sound stimulation and perform a variety of functions in the auditory cortex; 2. The primary auditory cortex may exist nuclei, which in deal with different sounds independently and express different biological significance by the specific pathway completing central information processing; 3. The threshold of neurons response to click shows is significantly different rules compared with other sounds in primary auditory cortex. Neuron response to click is significant different compared with tone and noise in the same layer, prompting that the treatment of neurons to click may be significant different with noise and tone; 4. Neurons response latency to the tone in the layer IV of primary auditory cortex is shortest, the second one in layers Ⅱ/Ⅲ and V, and the longest one in layer Ⅵ. The FSL of the neurons responded to noise between each layer was similar to tone, show that the first layer which receives the single frequency and broadband information in the primary auditory cortex is the layer Ⅳ, in which the signal spread to the shallow and deep;5. The latency of the neurons responded to click was increased from the layerVI to the layer of Ⅱ and Ⅲ, which indicated that the orientation of transfering message was from the deep cortex gradually to the surface; 6. The receptive field of the frequency-intensity is the type of "V" and "U" from the neurons responded to tone.From what has been discussed above, we verified that as lemniscus pathway the first layer in primary auditory corte, the layer IV is mainly responsible for the different frequency information transmission, processing and integration.In addition,we found the neurons response in primary auditory cortex to the transient and narrowband voice showed a different approach and pathways, there is no literature reports.We already know from literature that there is wide and dispersive projection of relation between primary auditory cortex and secondary auditory cortex.There is no tonotopically organized in secondary auditory cortex, whether are there response cells of sound advantage in secondary auditory cortex? Whether does neurons exist laminar response characteristics to different sound elements in the secondary auditory cortex? Whether is there also a function column structure in the secondary auditory cortex? How do different sounds conduct in the secondary auditory cortex? To discuss these problems, we have ordered a probe into the secondary auditory cortex in anesthesia rats.2.2 To explore the neurons of each layer of A2 area in anesthetic rats,157 discharging neurons were observed in A2 area, including 78 neurons which had a response to detect sound and 45 neurons had complete three sound stimulus programs. From the proportion of each layer of cells response to the voice, we got some conclusion:1.Most neurons in secondary auditory cortex in each layer response to three kinds of voice, suggesting that in most of the auditory neurons in secondary auditory cortex also react to a variety of sound stimulation and perform a variety of functions.2. Neurons of each layer had a good response to noise stimulation, while hardly only had a responses to pure tone. This seems to suggest that simple frequency information in the primary auditory cortex is over, and complex frequency information of acoustic signal need to pass to secondary auditory cortex for further analysis and integration.3.Neurons response to click only in the Ⅳ, Ⅴ, Ⅵ layer of secondary auditory cortex had a stable ratio, showing that in the secondary auditory cortex also had independent nucleus of dealing with the click sound.4.In each layer of A2 area, few neurons response to tone had a narrow and deep sharp "V" type receptive field, while the majority shape types of receptive field were the "V" type and "U" type with the width base. There was no function of column structure was found in the secondary auditory cortex area, while comparing the same puncture point of CF, MT and FSL response under pure tone neurons. Perhaps this may relate to secondary auditory cortex receive wide dispersion projection on nonspecific pathway.5. Secondary auditory cortex neurons in response to the noise of the FSL have clear change law showing that layer V was the shortest and longer in layer Wand VI and the longest in layer Ⅱ/Ⅲ. This may suggest that layer Ⅳ may not be the first layer that received noise in secondary auditory cortex.6. FSL of neurons to click in the neurons of secondary auditory cortex was similar with the response in primary auditory cortex showing that the shortest one was in layer Ⅱ/Ⅲ/ Ⅵ,the longer one was in layer V and the longest was in layer Ⅳ. This seems to show that A1 and A2 area are all accept the same input under a central nucleus.7. Neurons showed a different rule when response to the FSL to tone in secondary auditory cortex and in A1. It is in the Ⅱ/Ⅲ、Ⅵ layer shortest, longest inⅣ layer.This suggested that neuron of secondary auditory cortex accept tone stimulation passed from the primary auditory cortexAbove, the description about neurons response characteristics to sound elements in primary auditory cortex and secondary auditory cortex is under the condition of anesthesia, to eliminate the effects of anesthesia on auditory neurons we should add a awake contrast in rats, but because of experimental technology limited, we can’t stable record in awake rats. In order to understand response characteristics to sound elements in the primary auditory cortex of consicous animal, we have carried on the exploration to awake the mice primary auditory cortex.2.390 firing neurons were found in the detection of A1 from consicous mouse, 80 of them had a response to probe sound. The ratio of cells in different layers response to sound indicated a lot of information:1.The high proportion of mixed response neurons in all layers of primary auditory cortex demonstrated that most neurons of auditory cortex had the ability of react to various kinds of sound stimuli and perform different functions.2. Neurons showed a good response to noise in all layers of primary auditory cortex, but prefer to click in layer VI and V.this characteristic is in accordance with Al of anesthetic rat.3.The highest ratio of neurons that only react to click in layer Ⅱ/Ⅲ, especially layer VI and V, and the lowest in layer IV indicated that there was independent group of neurons to process click, and mainly distributed in superficial layer and layer VI.4. FSL of neurons to click was similar with the response in primary auditory cortex of anesthetic rat, indicated a pipeline from deep to surface that differ from the classical lemniscuses pathway. And the prolonged latency also hints a latent mechanism about uploading and process transient information.3. ConclusionBased on the results above, we can conclude that:1.Some neurons in primary auditory cortex would priority to a certain kind of sound, and pure tone is transmitted by the classical lemniscal pathway in primary auditory cortex.2. Noise and pure tone in second auditory cortex are transmitted from primary auditory cortex, and discrepancy existed in the response to noise and pure within different layers.3. The conduction and process of neurons to click are different from noise and pure tone between primary auditory cortex and second auditory cortex, it was transferred in senior auditory system through the non-lemniscal pathway.4. There is no structure of the functional column in second auditory cortex, lager difference of frequency selectivity existed within laminar structure. |
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