Response Characteristic Of Primary Auditory Cortex Neurons And Its Functional Development In Rat

The functional organization of the cerebral cortex is not rigidity or unchangeable but have great capacity for plasticity. Many studies have demonstrated that the plasticity of the cortex are most significant during the early life of the animal but still exist even in the adult animals. Some significant biological events are especially important in the development of the cortex since they have influence on the functional organization both in developing and mature periods. So, the mature cortex can also make adjustments in order to continuously adapt to environmental changes. However, many studies have demonstrated that early postnatal period is the most important period for the development of the cerebral cortex. During such period, the functional areas of the neocortex are quickly specified. Also, the neural plasticity is extreme and environment has the greatest influence on its development. So we call this period as"critical Period". Though, many studies have proved that acoustic experience in the critical period can change the normal tonotopic organization of the primary auditory cortex (A1) and influence its function to process auditory information, there is no study on the onset, duration and closing of the critical period. So, investigating the development of the functional organization of the primary auditory cortex and the time course of the critical period from a developmental aspect is significantly important.In this study, we firstly investigated the response characteristic of the neuron in the primary auditory cortex and analysis its excitory Frequency Response Area (eFRA) by extracellular recording technique. Then we explored the development of the primary auditory cortex in rat and the influence of the acoustic stimuli on the formation of the tonotopic map of A1 during critical period by tonotopic mapping technique. The results are described as follows:1. According to the shape of the (eFRA).we can classified them as 6 classes, that is, U-shaped (29.5%), V-shaped (20%) Lower-tailed- upper-sharp (19%), circumscribed (15%), slant-lower (7.5%) and multi-peaked (9%). Each class distributes differently in the A1.Besides, the relationship of the Rate-Intensity of most A1 neurons are monotonicity.2. We can record tone-evoked responses with high response threshold (60-80dB SPL), long minimum latency (31-35ms) and limited response frequency range (6-10 KHz) as early as postnatal day 13. Then, as the development of the auditory cortex both threshold and latency gradually decreased and reached their adult level at postnatal day30 when the response threshold decreased to 13.2±2.0dB SPL and response latency decrease to 13.1±1.2ms for neurons with BF high than 10KHz, also, they decreased to 32.3±2.5dB SPL and 16.1±2.2ms respectively for neurons with BF lower than 10KHz..3. Exposure to 8KHz tone stimuli during different period of early postnatal time resulted in frequency representation difference in A1 between different groups. After recording the tonotopic map of A1 in each exposure group, that is, P9-15, P16-22, P23-29, and P30-36 at postnatal day30-34 (37-40), We found that comparing with normal group frequency representation area that response to 8±1kHz was significantly enlarged in exposure group P9-15. But in exposure groupP16-22, P23-29 and P30-36 we can record such changes. Besides, when we record the tonotopic map at postnatal day 80, we found the result had no significant difference with result recorded in P30-34 (37-40).Conclusion: postnatal day 9-15 is the most important period for the development of the primary auditory cortex. The effects of the acoustic environment can significantly influence the organization of the tonotopic map in A1 during this short period and this effect seems lasting.