以神經群記錄探討大鼠大腦皮質初級體感覺區各分層之觸覺與傷害覺

Abstract

大腦皮質初級體感覺區 (primary somatosensory cortex , S1)為處理周邊體感覺之高等中樞核區。關於此腦區的細微結構多來自其解剖學上的證據，諸如細胞結構分佈、核區間之連結與其相對於周邊體感覺區的關係。於本研究中，我們將解析大鼠S1後肢區同一柱狀結構 (column)內各分層 (lamina)神經群對於不同形式體感覺之反應，並由功能面來洞悉大腦皮質細微結構存在之意義。 為同時獲得各分層神經群產生之神經訊號，我們採用16通道的神經探針，以單一次插入即可盡可能覆蓋所有分層之方式進行記錄。擷取之神經訊號將分別濾波以得到高頻多神經元之動作電位與低頻之場電位信號。並經由濾波積分法量化多神經元動作電位，以及利用電流源密度分析量化場電位。另外，藉由更換施予同一周邊體感覺區的觸覺與二氧化碳雷射刺激，我們可以獲得同一大腦皮質柱狀結構各分層對於非傷害覺與傷害覺之神經訊號。 由於多神經元動作電位以及場電位均會因雷射刺激而出現顯著且可被重覆引起的反應，因此證實大鼠的初級體感覺皮質區確實與處理周邊體表的傷害覺有關。同時，藉由比較柱狀結構內各分層神經群對於同一周邊出現的非傷害覺與傷害覺之反應，各分層的功能也相對顯而易見：第四層 (the layer Ⅳ)與第五層 (the layer Ⅴ)之淺層無論對哪一種刺激均有明顯反應，足見其功能與接收感覺路徑由視丘上行進來的訊號有關，亦再次支持初級體感覺皮質區具備定位周邊刺激位置之功能；表淺的皮質分層，即第二與第三層 (the layer Ⅱ/Ⅲ)，則只與非傷害性感覺的處理相關。對於大腦皮質之同一柱狀結構對非傷害性及傷害性刺激均有反應，除驗證大腦皮質柱狀結構之存在之外，也可能意謂著單一柱狀結構的功能與動物大腦皮質區所存在的柱狀結構數目息息相關。

The understanding of cortical microcircuits was traditionally based on the anatomical province, including cytoarchitecture, connections between nuclei, and the relationship to peripheral receptive fields. In this study, with insight into their responses to different sensory modalities, we explore the columnar and laminar functions in the hindpaw region of the primary somatosensory cortex (S1) in the rat. The 16-channel neural probe was used in this study to simultaneously acquire the neural responses among laminae in a column. High-frequency multi-unit activities (MUAs), as well as low-frequency field potentials (FPs), were recorded and filtered. Rectification and integration for MUAs and current-source-density analysis for FPs were performed to quantitatively analyze their responses respectively. Furthermore, to study the responses of the same column to different modalities, not only non-noxious mechanical stimuli, but also noxious CO2 laser stimuli were delivered to the same receptive field on the hindpaw of the rat. The results demonstrate that S1 of the rat is actually involved in the processing of nociception, because of the constant latency characteristics of the laser-evoked MUAs and FPs. Comparing non-nociceptive and nociceptive responses in the same column, laminar functions of cortical columns could be examined. The layer Ⅳ and the superficial layer Ⅴ with consistent higher activities, no matter which modality was applied to the periphery, receive the thalamocortical projection, while the superficial cortical lamina, the layer Ⅱ/Ⅲ, just processes innocuous input. The parallel coding of different sensory modalities in a single column further corroborates the columnar organization in S1 and implies the idea that the functional role of a column might depend on the number of cortical columns among different species.