Koralek K A, Olavarria J, Killackey H P
Department of Psychobiology, University of California, Irvine 92717.
J Comp Neurol. 1990 Sep 8;299(2):133-50. doi: 10.1002/cne.902990202.
The present study examines patterns of connectivity between the primary somatosensory cortex of the rat (SI) and surrounding cortical areas also implicated in the processing of somatosensory information. The impetus for the study was the recent reports of major differences in the organization of cortex lateral and caudal to the SI in two other rodent species; the mouse (Carvell and Simons, '86: Somatosens. Res. 3:213-237; '87: J. Comp. Neurol. 265:409-427) and the grey squirrel (Krubitzer et al., '86: J. Comp. Neurol 250: 403-430). Corticocortical connections between the somatosensory areas of the rat parietal cortex were examined by using the combined retrograde and anterograde transport of horseradish peroxidase as well as the retrograde transport of fluorescent tracers. Tracer injections were made into different locations within SI and dysgranular cortex as well as into more lateral regions of parietal cortex. The tangential patterns of distribution both of callosal connections and of cytochrome oxidase activity together provided points of reference in determining the relation between injection sites and the resultant patterns of label. The results indicate that two distinct somatosensory areas, SI and the dysgranular cortex, are interconnected with a further lateral somatosensory area referred to as the second somatosensory area (SII). These projections are organized in a topographic fashion, which we interpret as evidence for a single representation of the body surface in SII. The three somatosensory areas each exhibit unique laminar patterns of ipsilateral corticocortical projection neurons and terminations. In SI, projection neurons are found mainly in layers II, III, and Va, and terminations are largely restricted to the infragranular layers. In the dysgranular cortex, projection neurons and terminations are found in all layers except layer I in which only terminal label is detectable and layer Vb in which notably fewer neurons are labelled. In SII, projection neurons and terminations are found in all layers except layer I and are particularly dense in lower layer III and layer IV. Further, whereas the laminar and areal distributions of ipsilateral and contralateral corticocortical projections largely overlap in both SI and the dysgranular cortex, in SII they tend to be areally segregated. Neurons projecting bilaterally to both ipsilateral and contralateral somatosensory cortex were equally rare in all three somatosensory areas. These results are discussed in relation to the organization of SII in other rodent species, and it is concluded that in the rat, like the mouse, cortex lateral and caudal to SI contains a single representation of the body surface.
本研究考察了大鼠初级躯体感觉皮层（SI）与周围同样参与躯体感觉信息处理的皮层区域之间的连接模式。开展这项研究的契机是最近有报道称，在另外两种啮齿动物中，SI外侧和尾侧的皮层组织存在重大差异；这两种动物分别是小鼠（卡维尔和西蒙斯，1986年：《躯体感觉研究》3:213 - 237；1987年：《比较神经学杂志》265:409 - 427）和灰松鼠（克鲁比策等人，1986年：《比较神经学杂志》250:403 - 430）。通过使用辣根过氧化物酶的逆行和顺行联合运输以及荧光示踪剂的逆行运输，研究了大鼠顶叶皮层躯体感觉区域之间的皮质 - 皮质连接。将示踪剂注射到SI和颗粒下层皮质的不同位置以及顶叶皮层更外侧的区域。胼胝体连接的切线分布模式和细胞色素氧化酶活性共同为确定注射部位与最终标记模式之间的关系提供了参考点。结果表明，两个不同的躯体感觉区域，即SI和颗粒下层皮质，与另一个更外侧的躯体感觉区域相连，该区域被称为第二躯体感觉区（SII）。这些投射以拓扑方式组织，我们将其解释为SII中体表单一表征的证据。这三个躯体感觉区域各自表现出同侧皮质 - 皮质投射神经元和终末的独特分层模式。在SI中，投射神经元主要见于II、III和Va层，终末主要局限于颗粒下层。在颗粒下层皮质中，除了仅可检测到终末标记的I层和标记神经元明显较少的Vb层外，在所有层中都发现了投射神经元和终末。在SII中，除I层外，在所有层中都发现了投射神经元和终末，并且在III层下部和IV层中特别密集。此外，虽然同侧和对侧皮质 - 皮质投射的分层和区域分布在SI和颗粒下层皮质中大部分重叠，但在SII中它们往往在区域上是分离的。在所有三个躯体感觉区域中，双侧投射到同侧和对侧躯体感觉皮层的神经元同样罕见。结合其他啮齿动物物种中SII的组织情况对这些结果进行了讨论，得出的结论是，在大鼠中，与小鼠一样，SI外侧和尾侧的皮层包含体表的单一表征。
