Sillito A M, Murphy P C
Department of Visual Science, Institute of Ophthalmology, London.
Eye (Lond). 1988;2 Suppl:S221-32. doi: 10.1038/eye.1988.146.
The translation of the retinal input through the dorsal lateral geniculate nucleus (dLGN) to the visual cortex is highly dependent on a range of influences. This article reviews the available evidence. One of the influences, the corticofugal projection to the dLGN from layer VI of the visual cortex, provides a synaptic input which in magnitude exceeds that from the retina. This makes direct synaptic contact on relay cells and the intrinsic and perigeniculate inhibitory interneurones influencing their activity. The corticofugal system appears to be spatially organised in such a way that for any given region in the dLGN, there is a central zone comprising an overlying field with facilitatory effect, and a surrounding zone with inhibitory influence. The extent to which these overlap is open to question at present. The inhibitory effect of the corticofugal projection can be clearly seen in its contribution to the length tuning of dLGN cells when tested with drifting bars. On average dLGN cells exhibit a very high degree of length tuning, matching that of cortical hypercomplex cells. Removal of the corticofugal influence causes a radical reduction in this, shifting the mean reduction in peak response with increasing bar length from 71% to 43%. One consequence of this corticofugal effect is that the selectivity of the dLGN cell receptive field towards stimuli spatially restricted to the vicinity of the centre mechanism, is as good for moving bars as it is for stationary flashing spots. The retinal output to dLGN relay cells appears to be mediated by excitatory amino acid receptors, of both NMDA and non-NMDA categories. The non-NMDA receptors appear to provide an initial level of depolarisation which enables the operation of the voltage dependent NMDA receptor channels. The NMDA receptor however sits as a critical gate regulating the transmission of retinal information in the dLGN, when it is blocked visual responses are virtually eliminated. Its voltage dependency makes it crucially dependent on the complex pattern of excitatory and inhibitory influences from the cortex and the "non-specific" modulatory influence of the cholinergic system.
视网膜输入信号经背侧外侧膝状体核（dLGN）向视觉皮层的传递高度依赖于一系列影响因素。本文综述了现有证据。其中一种影响因素是视觉皮层第VI层向dLGN的皮质离心投射，它提供的突触输入在强度上超过了来自视网膜的输入。这与中继细胞以及影响其活动的内在和膝状体周围抑制性中间神经元形成直接突触联系。皮质离心系统似乎在空间上是这样组织的，即对于dLGN中的任何给定区域，都有一个中央区，其包括具有促进作用的覆盖区域，以及具有抑制影响的周边区域。目前，这些区域的重叠程度尚存在疑问。当用漂移光条进行测试时，皮质离心投射的抑制作用在其对dLGN细胞长度调谐的贡献中清晰可见。平均而言，dLGN细胞表现出非常高的长度调谐程度，与皮质超复杂细胞的调谐程度相当。去除皮质离心影响会导致这种调谐程度大幅降低，随着光条长度增加，峰值反应的平均降低幅度从71%变为43%。这种皮质离心效应的一个结果是，dLGN细胞感受野对空间上局限于中心机制附近的刺激的选择性，对于移动光条和静止闪烁点来说是一样好的。视网膜向dLGN中继细胞的输出似乎由NMDA和非NMDA两类兴奋性氨基酸受体介导。非NMDA受体似乎提供了初始的去极化水平，从而使电压依赖性NMDA受体通道能够发挥作用。然而，NMDA受体是调节dLGN中视网膜信息传递的关键闸门，当其被阻断时，视觉反应几乎完全消除。其电压依赖性使其关键地依赖于来自皮层的兴奋和抑制影响的复杂模式以及胆碱能系统的“非特异性”调节影响。
