Dürr V, Egelhaaf M
Lehrstuhl für Neurobiologie, Fakultät für Biologie, Universität Bielefeld, D-33501 Bielefeld, Germany.
J Neurophysiol. 1999 Dec;82(6):3327-38. doi: 10.1152/jn.1999.82.6.3327.
In this comparative in vivo study of dendritic calcium accumulation, we describe the time course and spatial integration properties of two classes of visual interneurons in the lobula plate of the blowfly. Calcium accumulation was measured during visual motion stimulation, ensuring synaptic activation of the neurons within their natural spatial and temporal operating range. The compared cell classes, centrifugal horizontal (CH) and horizontal system (HS) cells, are known to receive retinotopic input of similar direction selectivity, but to differ in morphology, biophysics, presence of dendrodendritic synapses, and computational task. 1) The time course of motion-induced calcium accumulation was highly invariant with respect to stimulus parameters such as pattern contrast and size. In HS cells, the rise of Ca(2+) can be described by a single exponential with a time constant of 5-6 s. The initial rise of Ca(2+) in CH cells was much faster (tau approximately 1 s). The decay time constant in both cell classes was estimated to be at least 3.5 times longer than the corresponding rise time constant. 2) The voltage-Ca(2+) relationship was best described by an expansive nonlinearity in HS cells and an approximately linear relationship in CH cells. 3) Both cell classes displayed a size-dependent saturation nonlinearity of the calcium accumulation. Although in CH cells calcium saturation was indistinguishable from saturation of the membrane potential, saturation of the two response parameters differed in HS cells. 4) There was spatial overlap of the calcium signal in response to nonoverlapping visual stimuli. Both the area and the amplitude of the overlap profile was larger in CH cells than in HS cells. Thus calcium accumulation in CH cells is spatially blurred to a greater extent than in HS cells. 5) The described differences between the two cell classes may reflect the following computational tasks of these neurons: CH cells relay retinotopic information within the lobula plate via dendritic synapses with pronounced spatial low-pass filtering. HS cells are output neurons of the lobula plate, in which the slow, local calcium accumulation may be suitable for local modulatory functions.
在这项关于树突钙积累的比较性体内研究中，我们描述了家蝇小叶板中两类视觉中间神经元的时间进程和空间整合特性。在视觉运动刺激期间测量钙积累，确保神经元在其自然空间和时间操作范围内的突触激活。所比较的细胞类型，即离心水平（CH）细胞和水平系统（HS）细胞，已知接收具有相似方向选择性的视网膜拓扑输入，但在形态、生物物理学、树突-树突突触的存在以及计算任务方面存在差异。1）运动诱导的钙积累的时间进程相对于刺激参数（如图案对比度和大小）高度不变。在HS细胞中，[Ca(2+)]i的上升可以用一个时间常数为5 - 6秒的单指数来描述。CH细胞中[Ca(2+)]i的初始上升要快得多（时间常数约为1秒）。两类细胞中的衰减时间常数估计至少比相应的上升时间常数长3.5倍。2）HS细胞中电压-[Ca(2+)]i关系最好用扩张性非线性来描述，而CH细胞中则是近似线性关系。3）两类细胞都表现出钙积累的大小依赖性饱和非线性。尽管在CH细胞中钙饱和与膜电位饱和难以区分，但在HS细胞中这两个响应参数的饱和情况有所不同。4）对不重叠视觉刺激的响应中，钙信号存在空间重叠。CH细胞中重叠轮廓的面积和幅度都比HS细胞中的大。因此，CH细胞中的钙积累在空间上比HS细胞中更模糊。5）所描述的两类细胞之间的差异可能反映了这些神经元的以下计算任务：CH细胞通过具有明显空间低通滤波的树突突触在小叶板内传递视网膜拓扑信息。HS细胞是小叶板的输出神经元，其中缓慢的局部钙积累可能适合局部调节功能。
