Baccus S A
Neuroscience Program, University of Miami, Miami, FL 33136, USA.
Proc Natl Acad Sci U S A. 1998 Jul 7;95(14):8345-50. doi: 10.1073/pnas.95.14.8345.
A rapid, reversible enhancement of synaptic transmission from a sensory neuron is reported and explained by impulses that reverse direction, or reflect, at axon branch points. In leech mechanosensory neurons, where one can detect reflection because it is possible simultaneously to study electrical activity in separate branches, action potentials reflected from branch points within the central nervous system under physiological conditions. Synapses adjacent to these branch points were activated twice in rapid succession, first by an impulse arriving from the periphery and then by its reflection. This fast double-firing facilitated synaptic transmission, increasing it to more than twice its normal level. Reflection occurred within a range of resting membrane potentials, and electrical activity produced by mechanical stimulation changed membrane potential so as to produce and cease reflection. A compartmental model was used to investigate how branch-point morphology and electrical activity contribute to produce reflection. The model shows that mechanisms that hyperpolarize the membrane so as to impair action potential propagation can increase the range of structures that can produce reflection. This suggests that reflection is more likely to occur in other structures where impulses fail, such as in axons and dendrites in the mammalian brain. In leech sensory neurons, reflection increased transmission from central synapses only in those axon branches that innervate the edges of the receptive field in the skin, thereby sharpening spatial contrast. Reflection thus allows a neuron to amplify synaptic transmission from a selected group of its branches in a way that can be regulated by electrical activity.
据报道，感觉神经元突触传递的快速、可逆增强是由在轴突分支点处反向或反射的冲动所引起并得到解释的。在水蛭的机械感觉神经元中，可以检测到反射，因为能够同时研究不同分支中的电活动，在生理条件下，中枢神经系统内分支点处的动作电位会发生反射。与这些分支点相邻的突触会被快速连续激活两次，首先是来自外周的冲动，然后是其反射。这种快速的双重放电促进了突触传递，使其增加到正常水平的两倍以上。反射发生在一定范围的静息膜电位内，机械刺激产生的电活动会改变膜电位，从而产生和终止反射。使用了一个房室模型来研究分支点形态和电活动如何促成反射。该模型表明，使膜超极化从而损害动作电位传播的机制可以增加能够产生反射的结构范围。这表明反射更有可能发生在其他冲动传导失败的结构中，例如哺乳动物大脑中的轴突和树突。在水蛭感觉神经元中，反射仅在那些支配皮肤感受野边缘的轴突分支中增加了中枢突触的传递，从而增强了空间对比度。因此，反射使神经元能够以一种可由电活动调节的方式增强其选定一组分支的突触传递。
