Blackwell Kim T
School of Computational Sciences, and the Krasnow Institute for Advanced Study, George Mason University, Fairfax, VA 22030, USA.
Anat Rec B New Anat. 2006 Jan;289(1):25-37. doi: 10.1002/ar.b.20090.
A breakthrough for studying the neuronal basis of learning emerged when invertebrates with simple nervous systems, such as the sea slug Hermissenda crassicornis, were shown to exhibit classical conditioning. Hermissenda learns to associate light with turbulence: prior to learning, naive animals move toward light (phototaxis) and contract their foot in response to turbulence; after learning, conditioned animals delay phototaxis in response to light. The photoreceptors of the eye, which receive monosynaptic inputs from statocyst hair cells, are both sensory neurons and the first site of sensory convergence. The memory of light associated with turbulence is stored as changes in intrinsic and synaptic currents in these photoreceptors. The subcellular mechanisms producing these changes include activation of protein kinase C and MAP kinase, which act as coincidence detectors because they are activated by convergent signaling pathways. Pathways of interneurons and motorneurons, where additional changes in excitability and synaptic connections are found, contribute to delayed phototaxis. Bursting activity recorded at several points suggest the existence of small networks that produce complex spatiotemporal firing patterns. Thus, the change in behavior may be produced by a nonlinear transformation of spatiotemporal firing patterns caused by plasticity of synaptic and intrinsic channels. The change in currents and the activation of PKC and MAPK produced by associative learning are similar to those observed in hippocampal and cerebellar neurons after rabbit classical conditioning, suggesting that these represent general mechanisms of memory storage. Thus, the knowledge gained from further study of Hermissenda will continue to illuminate mechanisms of mammalian learning.
当具有简单神经系统的无脊椎动物，如太平洋侧花海兔（Hermissenda crassicornis）被证明能表现出经典条件反射时，学习的神经元基础研究取得了突破。太平洋侧花海兔学会了将光与湍流联系起来：在学习之前，未受过训练的动物会朝着光移动（趋光性），并在感受到湍流时收缩足部；学习之后，受过训练的动物在感受到光时会延迟趋光行为。眼睛的光感受器从平衡囊毛细胞接收单突触输入，既是感觉神经元，也是感觉汇聚的第一个部位。与湍流相关的光的记忆以这些光感受器内在电流和突触电流的变化形式存储。产生这些变化的亚细胞机制包括蛋白激酶C和丝裂原活化蛋白激酶的激活，它们作为巧合探测器，因为它们是由汇聚的信号通路激活的。中间神经元和运动神经元的通路中发现了兴奋性和突触连接的其他变化，这些通路导致了趋光行为的延迟。在几个点记录到的爆发性活动表明存在产生复杂时空放电模式的小网络。因此，行为的变化可能是由突触和内在通道可塑性引起的时空放电模式非线性转换产生的。联想学习产生的电流变化以及蛋白激酶C和丝裂原活化蛋白激酶的激活与兔子经典条件反射后在海马体和小脑神经元中观察到的相似，这表明这些代表了记忆存储的一般机制。因此，从对太平洋侧花海兔的进一步研究中获得的知识将继续阐明哺乳动物学习的机制。
