Chagnaud B P, Wilkens L A, Hofmann M H
Center for Neurodynamics, Department of Biology, University of Missouri-St. Louis, St. Louis, MO 63121, USA.
J Physiol Paris. 2008 Jul-Nov;102(4-6):246-55. doi: 10.1016/j.jphysparis.2008.10.006. Epub 2008 Oct 17.
Paddlefish use their electrosense to locate small water fleas (daphnia), their primary prey, in three-dimensional space. High sensitivity and a representation of object location are essential for this task. High sensitivity can be achieved by convergence of information from a large number of receptors and object location is usually represented in the nervous system by topographic maps. However the first electrosensory center in the brain, the dorsal octavolateral nucleus in the hindbrain, is neither topographically organized nor does it show a higher sensitivity than primary afferent fibers. Here, we investigated the response properties of electrosensory neurons in the dorsal octavolateral nucleus (DON), the lateral mesencephalic nucleus (LMN) and the tectum mesencephali (TM). LMN units are characterized by large receptive fields, which suggest a high degree of convergence. TM units have small receptive fields and are topographically arranged, at least in the rostro-caudal axis, the only dimension we could test. Well-defined receptive fields, however, could only be detected in the TM with a moving DC stimulus. The receptive fields of TM units, as determined by slowly scanning the rostrum and head with a 5 Hz stimulus, were very large and frequently two or more receptive fields were present. The receptive fields for LMN units were located in the anterior half of the rostrum whereas TM units had receptive fields predominantly on the head and at the base of the rostrum. A detailed analysis of the prey catching behavior revealed that it consists of two phases that coincide with the location of the receptive fields in LMN and TM, respectively. This suggests that LMN units are responsible for the initial orienting response that occurs when the prey is alongside the anterior first half of the rostrum. TM units, in contrast, had receptive fields at locations where the prey is located when the fish opens its mouth and attempts the final strike.
匙吻鲟利用其电感应能力在三维空间中定位小型水蚤（水蚤属），这是它们的主要猎物。高灵敏度以及物体位置的表征对于这项任务至关重要。高灵敏度可通过大量感受器的信息汇聚来实现，而物体位置通常由神经系统中的地形图来表示。然而，大脑中的第一个电感应中枢，即后脑的背侧八分体外侧核，既没有按地形图组织，其灵敏度也不比初级传入纤维更高。在此，我们研究了背侧八分体外侧核（DON）、中脑外侧核（LMN）和中脑顶盖（TM）中电感应神经元的反应特性。LMN单元的特点是感受野大，这表明有高度的汇聚。TM单元的感受野小，并且至少在我们能够测试的唯一维度——前后轴上是按地形图排列的。然而，只有在使用移动直流刺激时，才能在TM中检测到明确的感受野。通过以5赫兹的刺激缓慢扫描吻部和头部来确定的TM单元的感受野非常大，并且经常存在两个或更多的感受野。LMN单元的感受野位于吻部的前半部分，而TM单元的感受野主要在头部和吻部基部。对捕食行为的详细分析表明，它由两个阶段组成，分别与LMN和TM中感受野的位置相吻合。这表明，当猎物位于吻部前半部分旁边时发生的初始定向反应是由LMN单元负责的。相比之下，TM单元的感受野位于鱼张开嘴并试图进行最后一击时猎物所在的位置。
