Snow P J, Plenderleith M B, Wright L L
Department of Anatomical Sciences, University of Queensland, Brisbane, Australia.
J Comp Neurol. 1993 Aug 1;334(1):97-103. doi: 10.1002/cne.903340108.
In order to assess the ability of sharks and rays to sense pain, the proportion of myelinated versus unmyelinated sensory fibres in the dorsal roots and the diameter spectrum of cells in the dorsal root ganglia of three species of elasmobranch fish were ascertained. Electron micrographs were used to count the numbers of myelinated and unmyelinated fibres in montages of whole dorsal roots of the long-tailed stingray (Himantura sp.), the shovelnose ray (Rhinobatus battilum), and small specimens of the black-tip shark (Carcharhinus melanopterus). The diameters of dorsal root ganglion cells in each species were measured by using the light microscope. Less than 1% of the dorsal root axons in the long-tailed stringray and a large specimen of the shovelnose were unmyelinated, whereas in smaller shovelnose rays and in the small black-tipped sharks, from 14% to 38% of axons were unmyelinated. Unmyelinated fibres differed from those in mammalian nerves in that there was a one-to-one association of the fibre with a Schwann cell. We conclude from these observations that myelination was incomplete in the black-tipped sharks and the smaller specimens of the shovelnose rays. The distribution of the diameter of cells of the dorsal root ganglia of these species was unimodal, resembling the diameter range that has been reported for the somata of myelinated fibres in the cat. We interpret these results as indications that sharks and rays lack the neural apparatus essential for the sensation of pain and we suggest that, to these life forms, the perception of pain might have little relevance to survival.
为了评估鲨鱼和鳐鱼感知疼痛的能力，研究人员确定了三种软骨鱼类背根中有髓与无髓感觉纤维的比例以及背根神经节中细胞的直径谱。利用电子显微镜照片，对长尾魟（Himantura sp.）、铲鼻魟（Rhinobatus battilum）和黑鳍鲨（Carcharhinus melanopterus）幼体的整个背根拼接图中的有髓和无髓纤维数量进行了计数。使用光学显微镜测量了每个物种背根神经节细胞的直径。在长尾魟和大型铲鼻魟中，背根轴突中不到1%是无髓的，而在小型铲鼻魟和小黑鳍鲨中，14%至38%的轴突是无髓的。无髓纤维与哺乳动物神经中的无髓纤维不同，其纤维与施万细胞存在一对一的关联。基于这些观察结果，我们得出结论，黑鳍鲨和小型铲鼻魟的髓鞘形成并不完全。这些物种背根神经节细胞直径的分布是单峰的，类似于猫中有髓纤维胞体所报道的直径范围。我们将这些结果解释为表明鲨鱼和鳐鱼缺乏感知疼痛所必需的神经结构，并且我们认为，对于这些生物来说，疼痛感知可能与生存几乎无关。
