Temereva Elena N, Kuzmina Tatyana V
Department of Invertebrate Zoology, Biological Faculty, Moscow State University, Leninskie Gory, 1-12, 119991, Moscow, Russia.
BMC Evol Biol. 2017 Jul 31;17(1):172. doi: 10.1186/s12862-017-1029-5.
The nervous system in brachiopods has seldom been studied with modern methods. An understanding of lophophore innervation in adult brachiopods is useful for comparing the innervation of the same lophophore type among different brachiopods and can also help answer questions about the monophyly of the lophophorates. Although some brachiopods are studied with modern methods, rhynchonelliform brachiopods still require investigation. The current study used transmission electron microscopy, immunocytochemistry, and confocal laser scanning microscopy to investigate the nerve system of the lophophore and tentacles in the rhynchonelliform Hemithiris psittacea.
Four longitudinal nerves pass along each brachium of the lophophore: the main, accessory, second accessory, and lower. The main brachial nerve extends at the base of the dorsal side of the brachial fold and gives rise to the cross nerves, passing through the extracellular matrix to the tentacles. Cross nerves skirt the accessory brachial nerve, branch, and penetrate into adjacent outer and inner tentacles, where they are referred to as the frontal tentacular nerves. The second accessory nerve passes along the base of the inner tentacles. This nerve consists of Ʊ-like parts, which repetitively skirt the frontal and lateral sides of the inner tentacle and the frontal sides of the outer tentacles. The second accessory nerve gives rise to the latero-frontal nerves of the inner and outer tentacles. The abfrontal nerves of the inner tentacles also originate from the second accessory nerve, whereas the abfrontal nerves of the outer tentacles originate from the lower brachial nerve. The lower brachial nerve extends along the outer side of the lophophore brachia and gives rise to the intertentacular nerves, which form a T-like branch and penetrate the adjacent outer tentacles where they are referred to as abfrontal nerves. The paired outer radial nerves start from the lower brachial nerve, extend into the second accessory nerve, and give rise to the lateroabfrontal tentacular nerves of the outer tentacles.
The innervation of the lophophore in the rhynchonelliform Hemithiris psittacea differs from that in the inarticulate Lingula anatina in several ways. The accessory brachial nerve does not participate in the innervation of the tentacles in H. psittacea as it does in L. anatina. The second accessory nerve is present in H. psittacea but not in L. anatina. There are six tentacular nerves in the outer tentacles of H. psittacea but only four in all other brachiopods studied to date. The reduced contribution of the accessory brachial nerve to tentacle innervation may reflect the general pattern of reduction of the inner lophophoral nerve in both phoronids and brachiopods. Bryozoan lophophores, in contrast, have a weakened outer nerve and a strengthened inner nerve. Our results suggest that the ancestral lophophore of all lophophorates had a simple shape but many nerve elements.
腕足动物的神经系统很少用现代方法进行研究。了解成年腕足动物触手冠的神经支配情况，有助于比较不同腕足动物中相同类型触手冠的神经支配，也有助于回答有关触手冠动物单系性的问题。虽然一些腕足动物已用现代方法进行了研究，但小嘴贝形腕足动物仍需进一步研究。本研究使用透射电子显微镜、免疫细胞化学和共聚焦激光扫描显微镜，研究了小嘴贝形腕足动物半纹海豆芽触手冠和触手的神经系统。
四条纵神经沿着触手冠的每条腕臂走行：主神经、副神经、第二副神经和下神经。主腕臂神经在腕臂褶背侧基部延伸，并发出横神经，穿过细胞外基质到达触手。横神经绕过副腕臂神经，分支并穿透相邻的外侧和内侧触手，在那里它们被称为额触手神经。第二副神经沿着内侧触手基部走行。这条神经由类似Ʊ的部分组成，反复绕过内侧触手的额侧和外侧以及外侧触手的额侧。第二副神经发出内侧和外侧触手的侧额神经。内侧触手的后额神经也起源于第二副神经，而外侧触手的后额神经起源于下腕臂神经。下腕臂神经沿着触手冠腕臂的外侧延伸，并发出触手间神经，这些神经形成一个T形分支并穿透相邻的外侧触手，在那里它们被称为后额神经。成对的外侧桡神经从下腕臂神经发出，延伸到第二副神经，并发出外侧触手的侧后额触手神经。
小嘴贝形腕足动物半纹海豆芽触手冠的神经支配在几个方面与无铰纲的鸭嘴海豆芽不同。在半纹海豆芽中，副腕臂神经不像在鸭嘴海豆芽中那样参与触手的神经支配。半纹海豆芽中有第二副神经，而鸭嘴海豆芽中没有。半纹海豆芽外侧触手有六条触手神经，而迄今为止研究的所有其他腕足动物中只有四条。副腕臂神经对触手神经支配的贡献减少，可能反映了帚虫动物和腕足动物中触手冠内部神经减少的一般模式。相比之下，苔藓虫的触手冠外侧神经减弱而内侧神经增强。我们的结果表明，所有触手冠动物的祖先触手冠形状简单但神经元件众多。
