Similar but different: Comparative neuroanatomy of the chemo- and mechanosensory pathways in scorpion pectines

化学感觉几乎是所有生命形式不可或缺的形态。节肢动物是最多样化的动物门，许多代表动物都拥有与头部第二节相关的初级化学感觉器官-触角I。下颚目动物（六足目、甲壳目和多足目）的嗅觉通路明显相似，它们的初级嗅觉中心在某些方面甚至与脊椎动物的嗅球相似。其余的真节肢动物分类单元，螯角目，没有与第二头节相关的专用化学感觉附属物。在蝎子身上发现了一种特别有趣的弥补触角缺失的方法。这些动物拥有所谓的“果胶”，这是一对专门的化学感觉附属物，也具有机械感觉功能，很像曼迪布拉塔的触角1。有趣的是，这些果胶位于身体的腹侧，就在行走的腿和生殖器盖的后面。它们是梳状的器官，镶嵌着成千上万的机械和化学感觉感受器，这些感受器排列在每颗牙齿或牙钉的腹侧。果胶作为接触化学感受器官在扫描基质中起着重要的作用。感觉传入投射到中枢神经果仁神经节的许多室室（小叶到肾小球）。这些隔室使人联想到下颌骨嗅觉中心的肾小球组织。最初的回填实验揭示了聚乙二醇感觉器的体位投射，这在化学感受系统中是相当罕见的，尽管在机械感觉神经丸中通常观察到。这就提出了一个问题，即化学和机械感觉输入是如何整合到果胶神经药丸中的。本项目旨在详细研究蝎子果仁神经丸的神经解剖学组织及其投影。我们将集中讨论这一通路的所有组成部分：化学感觉神经元从外周投射到中枢神经系统，假定的中枢神经系统肾小球的结构和解剖，局部中间神经元及其神经递质的特征，以及投射神经元与高级脑中枢的连接。我们将结果数据集与机械感觉果胶投影进行比较，从而表征两种感觉模式的整合模式。发展方面的问题将在所有的项目部分，重点是感官结构和他们的主要预测。这将扩大我们对化学和机械感觉神经药物组织及其相互作用的潜在功能需求的理解。

The chemical sense is an integral modality for virtually all forms of life. Regarding Arthropoda, the most diverse animal phylum, many representatives possess primary chemosensory organs – the antennae I - associated with the second head segment. Olfactory pathways of the Mandibulata (Hexapoda, Crustacea, and Myriapoda) are markedly similar, and their primary olfactory centers even resemble vertebrate olfactory bulbs in several respects. The remaining euarthropod taxon, the Chelicerata, do not possess dedicated chemosensory appendages associated with the second head segment. A particularly fascinating solution to compensate the lack of antennae is observed in scorpions. These animals possess so-called pectines as dedicated pairs of chemosensory appendages that also fulfil a mechanosensory function, much like the antenna I in Mandibulata. Intriguingly, the pectines are located on the ventral side of the body, just behind the walking legs and the genital operculum. They are comb-shaped organs studded with many thousands of mechano- and chemosensory sensilla that are arranged on the ventral sides of each tooth, or peg. Pectines have been suggested to play an important role in scanning the substrate as contact chemoreceptive organs. The sensory afferents project into numerous (lobular to glomerular) compartments of the central nervous pectine neuropil. These compartments are reminiscent of the glomerular organization of the olfactory centers in Mandibulata. Initial backfill experiments revealed a somatotopic projection of peg sensilla, which is quite uncommon for chemoreceptive systems, though typically observed in mechanosensory neuropils. This raises the question of how chemo- and mechanosensory inputs are integrated in the pectine neuropils.The present project sets out to investigate the neuroanatomical organization of the scorpion pectine neuropils and their projections in detail. We will focus on all components of this pathway: Projections of chemosensory neurons from the periphery into the CNS, structure and anatomy of putative glomeruli in the central nervous system, characteristics of local interneurons and their neurotransmitters, and connectivity to higher brain centers by projection neurons. We will compare the resulting data set to the mechanosensory pectine projections and thus characterize the mode of integration of the two sensory modalities. Developmental aspects will be addressed in all project parts, focusing on sensory structures and their primary projections. This will expand our understanding of potential functional necessities in the organization of the chemo- and mechanosensory neuropils and their interlacing.