In the mammalian olfactory system, there exist several parallel specialized subsystems, one of which is the necklace olfactory system. This subsystem has several interesting features in its anatomical organization and physiological responses. Its olfactory sensory neurons (OSNs) in the olfactory epithelium project their axons to a set of glomeruli in the caudal olfactory bulb, forming the shape of "beads-on-a-string" and thus being named as "necklace glomeruli." Physiologically, necklace OSNs lack components suggesting cAMP as the second messenger in the signal transduction cascade as those observed in the OSNs of the canonical olfactory system. In contrast, necklace OSNs possess several signaling components suggesting cGMP as the second messenger. Our recent studies demonstrate that one of the major functions of the necklace olfactory system is to detect atmospheric carbon dioxide (CO2) and mediate avoidance behavior, suggesting novel molecular and cellular mechanisms of CO2 sensing. Here, I will review recent progresses on our understanding of the organization and function of the necklace olfactory subsystem. These recent studies suggest the exciting potentials of using the necklace olfactory system as an advantageous model system for studying neural circuits underlying innate avoidance behavior.

中文翻译：

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哺乳动物的项链嗅觉系统。

在哺乳动物的嗅觉系统中，存在多个并行的专门子系统，其中之一是项链嗅觉系统。该子系统在其解剖结构和生理反应方面具有几个有趣的特征。嗅觉上皮中的嗅觉感觉神经元（OSN）将其轴突投射到尾嗅球中的一组肾小球上，形成“串珠状”的形状，因此被称为“项链肾小球”。从生理上讲，项链OSN缺少组成成分，暗示cAMP作为标准嗅觉系统的OSN中观察到的信号转导级联中的第二信使。相反，项链OSN具有几个信号成分，建议cGMP作为第二个使者。我们最近的研究表明，项链嗅觉系统的主要功能之一是检测大气中的二氧化碳（CO2）并介导回避行为，这提示了CO2传感的新型分子和细胞机制。在这里，我将回顾我们对项链嗅觉子系统的组织和功能的最新进展。这些最新研究表明，使用项链嗅觉系统作为研究先天回避行为背后的神经回路的有利模型系统的令人兴奋的潜力。