Oakland, California.
Environmental Physiology, Molecular, and Integrative Physiological Sciences Program, Department of Environmental Health, Harvard T. H. Chan School of Public Health, Boston, Massachusetts.
Physiology (Bethesda). 2021 May 1;36(3):183-194. doi: 10.1152/physiol.00040.2020.
Species throughout the animal kingdom use the Earth's magnetic field (MF) to navigate using either or both of two mechanisms. The first relies on magnetite crystals in tissue where their magnetic moments align with the MF to transduce a signal transmitted to the central nervous system. The second and the subject of this paper involves cryptochrome (CRY) proteins located in cone photoreceptors distributed across the retina, studied most extensively in birds. According to the "Radical Pair Mechanism" (RPM), blue/UV light excites CRY's flavin cofactor (FAD) to generate radical pairs whose singlet-to-triplet interconversion rate is modulated by an external MF. The signaling product of the RPM produces an impression of the field across the retinal surface. In birds, the resulting signal on the optic nerve is transmitted along the thalamofugal pathway to the primary visual cortex, which projects to brain regions concerned with image processing, memory, and executive function. The net result is a bird's orientation to the MF's inclination: its vector angle relative to the Earth's surface. The quality of ambient light (e.g., polarization) provides additional input to the compass. In birds, the Type IV CRY isoform appears pivotal to the compass, given its positioning within retinal cones; a cytosolic location therein indicating no role in the circadian clock; relatively steady diurnal levels (unlike Type II CRY's cycling); and a full complement of FAD (essential for photosensitivity). The evidence indicates that mammalian Type II CRY isoforms play a light-independent role in the cellular molecular clock without a photoreceptive function.
动物王国中的物种利用地球磁场 (MF) 通过两种机制中的任一种或两种来导航。第一种机制依赖于组织中的磁铁矿晶体，其磁矩与 MF 对齐，以将信号转换为传送到中枢神经系统的信号。第二种机制也是本文的主题，涉及位于视网膜中分布的视锥细胞中的隐花色素 (CRY) 蛋白，在鸟类中研究得最为广泛。根据“自由基对机制”（RPM），蓝光/紫外光激发 CRY 的黄素辅因子（FAD）产生自由基对，其单重态到三重态的转换速率受外部 MF 调制。RPM 的信号产物产生视网膜表面上磁场的印象。在鸟类中，视神经上产生的信号沿着丘脑传出途径传递到初级视觉皮层，该皮层投射到与图像处理、记忆和执行功能相关的大脑区域。最终结果是鸟类对 MF 倾斜度的取向：其相对于地球表面的向量角度。环境光的质量（例如，偏振）为罗盘提供了额外的输入。在鸟类中，IV 型 CRY 同工型似乎对罗盘至关重要，因为它位于视锥细胞内；其细胞溶胶位置表明它在昼夜节律钟中没有作用；相对稳定的昼夜节律水平（与 II 型 CRY 的循环不同）；并且具有完整的 FAD 补充（对光敏性至关重要）。证据表明，哺乳动物的 II 型 CRY 同工型在没有光感受器功能的情况下在细胞分子钟中发挥光独立作用。
