[What astrocytic microstructure tells us about its physiology]

In neurons, fine morphological structures such as dendritic spines play a key role in maintaining synaptic input independence and enabling local, parallel signal processing, forming a fundamental basis of neuronal computation. In this context, it is well established that neuronal microstructure shapes the spatiotemporal properties of signaling. In contrast, although astrocytes, a type of glial cells, possess highly elaborate, densely ramified processes, the functional significance of their morphology has remained less clear. Astrocytic Ca²⁺ signals have long been described as slow and spatially diffuse, seemingly at odds with their fine structures. Recent advances in live super-resolution imaging have begun to bridge this gap by allowing direct visualization of perisynaptic astrocytic processes. Our studies suggest that astrocytic processes are not merely passive structural elements, but actively localize Ca²⁺ signals at individual synapses by regulating the diffusion of signaling molecules. Thus, astrocytes may, like neurons, exploit their morphology to control spatiotemporal signaling. Furthermore, such compartmentalization strategies may extend beyond the intracellular structure to the extracellular space surrounding neurons and astrocytes. The extracellular space has traditionally been examined only at a coarse, tissue-level resolution; however, advances in live super-resolution imaging have revealed a highly complex, heterogeneous geometry. This architecture is closely coupled to astrocytic microstructure and may influence local dynamics of transmitters and ions. In this review, we propose that microstructures outside neurons also contribute to shaping spatiotemporal signaling patterns through the regulation of molecular diffusion, and may even constitute the structural basis of brain information processing.