Emergence of a somatosensory tonotopic map for substrate vibration in the brainstem

Abstract

Perceiving substrate vibrations is a fundamental component of tactile perception. The wide frequency spectrum of vibrations is covered by integrating responses of multiple mechanoreceptors that innervate various subtypes of mechanosensitive end organs, each preferring a specific range: Merkel cells (0.5-10Hz), Meissner corpuscles (10-150Hz) and Pacinian corpuscles (150-1000Hz) in primates. As the density of different end organs greatly varies across the body, each body part potentially has a specific frequency preference. How location (somatotopy) and frequency tuning (tonotopy) are processed along the ascending neuraxis and how they converge to drive responses of individual neurons is poorly understood. In this study, we address this question by combining in vivo peripheral electrophysiology and two-photon calcium imaging along the entire dorsal column-medial lemniscal pathway, including the dorsal root ganglia, dorsal column nuclei (DCN), the thalamus and the cortex. Surprisingly, we found that both frequency, as well as location, are organised into structured maps in the DCN. Furthermore, both maps are intimately related at the fine spatial scale with parallel map gradients that are consistent across the depth of the DCN and preserved along the ascending pathway. Additional sensory mapping experiments based on peripheral characterisation revealed that the tonotopic map only partially reflects the distribution of end organs in the skin and deep tissue. Instead, we show that the emergence of the finescale tonotopy is probably due to the selective dendritic sampling from axonal afferents, right at the first synaptic relay. Taken together, we conclude DCN neural circuits are key to the emergence of these two fine-scale topological organisations in early somatosensory pathways. The underlying computational principle is intriguingly similar to the integration of multiple functional maps along the ascending visual pathways, suggesting a universal law governing the optimization of sensory systems.