Leonardo Manzari
Background: The vestibular system encodes head motion through specialized Type I and Type II hair cells, which differentially respond to acceleration and its temporal derivative, jerk. Molecular gradients of retinoic acid establish zonal distributions of these hair cells, prefiguring their functional specialization. Objectives & Methods: Here I integrate developmental, synaptic, biomechanical, and neural evidence to propose that Type I hair cells, via multimodal synaptic transmission, are particularly well suited for ultrafast detection of transient inertial deformation (jerk), whereas Type II cells play a greater role in encoding sustained acceleration through viscous-flow mechanisms. Molecular gradients of retinoic acid help establish central–peripheral zonal patterning in the otolith and canal epithelia, which in turn underlies differential mechanical and synaptic specialization rather than a simple redistribution of hair-cell types. Computational and experimental studies reveal that the vestibular organs operate in dual mechanical regimes, enabling the dynamic encoding of motion onset and continuity. In systems terms, these viscous and inertial activation modes correspond to distinct temporal filters, whose different time constants naturally give rise to distinct frequency responses. What has traditionally been described as ‘low- vs. high-frequency’ tuning therefore emerges as the frequency-domain signature of acceleration- versus jerk-sensitive pathways. Conclusions: This hierarchical organization elucidates the selective activation observed in clinical vestibular tests and informs novel diagnostic and rehabilitative strategies targeting specific receptor pathways. Together, these findings redefine vestibular transduction as a multimodal dynamic sensor, enhancing our understanding of balance and spatial orientation under complex motion conditions.