An elastic segment of the whisker shaft enables coding of the whisking phase via whisker torsion in rats and mice

Rodents' ability to encode the whisking phase has been extensively documented through neuronal recordings from ascending sensory pathways. Yet, while indicating that reafference originates from the mechanoreceptors, the mechanistic underpinnings of the whisking phase encoding within the follicle remain unclear. Here we present anatomical, histological, and biomechanical evidence for the presence of a distinctive elastic segment (ES) within the basal part of the whisker shaft inside the follicle. This ES, composed of immature keratin, is capable of both bending and twisting. Forces generated by whisker movement deform this segment, causing whisker shaft deflections that can stimulate specific mechanoreceptor subsets within the follicle at different phases of the whisking cycle. This mechanism appears to operate during both free-air whisking and object contact. We propose that the ES enables torsion-based mechanoreceptor activation, allowing encoding of the whisking phase.