Follicle architecture and innervation of functionally distinct rat vibrissae.

The vibrissa follicle is a complex mechanotransducer with intricate accessory structures such as vibrissa, ring sinus and ringwulst as well as rich innervation by diverse afferent types. Establishing how afferent types and accessory structures operate together to derive specific kinds of sensory information has been challenging, because we often lack precise information on afferent types, accessory structures and vibrissa function. Here we address this challenge by synchrotron X-ray imaging of vibrissa follicles of rat vibrissae with distinct function. Specifically, we characterize accessory structures and trace myelinated axons of the all-purpose-sensing C2-, an object-sensing micro-, the wind-sensing supraorbital- and the ground-sensing trident-vibrissa. We find that while vibrissa length and follicle size differ widely across these vibrissae, the ringwulst and the associated club-like afferents are of near constant diameter and height and appear to form a non-scalable sensory module. The two longer vibrissae (supraorbital and C2 vibrissa) have noticeably more club like afferents, suggesting a special role of the ringwulst in transducing presumably smaller deflection angles encountered by long sensory hairs. The trident vibrissa receives overall few afferents, which are strongly polarized to the posterior vibrissa-shaft, a putative specialization to sensing forward-egomotion. We conclude that high-resolution structural analysis allows relating follicle architecture and function.