Constraints on the deformation of the vibrissa within the follicle

Abstract

Nearly all mammals have a vibrissal system specialized for tactile sensation, composed of whiskers growing from sensor-rich follicles in the skin. Because a whisker has no sensors along its length, an open question is how mechanoreceptors in the follicle transduce sensory signals. These mechanoreceptors are activated by whisker deflection, so it is essential to understand how the whisker deforms within the follicle and so how it may activate different populations of mechanoreceptors in different ways. During active whisking behaviors, muscle contractions and increases in blood pressure in the ring sinus will likely affect the whisker deformation profile. Directly recording from mechanoreceptors under these conditions is difficult due to their small size, location within intricate and delicate membranes, and movement during sensation. Using data from a previous experimental study on whisker deflection, and from histological analysis of follicle tissue, we develop a mechanical model of the follicle sinus complex. With this model we first simulate passive whisker contact, replicating previous results of ex vivo experiments on deformation of a whisker within the follicle. We then simulate whisker deformation within the follicle during active whisking. Results of these simulations predict that both intrinsic muscle contraction and elevated hydrostatic pressure within the ring sinus may be regulatory mechanisms to enhance tactile sensitivity during active whisking. The mechanical model presented in this study is an important first step in simulating mechanical interactions within whisker follicles, and aids in the development of artificial robotic follicles. Author summaryMany mammals rely on whiskers as a mode of tactile sensation, especially when exploring in darkness. Active, rhythmic protraction and retraction of the whiskers, commonly referred to as whisking, is observed among many whisker specialist animals. Whisker-based sensing requires the forces and moments generated by external stimuli to be transduced into neural signals inside the follicle, which holds the base of the whisker shaft. Within the follicle, the interaction between the whiskers deformation and the surrounding tissue determines how different groups of mechanoreceptors along the inside of a follicle will deformed. However, experimental measurement of this interaction is challenging to perform in active animals. We therefore created a mechanical model for the follicle sinus complex to simulate whisker deformation within the follicle resulting from external whisker deflection. Our simulations replicate results from previous ex vivo experiments that have monitored whisker deformation in the follicle ring sinus. We extend these results by predicting whisker deformation profiles during active whisking. Our results suggest that both intrinsic muscle contraction and an increase in blood pressure will affect the whisker deformation profile within the follicle, and in turn, the tactile sensitivity of the whisker system.