Functional anatomy of mystacial active sensing in rats

Rats' whisking motion and objects' palpation produce tactile signals sensed by mechanoreceptors at the vibrissal follicles. Rats adjust their whisking patterns to target information type, flow, and resolution, adapting to their behavioral needs and the changing environment. This coordination requires control over the activity of the mystacial pad's intrinsic and extrinsic muscles. Studies have relied on muscle recording and stimulation techniques to describe the roles of individual muscles. However, these methods lack the resolution to isolate the mystacial pad's small and compactly arranged muscles. Thus, we propose functional anatomy as a complementary approach for studying the individual and coordinated effects of the mystacial pad muscles on vibrissae movements. Our functional analysis addresses the kinematic measurements of whisking motion patterns recorded in freely exploring rats. Combined with anatomical descriptions of muscles and fascia elements of the mystacial pad in situ, we found: (1) the contributions of individual mystacial pad muscles to the different whisking motion patterns; (2) active touch by microvibrissae, and its underlying mechanism; and (3) dynamic position changes of the vibrissae pivot point, as determined by the movements of the corium and subcapsular fibrous mat. Finally, we hypothesize that each of the rat mystacial pad muscles is specialized for a particular function in a way that matches the architecture of the fascial structures. Consistent with biotensegrity principles, the muscles and fascia form a network of structural support and continuous tension that determine the arrangement and motion of the embedded individual follicles.