Models of rodent facial musculature for the study of active tactile sensing

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Project Summary: The rodent vibrissal (whisker) system is one of the most widely-used models in neuroscience to study how information about movement and touch are combined. During many exploratory behaviors, rats and mice sweep their whiskers back and forth in a rapid, rhythmic motion called “whisking” to actively gather touch information. Although whisking is rhythmic, rodents can also change how their whiskers move depending on the desired sensory information, and on their particular behavior. Researchers are nearly able to begin to “close-the-loop” between movement and touch for the whisker system, except for one critical gap: we do not yet have a three dimensional (3D) model of rodent facial musculature. Without such a model, we cannot identify how the rat changes its muscle activity to change whisker motion and acquire particular types of sensory information. We cannot know which whisker motions are fixed via the biomechanics, versus which motions the rat can actively control. We cannot fully understand the motor commands sent to the whisker muscles. The central goal of this proposal is to develop three-dimensional (3D) models of rodent facial musculature that close this gap. We will first use a novel combination of tactile profilometry, histology, MRI, and CT-scans to quantify the anatomy of rodent facial muscles and the follicles that hold the whiskers. Using this anatomy, we will then construct 3D biomechanical models of the whisker muscles and follicles to simulate the motion of all whiskers. These models will be validated and tested in several different complementary software systems, and then be used to test eleven specific predictions for the particular function of each whisker-related muscle. Finally, we will integrate the 3D models of rodent facial muscles with existing models that describe the sensory, tactile side of whisker motion. These combined muscle-sensory simulations will be directly compared with active animal behavior. This work takes a step towards closing the loop between motor action and the sensory data acquired, and helps disentangle the relative roles of biomechanics and neural control during different types of whisking. The proposed work will inform all levels of study of whisker neural pathways, from primary sensory neurons to sensory and motor cortical areas, to brainstem regions involved in controlling whisker motions. More generally, whisking represents a unique window into how volitional control can modulate or override centrally-patterned movement. The transition between varieties of rhythmic and non- rhythmic movement has important implications for the coordination of sniffing, breathing, olfaction, chewing, swallowing, and suckling, and the proposed work could thus shed light on the neuromechanical basis for some pediatric and geriatric dysphagias.