Exercise-based therapies are currently used to treat voice and swallowing disorders without a clear understanding of the mechanisms that alter the cranial neuromuscular system. For instance, it is not known if tongue exercises that target improved swallowing alter neural substrates in targeted structures (tongue) alone or if these exercises also affect other swallow-related (laryngeal) structures (cross-training). It is also unknown if behavioral and neuroplastic changes endure after therapy is discontinued. The recent application of principles of neuroplasticity to rehabilitation has revolutionized how we think about treatment, highlighting the need for change in both behavior and neural substrates for creating lasting benefits. It is difficult, however, to study neural substrates in human patiens while controlling for factors that may influence plasticity such as genetic and environmental differences. The use of a rat model allows these controls. The proposed research aims to further our understanding of the neuroplastic potential of exercise in the cranial sensorimotor system with the ultimate long-term goal of guiding care of individuals with voice and swallowing problems. Neurotrophins are mediators of neuroplasticity. Research from our laboratory has shown that 8 weeks of tongue exercise in adult rats leads to increased tongue forces and neurotrophin up-regulation in the hypoglossal nucleus. However, it is unknown if tongue exercise creates lasting and generalizable therapeutic benefits throughout the cranial sensorimotor system and across the lifespan. It is hypothesized that: 1) tongue exercise will up-regulate neurotrophins in the cranial sensorimotor system across the lifespan with greater effects in lingual versus laryngeal structures, and 2) that exercise effects will diminish when the exercise program is discontinued (detraining). We will test these hypotheses in a rat model by comparing behavioral and molecular neurotrophin parameters in rats of different ages that have undergone a tongue exercise program, a detraining program, or a control condition. The proposed research has 2 specific aims. In young adult and old rats, we will: 1) determine the effect of tongue exercise on neurotrophins and behavioral measures in the cranial sensorimotor system to examine cross-training effects and neuroplastic potential, and 2) examine the enduring effects of exercise by quantifying both neurotrophins and behavioral measures following a 2 or 4 week detraining period. The proposed research is significant because it will examine the cross-training and neuroplastic potential of exercise in the cranial sensorimotor system in both muscle and the central nervous system, along with the enduring effects of exercise (detraining) with the long term goal of using our results to guide current therapy timelines and protocols used in clinical populations with voice and swallowing problems.
Exercise is used to treat voice and swallowing disorders, but without a strong scientific basis or true understanding of mechanism, which is necessary to move the field forward. In a rat model, we will study changes in nerve growth factors, called neurotrophins, to determine if tongue exercise has the potential to create cross-system, enduring improvements in measures of tongue and laryngeal function. This work will add to current theories regarding the plastic nature of the brain and principles of rehabilitation and wil have implications for future development of exercise-based voice and swallowing therapy approaches.