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The Effect of Acetylcholine on the Reduction of the Required Voltage for Muscle Contraction

Abstract

Facial nerve dysfunction and the subsequent inability to blink is a common complication caused by numerous medical problems, such as stroke. Denervation and dysfunction of the orbicularis oculi, the muscle responsible for eye blinks can result in corneal abrasions and breakdown. Direct electrical stimulation of the muscle is not a viable solution, as the required electrical stimulus needed to induce a blink in denervated muscle is 3 mA while the human pain threshold is 1 mA. The insufficiency of electrical stimulation alone in triggering painless muscle contraction in denervated orbicularis oculi muscles indicates a need to investigate other methods of stimulation. The focus of this dissertation has been to investigate the response of skeletal muscle to combined electrochemical stimulation by initiating the intricate cascade that leads to muscle contraction with the application of acetylcholine (ACh) in order to reduce the required electrical stimulus. Initial in vivo testing in rabbits demonstrated that the required current to stimulate a muscle contraction could be reduced with the incorporation of ACh, supporting the feasibility of the concept of electrochemical stimulation. Focus was then shifted to in vitro testing using cultured C2C12 cells grown on microposts to determine the correlation between electrical stimulation to myotube excitability, indicated by the contraction force generated. Insufficient myotube growth and cell detachment upon electrical stimulation resulted in the decision to continue experiments using excised skeletal muscle from laboratory rats. Image analysis testing of excised biceps femoris demonstrated that the change in area of samples increased in response to electrochemical stimulation in comparison to electrical stimulation. Force analysis testing of excised extensor digitorum longus demonstrated that the application of ACh for electrochemical stimulation clearly enhanced the contraction force, thus allowing for a lower level of electrical stimulation. The influence of electrochemical testing in enhancing contraction forces in denervated muscle has been demonstrated. Thus potentially creating a new platform for the control of muscles that no longer have a functional neural control.

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