Emerging evidence demonstrates that animals and people can exert control over the level of excitability in spinal and corticospinal neural circuits that contribute to movement. This discovery has important implications, as it represents a new strategy to improve motor control in people of all ability levels, including those with neurological conditions. Operant conditioning is a well-studied mechanism of learning, in which the modification of a behavior can be brought about by the consequence of the behavior, and reinforcement causes behaviors to become more frequent. In recent years, operant conditioning has been applied to spinally-mediated reflex responses in mice, rats, monkeys and people. By electrically stimulating a peripheral nerve, recording the muscle response, and rewarding responses that are within a desirable range, it is possible to increase or decrease the neural circuit's excitability. This may alter the level of resting muscle tone and spasticity, as well the muscle's contribution to planned movements and responses to unexpected events. Operant conditioning of spinal reflexes has been applied to a lower limb muscle in healthy people and those with spinal cord injuries. In this project, we will expand the use of operant conditioning to muscles of the upper limb, demonstrating feasibility and efficacy in healthy people and people post-stroke. We will determine whether operant conditioning can be used to decrease excitability of spinal reflexes that activate a wrist flexor muscle. Additionally, in a separate group of healthy people, we will determine whether operant conditioning can be used in a similar way to increase corticospinal excitability. We will stimulate the motor cortex with transcranial magnetic stimulation to elicit motor evoked potentials in the same wrist flexor muscle, and will reward responses that exceed a threshold value. We will examine the effects of these interventions on motor control at the wrist, using an innovative custom-designed cursor-tracking task to quantify movement performance. We will determine whether changes in spinal reflex excitability or corticospinal excitability alter motor control. The overall goal of this research is to develop a new, evidence-based strategy for rehabilitation that will improve recovery of upper limb function in people after stroke.