Peripheral nerve injuries affect hundreds of thousands of Americans every year, including many Veterans. Even though injured nerves can regenerate and reinnervate peripheral targets, complete recovery of normal motor function is unusual. A large majority are left with permanent motor deficits. Inappropriate sensorimotor connections in the spinal cord contribute to these deficits: the spinal reflex circuits that support motor functions such as locomotion are disordered. For example, after peripheral nerve and regeneration, Group IA primary afferent input from muscle spindles no longer strongly excites the spinal motoneurons of homonymous and synergist muscles. A new therapeutic method that can guide the restoration of appropriate sensorimotor connections in the spinal cord could improve functional recovery after peripheral nerve injury and regeneration. Over the past 30 years, we have developed and applied a unique operant conditioning protocol for inducing activity-dependent CNS plasticity, exploring its mechanisms, and using it therapeutically. This protocol induces activity in descending pathways from the brain that can modify specific spinal reflex pathways, such as the wholly spinal and largely monosynaptic pathway of the H-reflex, the electrical analog of the spinal stretch reflex (i.e., the knee-jerk reflex). In animals or people with incomplete spinal cord injuries that have impaired locomotion, an appropriate reflex conditioning protocol can restore more normal locomotion. Furthermore, recent preliminary data suggest that reflex conditioning can also improve locomotion in rats that have undergone sciatic nerve transection and regeneration. Based on this work, we hypothesize that an appropriate reflex operant conditioning protocol can improve locomotion after peripheral nerve injury and regeneration. To test this hypothesis, we will transect the right sciatic nerve in rats, repair (i.e., reoppose) the nerve so that regeneratin occurs, and assess the impact of up-conditioning (i.e., increasing) or down-conditioning (i.e, decreasing) the right soleus H-reflex. In Aim 1, we will determine the locomotor impact of up- or down-conditioning the soleus H-reflex during the period of sciatic regeneration. In Aim 2, we will determine the locomotor impact of up- or down-conditioning the soleus H-reflex after sciatic regeneration has already occurred. Each aim will study three rat groups: up-conditioned; down-conditioning; and control (i.e., the H-reflex is simply measured). We will assess these groups physiologically (i.e., spontaneous EMG activity, H-reflexes), functionally (i.e., EMG and kinematics during treadmill locomotion), and immunohistochemically (i.e., putative primary afferent terminals (i.e., terminals containing vesicular glutamate transporter-1 (VGLUT1)) on soleus motoneurons). At the end of data collection for each aim, we expect that the soleus H-reflex will be larger and locomotion will be better (e.g., longer steps, better right/left symmetryin timing and hip heights) in up- conditioned rats than in control rats or down-conditioned rats. Furthermore, we expect that VGLUT1 (i.e., putative primary afferent terminals) will be more numerous and/or larger on soleus motoneurons of up- conditioned rats. Thus, this proposal seeks to evaluate a novel and clinically practical therapeutic approach to reducing the functional impairments associated with peripheral nerve injury and regeneration. If this work is successful, it will validate reflex operant conditioning as a new method that can compliment standard rehabilitation regimens and enhance the recovery of useful function after peripheral nerve injury and regeneration; and it will prepare the way for clinical translation of this novel therapy.