Damage to the central nervous system caused by traumatic injury or neurological disorders can lead to permanent loss of voluntary motor function and muscle paralysis. Here, we describe an approach that circumvents central motor circuit pathology to restore specific skeletal muscle function. We generated murine embryonic stem cell-derived motor neurons that express the light-sensitive ion channel channelrhodopsin-2, which we then engrafted into partially denervated branches of the sciatic nerve of adult mice. These engrafted motor neurons not only reinnervated lower hind-limb muscles but also enabled their function to be restored in a controllable manner using optogenetic stimulation. This synthesis of regenerative medicine and optogenetics may be a successful strategy to restore muscle function after traumatic injury or disease.

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Motor neurons project axons from the hindbrain and spinal cord to muscle, where they induce myofibre contractions through neurotransmitter release at neuromuscular junctions. Studies of neuromuscular junction formation and homeostasis have been largely confined to in vivo models. In this study, three powerful tools have been merged—pluripotent stem cells, optogenetics, and microfabrication—and an open microdevice is designed in which motor axons grow from a neural compartment containing embryonic stem cell‐derived motor neurons and astrocytes through microchannels to form functional neuromuscular junctions with contractile myofibres in a separate compartment. Optogenetic entrainment of motor neurons in this reductionist neuromuscular circuit enhances neuromuscular junction formation more than twofold, mirroring the activity‐dependence of synapse development in vivo. An established motor neuron disease model is incorporated into the system and it is found that coculture of motor neurons with SOD1G93A astrocytes results in denervation of the central compartment and diminishes myofibre contractions, a phenotype which is rescued by the receptor interacting serine/threonine kinase 1 inhibitor necrostatin. This coculture system replicates key aspects of nerve–muscle connectivity in vivo and represents a rapid and scalable alternative to animal models of neuromuscular function and disease.