synapsed to alpha motor neuron 2020-09-23T20:16:26Z alpha motoneuron A large, multipolar lower motor neuron of the brainstem and spinal cord that innervates the extrafusal muscle fibers of skeletal muscle and are directly responsible for initiating their contraction. While their cell bodies are in the CNS (in the anterior gray horn of the spinal cord), they are part of the somatic nervous system - a branch of the PNS. lower motor neuron extrafusal muscle fiber David Osumi-Sutherland