Axon Guidance in the Spinal Cord

Abstract

Neural circuit assembly requires the coordination of various developmental processes, including axon guidance and synapse formation. Growth cones, the leading edges of axons, navigate by interacting with different kinds of attractive and repulsive axon guidance cues along their trajectories and at target areas.Spinal cords consist of different types of neural circuits, such as sensory-motor, commissural, and sympathetic circuits. Each neural circuit provides an unique model system to understand the cellular and molecular mechanisms underlying neural circuit formation. Studies of sensory-motor circuits have outlined the basic events leading to circuit formation. First, both sensory and motor axons project to the appropriate peripheral muscles. Sensory axons then form precise monosynaptic connections with a select group of motor neuron pools in the ventral spinal cord, and these pools of motor neurons ultimately develop their appropriate dendrite morphologies. Examination of commissural neurons and their axon trajectories in the spinal cord has contributed important data on how attractants and repellents influence the midline crossing of axons. Finally, studies of sympathetic neurons have led toward a greater overall understanding of the cellular and molecular mechanisms underlying neuronal survival, retrograde signaling, neuronal migration, and axon guidance.Semaphorin signaling in neural circuit assembly has been studied extensively in the spinal cord. Importantly, the first mammalian semaphorin, Sema3A, was identified by monitoring growth cone collapse in sensory neurons in vitro. Here, we review the current knowledge on semaphorin signaling in various steps of vertebrate neural circuit assembly in the spinal cord, including axon guidance, synaptic specificity, synapse formation, and dendrite development, highlighting the different neural circuit systems that have helped to broaden our understanding of semaphorins and their functions in the developing nervous system.