Development of a functional nervous system requires the generation of precise patterns of connections between neurons and their targets. Sensory neurons in the dorsal root ganglia (DRG) send peripheral processes to the skin and central axons to the dorsal spinal cord, thereby establishing a circuit that allows animals to detect and respond to noxious stimuli in the environment. A second set of DRG neurons carry proprioceptive information from muscles to interneurons and motor neurons in the ventral spinal cord. This circuit is used for movement control and balance. Very little is known about how these two types of sensory neurons find and synapse with distinct targets in the spinal cord. Several molecules have been implicated, but much remains to be learned about how such molecules act and what other signals may direct these complex axon guidance events. Understanding how these two kinds of neurons establish unique circuits serves as a useful model for how neurons wire together in the nervous system in general. This research may lead to improved treatment of neural tube defects and spinal cord injuries. In addition, since migrating cancer cells probably employ similar signaling systems, these studies may elucidate mechanisms of tumor metastasis.
