Publisher Summary Axotomizing peripheral cutaneous and muscle nerves and thus depriving them of contact with their peripheral targets result in slowed conduction of their action potentials. Reinnervation of their peripheral targets results in the recovery of conduction velocity. These results suggest the sufficiency of the normal peripheral target for the maintenance of at least one measure of normal physiological function (conduction velocity), and further suggest the presence of some substance(s) in the target tissue that is capable of acting in a supportive trophic manner on the innervating afferent fibers. Groups I and II muscle afferents of cats uniformly exhibit several physiological properties: they are rapidly conducting (24–120 m/s), have mechanosensitive receptive fields in muscle, are slowly adapting in response to a maintained stimulus and are stretch-sensitive, and generate monosynaptic excitatory postsynaptic potentials (EPSPs) in homonymous and heteronymous spinal motoneurons. In the experiments described in this chapter, it has been investigated (1) whether maintenance of these various other physiological properties of the peripheral sensory nerves also depends upon target innervation and (2) whether the reinnervation by afferents of foreign and native target tissues is sufficient for the maintenance of these properties. The neurotrophin NT-3, along with its high affinity receptor (tyrosine kinase-C, trkC), has been shown to be essential for the aspects of normal development during the prenatal period.