Response of "naive" cutaneous and muscle afferents to potential targets in vitro.

It is now well documented that motoneurons are specified to innervate particular target muscles prior to axon outgrowth. Here we investigate whether sensory neurons are similarly specified to innervate target skin or muscle, taking advantage of the avian trigeminal system where cutaneous and muscle afferents are anatomically separate. Using this system, we have previously shown that by embryonic day 10 (E10) (approximately 4-5 days after target innervation), regenerating cutaneous and muscle afferents differ in their response to various potential targets in vitro, in manners consistent with their normal innervation patterns in vivo. Thus, by E10 these two populations of sensory neurons have distinct identities as skin and muscle afferents. In contrast, we report here that the responses of younger, naive cutaneous and muscle afferents that have not yet, or only recently, innervated peripheral targets are indistinguishable, regardless of the target tissue tested. These findings suggest that at stages when innervation is being established, cutaneous and muscle afferents, unlike motoneurons, may not yet have acquired rigidly specified identities and/or the ability to recognize and respond selectively to their appropriate peripheral targets.