TrkA mediates developmental sympathetic neuron survival in vivo by silencing an ongoing p75NTR-mediated death signal […] The results described in this paper indicate that p75NTR provides an apoptotic signal for developing sympathetic neurons in the presence or absence of TrkA, and one of the major ways that TrkA supports neuronal survival is by suppressing this death signal. […] the data reported here support a model of naturally occurring neuronal death where an ongoing, receptor-mediated apoptotic signal destines cells to die, and where one of the major roles of exogenous survival ligands is to silence this ongoing apoptotic signal. In the case of sympathetic neurons, p75NTR provides the death signal and TrkA the survival signal. 