Nerve growth factor (NGF), the prototypical growth factor, is a protein secreted by a neurons target cell. NGF is critical for the survival and maintenance of sympathetic and sensory neurons. NGF is released from the target cells, binds to and activates its high affinity receptor TrkA on the neuron, and is internalized into the responsive neuron. The NGF/TrkA complex is subsequently trafficked back to the neurons cell body. This movement of NGF from axon tip to soma is thought to be involved in the long-distance signaling of neurons.