 These neurotrophic factors have both autocrine and paracrine effects, as they promote growth of the damaged neurons as well as the adjacent Schwann cells.Nerve growth factor (NGF) typically has a low level of expression in nerves that are healthy and not growing or developing, but in response to nerve injury NGF expression increases in Schwann cells.This is a mechanism to increase growth and proliferation of Schwann cells at the distal stump in order to prepare for reception of the regenerating axon.