BDNF and bone fracture: For example, Li et al demonstrated that neurotrophic factors such as brain-derived neurotrophic factor (BDNF) or neuropeptides such as Substance P (SP) released from afferent C-fibers can stimulate bone formation during fracture healing.90 Glia cells also contribute to the healing process by myelination and pain signals transmission.91 Following bone fracture, mechanical injury to nerve fibres may also occur, generating neuropathic pain states.92