Mechanisms and consequences of microglial responses to peripheral axotomy.

Microglia respond rapidly to injury of peripheral nerve axons (axotomy). This response is integrated into the responses of the injured neurons, i.e. processes for neuron survival, axon regeneration and restoration of target contact. The microglial response is also integrated in changes in presynaptic terminals on axotomized motor or autonomic neurons and in changes in the central terminals of peripherally axotomized sensory neurons. Microglia also has an established role in interacting with astrocytes to shape their response to peripheral axotomy. Axotomy models in mice have demonstrated a role for microglia in regulating the entry of lymphocytes into motor nuclei or sensory areas following peripheral axotomy. Whether this is a universal component of peripheral nerve injury remains to be determined. Under certain circumstances, microglia activated by axotomy are major contributors to CNS pathology, e.g. in models of neuropathic pain. However, the general roles played by microglia after peripheral nerve injury are still incompletely understood. Early proposals that the microglial reaction to peripheral nerve injury is preparatory for the eventuality of neuron degeneration may still have relevance.