ABSTRACT
When tissue damage occurs, astrocytes and microglia are the first cells to be activated. Until recently, the role of astrocytes in the injured brain was believed to be limited to scar tissue formation, separating dying from healthy tissue. Microglial cells were thought to be confined to a primarily phagocytic function, i.e., removing tissue debris after trauma. Now it is widely recognized that astrocytes and microglia play a more pivotal role in the recovery process by reestablishing homeostasis, synthesizing and releasing neuroactive substances, and triggering an orchestrated immune reaction within the brain itself. These events are called “glial activation.” Furthermore, activated astrocytes can enhance
axonal sprouting
and
reactive synaptogenesis
, events that are known to correlate well with behavioral recovery. Under the appropriate conditions, glial cells can intervene in neuronal signal transduction by absorbing or releasing neurotransmitter molecules such as glutamate and aspartate. Although the activation of both glial populations occurs
after trauma, the reported time delays between the appearance of microglial and astroglial cells led to the assumption that microglia respond first and then they, in turn, trigger the increased metabolic and biological activity of astrocytes (see Dietrich et al., 2000).
