they have been traditionally thought to function as the immunocompetent cells of the brain and spinal cord   and to be the sensors of injury and infection in the tissue [ 2 , 3 ] . they derive from primitive c - kit(+ ) erythromyeloid precursors from the yolk sac [ 46 ] , migrate into the brain during the period of early embryonic development prior to the formation of the blood - brain barrier ( bbb ) , and remain there once the bbb is formed . it is notable that this population is self - sustaining , and peripheral macrophages only contribute to this population in disease states , in which the bbb becomes compromised . within the brain , microglia have been described to have the ability to detect and mount an inflammatory response to various insults . sensing neuronal injury has been ascribed primarily to purinergic and chemokine receptors on the surface of microglia , as they monitor the levels of extracellular atp and secreted chemokines , respectively . their reaction to neuronal injury is accomplished as they undergo a process collectively called  activation . activation consists of several biological events that include migration to the site of injury , local proliferation , a change in morphology and gene expression , antigen presentation , and phagocytosis of dead cells and cell debris [ 10 , 11 ] . during activation , some of the changes in gene expression involve the secretion of cytokines and chemokines , which modulate the cns environment and regulate the state of inflammation . microglia can secrete both proinflammatory mediators , such as tumor - necrosis - factor- ( tnf- )  [ 12 , 13 ] or interleukin- ( il- ) 1 , nitric oxide ( no ) [ 14 , 15 ] , and glutamate , and anti - inflammatory effectors , such as il-4 and il-13 , which can enhance neuronal survival [ 17 , 18 ] .   depending on the predominance of factors secreted , microglia have classically been characterized , similarly to macrophages , as m1 ( proinflammatory ) or m2 ( anti - inflammatory ) cells . the m1 and m2 distinctions serve to separate activated microglia into the two broad categories . it is argued , however , that no specific marker designates a microglial response as definitively m1 or m2 . moreover , microglia differentially express pro- and anti - inflammatory markers making the m1 and m2 phenotypes the extremes of the categories , respectively , with variable phenotypes seen in between the two . with that in mind , m1 microglia have been associated with neurotoxic and neurodegenerative outcomes , as they are observed in a variety of chronic neurodegenerative diseases , such as alzheimer 's , end - stage amyotrophic lateral sclerosis ( als ) , and multiple sclerosis ( ms ) . stroke   and traumatic brain injury   show a characteristic accumulation of m1 microglia as well . a shift to an m2 phenotype of activated microglia has been correlated with neuroprotection , recovery , and repair in various disease settings [ 2527 ] . until recently , initial studies to understand neuronal - microglial interactions had described that a variety of neuroactive substances , such as no   and tnf- , have potent effects on neuronal function , in particular , synaptic plasticity . however , the cellular origin of these molecules had not been attributed to microglia but , rather , to astrocytes . the focus was maintained on the effect that inflammatory processes have on synaptic plasticity . in neuroinflammatory diseases , such as experimental autoimmune encephalomyelitis ( eae ) , a rodent model of ms , alterations in synaptic plasticity specifically , in the hippocampus of diseased animals , there is greater induction of long - term potentiation ( ltp ) , an electrophysiological measurement that relates to the connectivity and strength of synapses . this change in ltp was attributed to the secretion of il1 from the accumulated microglial cells . it is notable , however , that prolonged exposure to inflammatory cytokines can result in priming or sensitization of microglia so they more readily adopt an m2 , rather than m1 , phenotype in response to inflammation . , chronic inflammation can be induced by lps infusion and has been reported to attenuate ltp in the dentate gyrus ( dg ) of the hippocampus . found that ltp , dependent on either nmda receptors ( nmdar ) or on voltage - dependent calcium channels , was impaired . the cytokine , tnf- , increases the surface expression of ampa receptors in neuronal cultures , which is accompanied by the enhancement of synaptic strength . in a model of neuropathic pain in the c fibers of the dorsal horn of the spinal cord , inhibition of microglial activation by minocycline resulted in the induction of long - term depression ( ltd ) rather than ltp . this change in plasticity was found to involve src family kinases and to be mediated partially by tnf- . some studies have found that microglial activation , when both genetically and pharmacologically induced , results in an increase of ampar / nmdar ratio and an enhanced ratio of ampar- over nmdar - mediated currents . these studies demonstrated that when microglia were activated under pathological inflammatory conditions , they caused synaptic alterations via secretory mediators . the precise role of microglia on synaptic activity in the normal brain remained unclear . imaging studies have shown that microglia extend and retract their processes continuously to survey their local environment in the healthy brain [ 37 , 38 ] . moreover , interactions between microglia and neuronal synapses in the visual cortex have been directly visualized by electron microscopy ( em ) and by in vivo two - photon microscopy . the availability of visual stimuli resulted in enhancement of the duration of these contacts and the preservation of the synapse . these intriguing imaging observations first indicated the possibility that microglia could modulate neuronal functions by direct physical contacts . on the other hand , wake et al . demonstrated that , under conditions of prolonged ischemia , contact time between microglial processes and synapses increased , associated with a greater chance for elimination of presynaptic boutons . it is likely that the mechanism of synaptic pruning is modulated by a distinct molecular mechanism in each of these states and not simply by the longevity of contact . paolicelli et al .   explored whether there is a functional role for microglial interactions with synaptic structures during the development of the postnatal brain , using imaging and electrophysiological approaches . they used a transgenic mouse line expressing gfp in microglia , under the control of the chemokine receptor cx3cr1 promoter . thus , they were able to label and visualize microglia as well as manipulate them . the authors found that the number of synaptic elements and dendritic spines expressing the postsynaptic marker psd95 in cx3cr1 mice was about 3-fold higher than that in mice deficient in cx3cr1  ( cx3cr1 ) . their result provided some insight into potential roles of microglia in synapse maturation , along with the possibility that this may be a direct cx3cr1-mediated event . these microglial properties , thus , extend beyond immune surveillance and indicate modulatory roles during normal brain development . provided direct evidence , via confocal microscopy and electron microscopy , that microglia phagocytose synapses in the dorsal lateral geniculate nucleus ( dlgn ) as well . the authors proposed that the classical complement cascade , which includes members c1q and c3 , was a potential molecular pathway of microglia - synapse interactions in postnatal brain development . according to the experimental data , the synapses that were tagged with c1q and c3 were phagocytosed by microglia that expressed complement receptor cr3 . in mice deficient in the receptor or the ligand , higher numbers of synaptic inputs moreover , these animals showed deficits in their ability to segregate the territories of each eye . therefore , the microglia - mediated engulfment was important to drive synaptic stripping during normal development . together , these observations reveal that complement - mediated phagocytic activity of microglia is crucial in microglia - synapse interactions during normal brain development . to further address whether microglia contribute to synaptic activity in the normal young adult brain , ji et al .   used an electrophysiological approach in organotypic hippocampal brain slices and primary neuronal cultures . in this system , they manipulated the presence of microglia by either depleting them using clodronate , replenishing them in previously depleted cultures , or by using cocultures of neurons and microglia . the absence of microglia resulted in a robust increase of synaptic frequencies known as spontaneous and miniature excitatory postsynaptic currents ( sepsc and mepsc ) from the ca1 region of the hippocampus . this increase was subsequently reversed when microglia were replenished in the organotypic slices . in the complementary approach , the addition of microglia to neuronal cultures decreased the synaptic activity measured compared to cultures of neurons alone . the change in synaptic activity coincided with changes in synaptic numbers , which suggested that microglia could participate in the control of synaptic activity by regulating synaptic numbers ( figure 1 ) . as shown in previous reports , they also observed that the phagocytic activity of microglia drove synapse elimination when microglia were coincubated with neurons . this could be one mechanism by which synapse numbers are regulated in the normal brain ; however , it is still undefined whether synaptic pruning and phagocytic engulfment by microglia occur via a universal mechanism under normal and pathological conditions . a proposed mechanism by which microglia could regulate synaptic activity was suggested by the same study . overall expression of synaptic adhesion molecules , such as protocadherin and syncam1 , which determine synapse remodeling , stability , and synaptic activity , was decreased in neurons incubated with microglia compared to neurons alone ( figure 1 ) . the decreased levels of the synaptic adhesion molecules were recovered to wild - type levels when neurons were incubated with microglia deficient in the serine protease tissue plasminogen activator ( tpa ) ( figure 2 ) , potentially implicating serine proteolytic functions in the stability of these proteins . structural changes at the synapse are closely associated with synaptic stability . in particular , numerous synaptic adhesion molecules , such as classic cadherins ( e - cadherin and n - cadherin ) , protocadherins , and ncam , have been studied in modulating structural and functional synaptic plasticity . hippocampal slices pretreated with antibodies against the extracellular domain of n- and e - cadherins or with antagonistic peptides that inhibit cadherin dimerization exhibit a significantly reduced ltp . moreover , expression of mutant n - cadherin or short hairpin rna - mediated knockdown of n - cadherin prevents ltp - induced long - term stabilization of synapses . additionally , yamagata et al . showed that blocking antibodies to protocadherins or ncam in hippocampal slices diminished synaptic transmission and ltp induction . proteases in a synaptic microenvironment are important in the regulation of dynamic changes in the adhesion molecules associated with synaptic plasticity . proteases , such as matrix metalloproteinases ( mmps ) and tpa , secreted from neurons , astrocytes , or microglia under basal or pathological conditions of the cns have been associated with the targeted degradation or proteolytic processing of extracellular matrix ( ecm ) and cell adhesion molecules on the cell surface and at the synapse [ 4855 ] . in particular , application of tpa or mmp-9 emerging evidence has shown that application of mmp-9 or tpa decreases the levels of n - cadherin and diminishes synaptic transmission . moreover it is possible that proteases secreted from microglia could regulate synaptic activity by remodeling the ecm which is known to affect synaptic connectivity . each of the connected cells provides an array of cx isoforms , which form oligomers containing 6 of these cx proteins . cx36 and cx43 have been reported to be expressed in microglia [ 60 , 61 ] , where they are thought to be involved in the local release of proinflammatory cytokines ( tnf- and il1 )   and metabolites . during inflammatory events , the expression of cx43 this increase results in the formation of a functional syncytium among microglial cells , confirmed by the diffusion of the fluorescent dye , lucifer yellow . however , the syncytium neither forms in nonactivated microglia , nor happens if the gap junction formation is inhibited by inhibitors of cx43 , indicating the involvement of cx43 in the process . cx36 remains active in resting microglia and does not become upregulated during microglial activation .   in disease settings , it has been reported that blocking cx hemichannels resulted in the blockade of the microglial release of glutamate , which led to the subsequent exaggerated activation of neurons ( excitotoxicity ) . in a model of spinal cord injury ( partial cord transection ) , inhibition of cx resulted in improved functional recovery . similar to cx proteins , large pore channels are formed in microglia and consist primarily of pannexins and p2x channels . they are purinergic and activated by extracellular atp . among them , p2x4 is the channel that becomes primarily upregulated in activated microglia . in a recent report , li et al . state that in the optic tectum of larval zebrafish , neuronal activity drives the activation of pannexin-1 hemichannels . these can then  steer  the processes of resting microglia and facilitate their contact with highly active neurons . in turn , when resting microglia are in contact with neurons or neurites , a decrease in both spontaneous and visually evoked neuronal activities is observed , specifically for the neurons contacted . these results indicate that connexins and large pore channels could constitute one way by which microglia interact directly with neurons , especially during neuronal insult and inflammation , and could directly affect neuronal activity and survival . they also respond to changes in neurotransmitters by changing morphology and the motility of their processes . application of the glutamate receptor inhibitors nbqx and gyki , as well as the gabaergic signaling inhibitor bicuculline , has been shown to decrease microglial process motility . although there is debate on whether microglia express glutamate receptors , the presence of gabaa receptors on the surface of human microglia has been documented . however , there is no concrete evidence that microglia respond in an obvious way to direct application of agonists of glutamate or gaba receptors , that is , in a pure microglial cell culture . rather , they seem to respond indirectly to such application , on a slice or tissue , suggesting that these agonists potentially have indirect effects on the cells . these indirect effects have been postulated to be mediated through atp 's effect on purinergic receptors since they are expressed on the surface of the cells . although the source of the atp release is not entirely determined , the most likely mechanism involves release through neuronal pannexin channels . in models of disease , specifically in the eae model of ms , the presence and accumulation of activated microglia have been correlated with decreases in the purkinje cell survival , connectivity in the cerebella of the eae animals , and attenuation of gabaergic transmission . this has also been observed in the eae striatum   and hippocampus , where a decrease in gabaergic interneurons was also noted , accompanied by induction of ltp . these results suggest that , in this context , microglia may be direct regulators of the numbers of gabaergic neurons and the subsequent attenuation of gabaergic inhibitory transmission . a new way of communication in the cns has been described involving the release of microvesicles , also referred to as shed vesicles or ectosomes , from the plasma membrane . these materials were originally thought to be inert but have been recently recognized as critical in mediating cell - to - cell communication . the vesicles contain lipids , cell surface proteins , and material from the cytoplasm or nucleus of the cell . the vesicles are recognized by the recipient cell through the presence of phosphatidylserine on their surface   and interact with the relevant receptors . they can also directly fuse with the recipient cell .   on the surface of microglia , p2x7 receptors , which respond to the release of atp , mediate the shedding of ectosomes . this process is triggered by the activity of acid sphingomyelinase and involves the activation of the effector protein p38 . although this is not a mechanism exclusive to microglia ( as astrocytes also have been shown to express the p2x7 receptors ) , microglia constitute a significant source of these shed vesicles . one of the factors thought to contribute to such signaling is annexin a2 , a protein expressed by microglia that affects their activation [ 78 , 79 ] . annexin a2 has been shown in different systems to affect neuronal ion channels and neuronal functioning [ 80 , 81 ] , either directly or through its interaction with p11 . in a recent report , it was noted that microvesicles derived from microglia were able to increase the frequency and amplitude of epscs . this effect required interaction between microglia and neuronal cells and did not involve secretion of cytokines . it did involve , however , an increase in the metabolism of sphingolipids in neuronal cells . although in a more chronic exposure to these shed particles the release of cytokines as regulators of neuronal activity can not be excluded , these data provide another possible pathway by which microglia affect neuronal activity . given the evidence from imaging , cellular , and electrophysiological approaches , the physical proximity between neurons and microglia seems to result in synaptic maturation and synaptic activity ( figure 3 ) . several different mechanisms , either involving direct contact and interaction between the two cell types , or mediated through chemical ligands and effectors , are described as potential regulators of these microglial functions . the findings indicate that microglia affect both the maturation of the cns during development and the acute and dynamic regulation of neuronal activity in the mature , healthy , or unhealthy cns and suggest that they are active contributors to a potential quad - partite synapse .