Glia cells play a supportive role to the neuron. A few of the functions of the glial cells include providing nutrition, maintaining homeostasis, stabilizing synapses, and myelinating axons. The glial cells are categorized as microglia and macroglia. Of these two cell types, which one plays an active and critical role in glutamatergic neurotransmission by providing a co-agonist required for glutamate receptor function?