Astrocytes provide structural isolation of neurons and their synapses and provide ionic (K+) sequestration, trophic support, and support for growth and signaling functions to neurons. Oligodendroglia provide myelination of axons in the CNS. Microglia are scavenger cells that participate in phagocytosis, inflammatory responses, cytokine and growth factor secretion, and some immune reactivity in the CNS. Perivascular cells participate in similar activities at sites near the blood vessels. Schwann cells provide myelination, ensheathment, trophic support, and actions that contribute to the growth and repair of peripheral neurons. Activated T lymphocytes normally can enter and traverse the CNS for immune surveillance for a period of approximately 24 hours.
NEURONS AND GLIAL CELLS
Recent years have witnessed a growing appreciation for functional roles astrocytes play within the CNS. It increasingly appears to be the case that astrocytes are integral to brain energy utilization. For example, at glutamate synapses astrocytes take up the glutamate that is released into the synaptic space by the presynaptic neuron. The glutamate is co-imported into the astrocyte along with a sodium cation. The sodium cation, in turn, is removed from the astrocyte by the action of the plasma membrane adenosine triphosphate (ATP)-dependent Na+-K+ pump. This consumes ATP, which activates astrocyte glycolysis, and, in turn, this stimulates glucose uptake from neighboring capillaries. By consuming more glucose through glycolysis, the astrocyte restores its energy supply but, in the process, also generates lactate. Astrocyte- generated lactate is then exported to the recently activated synapse neurons to help meet its increased energy needs. This relationship defines what is called the “astrocyte-neuron lactate shuttle hypothesis,” and suggests the classic bipartite synapse of a presynaptic and postsynaptic neuron might more accurately be thought of as a tripartite synapse consisting of a presynaptic neuron, postsynaptic neuron, and associated astrocyte. It is also important to note that glutamate absorbed by the synaptic astrocyte is recycled back to the presynaptic neuron. This is accomplished by converting it to glutamine before releasing it into the extracellular space. The presynaptic neuron is able to take up the glutamine, and once it is back inside the neuro, the glutamine is converted back to glutamate.