Glia and Other Non-neuronal Cells

Glial cells and other non-neuronal cells support nervous system function. Glial cells are currently viewed as active partners of neurons in synapse formation. They promote neurogenesis and synaptogenesis, sensing nutrients and metabolic signal molecules crucial for neuronal survival and function, and critically mediating immune responses. Essentially, the function of glial cells is to maintain a homeostatic environment for neuronal circuits to work properly in response to various environmental alterations. These homeostatic regulations provide by the neuroglia range from systemic homeostasis involving bidirectional communications between the brain and the periphery and defensive homeostasis of surveilling the environment, clearing the damage to cellular and molecular homeostasis of establishing, and maintaining a proper neuronal network in response to developmental cues and environmental triggers.

Fig.1 Summary of proposed mechanisms of glial regulation of synaptic transmission. (Newman, 2003)

Functions of Glia in Central Nervous System (CNS)

Types of Glia and Their Functions

• Astrocytes

Astrocytes are important in brain functions as diverse as ion and fluid balance in the interstitial space, contributing to the integrity of the neurovascular unit (blood-brain barrier), neurotransmitter regulation, metabolism of energy substrates, and possibly even axonal regeneration. Given their ubiquitous distribution and extensive network of processes, these cells are ideally suited for CNS-wide support of homeostatic mechanisms. Overall, it is becoming clear that astrocytes regulate CNS function from the molecular-microenvironment level (e.g., neurotransmitter turnover) to the physiologic level (e.g., brain states).

• Myelinating Glia

The myelinating glia has two different types: the oligodendrocytes in the CNS and the Schwann cells in the PNS. These two different types of glial cells share a similar function and are responsible for the production of myelin and myelinating the PNS and the CNS, respectively.

• Microglia

Microglia are of monocyte origin and fill a role as immune effector cells in the CNS. Microglia can regulate neuronal survival and have multiple parts in refining CNS formation and function. Microglia have been involved in phagocytosing cellular and myelin components and implicated in synapse formation. In addition, microglia have key roles in regulating the development and responses of other cells; finally, microglia direct endothelial cells to increase brain vascular complexity.

• Ependymal Cells

Ependymal cells are relatively abundant and are involved in the connection between the CSF and nervous tissue. The ependyma detoxifies various substances in the CSF as a protective barrier between the brain and the CSF. The ependyma is also a source of NSCs, which can differentiate into neurons and glial cells. In inflammation or ventricular dilation, ependymal cells can combine with subependymal astrocytes to form ependymal granulations.

Fig.2 Myelinating oligodendrocyte in the mouse cortex. (Allen, 2018)

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References

1. Newman, E. A. New roles for astrocytes: regulation of synaptic transmission. Trends in neurosciences. 2003, 26(10), 536-542.
2. Allen, N. J.; Lyons, D. A. Glia as architects of central nervous system formation and function. Science. 2018, 362(6411), 181-185.