Glial cells

Glial cellsNeuroglia1/5Synonyms: Neuroglia

Glial cells, also called neuroglia or simply glia, are smaller non-excitatory cells that act to support neurons. They do not propagate action potentials. Instead, they myelinate neurons, maintain homeostatic balance, provide structural support, protection and nutrition for neurons throughout the nervous system. 

This set of functions is provided for by four different types of glial cells;

• Myelinating glia produce the axon-insulating myelin sheath. These are called oligodendrocytes in the CNS and Schwann cells in the PNS. Remember these easily with the mnemonic “COPS” (Central – Oligodendrocytes; Peripheral – Schwann)
• Astrocytes (CNS) and satellite glial cells (PNS) both share the function of supporting and protecting neurons. 
• Other two glial cell types are found in CNS exclusively; microglia are the phagocytes of the CNS and ependymal cells which line the ventricular system of the CNS. The PNS doesn’t have a glial equivalent to microglia as the phagocytic role is performed by macrophages.

Most axons are wrapped by a white insulating substance called the myelin sheath, produced by oligodendrocytes and Schwann cells. Myelin encloses an axon segmentally, leaving unmyelinated gaps between the segments called the nodes of Ranvier. The neural impulses propagate through the Ranvier nodes only, skipping the myelin sheath. This significantly increases the speed of neural impulse propagation.