The Central Nervous System
In addition to neurons, the central nervous system contains supporting cells. Supporting cells, generically called glia, can outnumber neurons by a factor of ten. Historically, glia were defined as the “nerve glue” that holds neurons together in the central nervous system. However, glia are now known to have key dynamic functions in the central nervous system, from myelin synthesis, to synapses, to serving as the innate brain defensive system against pathology. Glia consist of three types of supporting cells: oligodendrocytes, astrocytes, and microglia.
Oligodendrocytes synthesize myelin and provide an expedient way, via the myelin sheath, to significantly increase how fast an axon can conduct an action potential. Myelin is a long plasma membrane sheet that wraps around each axonal segment, leaving bare axons between myelin segments, known as the nodes of Ranvier (Figure 2.3). Myelin effectively forms insulation that allows the action potential to jump from node to node, known as salutatory conduction.
Astrocytes are star-shaped cells that have processes (branches) and both physical and biochemical support functions in the central nervous system. They physically isolate neurons and oligodendrocytes with long processes by making a cover over the nodes of Ranvier and covering the surface of capillaries, forming part of the blood–brain barrier. Astrocytes play a key role in the migration and guidance of neurons during neural circuit development. They control the formation, maturation, function, and removal of synapses. They also regulate neurotransmission and participate in reuptake processes, particularly for the excitatory neurotransmitter glutamate. Astrocytes produce growth factors and signals for activating cytokines, which can also regulate neurotransmission. They can be activated in a wide range of central nervous system disorders, ranging from neurodegenerative disorders and brain trauma to drug addiction (for further reading, see Clarke and Barres, 2013).
Microglia are immune-like cells in the central nervous system, comparable to macrophages in the immune system. A macrophage is a large cell that removes waste products, harmful microorganisms, and foreign material from the bloodstream. Immune cells are unlikely to enter the central nervous system because they cannot cross the blood–brain barrier making the brain an immunologically privileged site. However, microglia may be recruited in the brain to serve similar functions. Microglia in the central nervous system are activated by any form of central nervous system injury. They not only remove damaged cells in the brain but also remove synapses that are no longer functioning. When activated, microglia act as macrophages and, similarly to astrocytes, secrete growth factors and cytokines, both of which can modulate and regulate neurotransmission (for further reading, see Kettenmann et al., 2013).