CELLS OF THE NERVOUS SYSTEM AND ELECTRICAL STIMULATION - Advanced Biomaterials: Fundamentals, Processing, and Applications - page 603

neurons and direct the outgrowth of axons in development; regulate presynaptic
terminals at the nerve-muscle junction; and more. Not forgetting that neurons
have subcategories, the glial cells within the nervous system are further catego-
rized into microglia and macroglia. Macroglia are further subcategorized in the
CNS and the peripheral nervous system (PNS) into oligodendrocytes, astrocytes,
and Schwann cells, respectively.
First, consider microglia. As the name implies, microglia are typically much
smaller than the cells found in the CNS and are the smallest of the glial cells.
Microglia are phagocytes produced outside the nervous system and are unrelated
physiologically and developmentally to the other cells of the CNS. Microglia are
not phagocytotic until they are mobilized or become activated because of injury,
disease, or infection of the nervous system. The brain owes its limited immuno-
logical response to microglia, because microglia remove apoptotic cells and are
activated during injury.
Second, oligodendrocytes in the CNS and Schwann cells in the PNS are
responsible for making the transmission of neuronal signals more effi cient. They
do this by insulating axons with sheaths of a lipid-based substance called myelin.
The sheaths are created by oligodendrocytes/Schwann cells densely wrapping
their membranous processes around the axon repeatedly. A single oligodendro-
cyte can myelinate an average of 15 internodes/axons. Schwann cells, by compari-
son, only envelop one internode/axon. The myelin produced by Schwann cells is
not chemically the same as that produced by oligodendrocytes, but are very
similar.
Lastly, astrocytes derive their name from the Greek word astron , which means
of the stars. The reason for this name is easily understood when observing astro-
cytes in vitro , since their cell bodies resemble stars. Astrocytes are the most
numerous cells in the CNS and were once consider solely supportive cells. One
thought is that astrocytes fi lter nutrients to neurons, since astrocytes have end-
feet at both capillaries and neurons. These end-feet encircle capillaries in the
brain, helping make an impermeable blood barrier know as the blood-brain
barrier. Neurotoxic substances and high concentrations of neurotransmitters such
as norepinephrine and glutamate are prevented from entering or accumulation in
damaging concentrations in the brain, respectively, by the blood-brain barrier.
The functions that astrocytes perform are indispensable for the survival of
neurons in vivo . Astroglial functions were once considered to be purely sup-
portive. What is known about glia and how they are viewed is partly due to prece-
dence given to neurons over other cells when studying the nervous system.
Previous attention was mainly given to neurons because of their intriguing capa-
bilities such as the transmission of signals via the action potential. Therefore,
astrocytes were studied for their relevance to neurons. Recently, however, astro-
cytes and other cells in the CNS are shown to serve a more profound role due to
the discovery of neurogenesis (Ma et al. 2005). This adds to the problem of clearly
categorizing or defi ning what astrocytes are (Kimelberg 2004). In other words, the
neurogenic niche is infl uenced by astrocytes, making astrocytes more than mere
support cells for neurons.