Page 362 - Atlas of Histology with Functional Correlations

and released neurotransmitters, such as glutamate, at active synaptic sites

to maintain a proper ionic environment for their function. If these metabolic
chemicals are not removed from these sites, they can interfere with neuronal
functions. Astrocytes inactivate glutamate and convert it to glutamine, which
is returned to the neurons. Astrocytes also contain reserves of glycogen that
they release as glucose, contributing to the energy metabolism of the CNS.

Also, with the presence of gap junctions, the astrocytes form a structural
syncytium and a communicating network in the CNS. In response to brain
injury, the astrocytes divide, proliferate, and form a scar.

Oligodendrocytes are smaller than astrocytes with fewer cytoplasmic

processes. Oligodendrocytes produce and myelinate the axons in the CNS
for insulation. Because of several cytoplasmic processes, a single
oligodendrocyte surrounds and myelinates several axons. As a result,
oligodendrocytes do not surround multiple unmyelinated axons. During

myelination, the plasma membrane of the oligodendrocyte is wrapped around
the adjacent axons and at intervals exhibits the nodes of Ranvier. In the
PNS, Schwann cells myelinate the axons, and in contrast to
oligodendrocytes, a Schwann cell myelinates only a single axon.

Microglia are the smallest neuroglial cells and are considered to be part
of the mononuclear phagocyte system of the CNS derived from the
circulating monocytes that originate in the bone marrow. Microglia enter the
CNS through the vascular system, and their main function is similar to that of
the macrophages of the connective tissue. During nervous tissue injury or

damage, microglia migrate to the region, proliferate, become phagocytic, and
remove dead or foreign tissue. Microglia constitute the brain’s major immune
system and, when activated, function as antigen-presenting cells and secrete

immunoregulatory cytokines.

Ependymal cells are simple cuboidal or low columnar epithelial cells
that line the ventricles of the brain and the central canal in the spinal cord.
Their apices contain cilia and microvilli. Cilia facilitate the movement of the
CSF through the central canal of the spinal cord, whereas microvilli may

have some absorptive functions.

Summary

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