Chapter 11
Fundamentals of the Nervous System and Nervous Tissue
389
11
that supports the neurons, neuroglia are now known to have
many other important and unique functions.
Neuroglia in the CNS
Neuroglia in the CNS include
astrocytes
,
microglial cells
,
ependy-
mal cells
, and
oligodendrocytes
(Figure 11.3a–d). Like neurons,
most neuroglia have branching processes (extensions) and a
central cell body. Tey can be distinguished, however, by their
much smaller size and their darker-staining nuclei. Tey out-
number neurons in the CNS by about 10 to 1, and make up
about half the mass of the brain.
Astrocytes
Shaped like delicate branching sea anemones,
astrocytes
(as
9
tro-sītz; “star cells”) are the most abundant and
versatile glial cells. Teir numerous radiating processes cling
to neurons and their synaptic endings, and cover nearby capil-
laries. Tey support and brace the neurons and anchor them to
their nutrient supply lines (Figure 11.3a).
Astrocytes play a role in making exchanges between capil-
laries and neurons, helping determine capillary permeability.
Tey guide the migration of young neurons, and formation of
synapses (junctions) between neurons. Astrocytes also control
the chemical environment around neurons, where their most
important job is “mopping up” leaked potassium ions and re-
capturing and recycling released neurotransmitters. Further-
more, astrocytes have been shown to respond to nearby nerve
impulses and released neurotransmitters.
Connected by gap junctions, astrocytes signal each other
both by taking in calcium, creating slow-paced intracellular
calcium pulses (calcium waves), and by releasing extracellular
chemical messengers. Recent research shows they also influence
neuronal functioning and therefore participate in information
processing in the brain.
Microglial Cells
Microglial cells
(mi-kro
9
gle-al) are small and
ovoid with relatively long “thorny” processes (Figure 11.3b).
Teir processes touch nearby neurons, monitoring their health,
and when they sense that certain neurons are injured or in other
trouble, the microglial cells migrate toward them. Where invad-
ing microorganisms or dead neurons are present, the microglial
cells transform into a special type of macrophage that phago-
cytizes the microorganisms or neuronal debris. Tis protective
role of the microglial cells is important because cells of the im-
mune system have limited access to the CNS.
Ependymal Cells
Ependymal cells
(ĕ-pen
9
dĭ-mul; “wrapping
garment”) range in shape from squamous to columnar, and
many are ciliated (Figure 11.3c). Tey line the central cavities
of the brain and the spinal cord, where they form a fairly per-
meable barrier between the cerebrospinal fluid that fills those
cavities and the tissue fluid bathing the cells of the CNS. Te
beating of their cilia helps to circulate the cerebrospinal fluid
that cushions the brain and spinal cord.
(a) Astrocytes are the most abundant CNS neuroglia.
(d) Oligodendrocytes have processes that form myelin
sheaths around CNS nerve fibers.
(e) Satellite cells and Schwann cells (which form myelin)
surround neurons in the PNS.
(b) Microglial cells are defensive cells in the CNS.
Schwann cells
(forming myelin sheath)
Cell body of neuron
Satellite
cells
Nerve fiber
Capillary
Neuron
Astrocyte
Neuron
Microglial
cell
Brain or
spinal cord
tissue
Ependymal
cells
Cilia
Nerve
fibers
Myelin sheath
Process of
oligodendrocyte
Fluid-filled cavity
(c) Ependymal cells line cerebrospinal fluid–filled cavities.
Figure 11.3
Neuroglia.
(a–d)
The four types of neuroglia of the
CNS.
(e)
Neuroglia of the PNS.
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