Page 339 - Atlas of Histology with Functional Correlations

contrast, each Schwann cell cytoplasm can envelope numerous unmyelinated

axons. Unmyelinated axons enveloped by Schwann cells do not show nodes of
Ranvier. Smaller axons in the peripheral nerves, such as those in the autonomic
nervous system (ANS), are unmyelinated and surrounded only by the Schwann
cell cytoplasm.

There are no Schwann cells in the CNS. Instead, neuroglial cells called

oligodendrocytes myelinate the axons in the CNS. Oligodendrocytes differ from
Schwann cells in that the cytoplasmic branching processes extend radially from
one oligodendrocyte and myelinate numerous adjacent axons.

GRAY AND WHITE MATTER

The brain and the spinal cord contain gray matter and white matter. The gray
matter of the CNS consists of neurons, their dendrites, and the supportive cells

called neuroglia. This region also represents the site of connections or synapses
between a multitude of neurons and dendrites. Gray matter forms the outer
surface of the brain (cerebrum) and cerebellum. The size, shape, and mode of

branching of these neurons are highly variable and depend on which region of
the CNS is examined.

The gray matter also contains a meshwork of neural tissues such as axonal,
dendritic, and glial processes that are packed very tightly together and that fill
the interneural spaces. This associated meshwork of neural processes in the gray

matter is called the neuropil. White matter in the CNS is devoid of neuronal
cell bodies and consists primarily of myelinated axons, some unmyelinated
axons, the supportive neuroglial oligodendrocytes, and blood vessels. The
myelin sheaths around the axons impart a white color to this region of the CNS.

SYNAPSES

Synapses are specialized sites for chemical or electrical transmission for
communication between neurons, interneurons, and effector cells, such as the
muscle fibers or glands. Synapses are too small to be visible with routine
histologic preparations but can be seen ultrastructurally with transmission

electron microscopy. The transmission of an impulse at the synapse is from one
presynaptic cell to a postsynaptic cell and is always unidirectional. Synapses

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