NERVOUS TISSUE

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Essentials of Histology 

     The nervous tissue is composed by neurons and supporting cells, called neuroglia. The neurons are
highly specialized cells that transmit stimuli, helping in the coordination and integration of all organic
systems. The neuron consists of a cell body, the perikaryon, and nerve processes. There are two types
of nerve processes: the dendrites and the axon. The dendrites appear in greater amount and are shorter
comparing to the axons, and are specialized to receive stimulus. The axon is a single nerve extension
that carries the impulse from the cell body. The axons can be extremely long and present several side
branches near its end. The terminal portion of the axon is known as terminal bouton, from where it
transmits the impulse to another neuron. This site is called synapse. The neuroglia are supporting cells
that can be separated in 4 types: the astrocytes, oligodendrocytes, microglia and ependymal cells. The
glia cells are responsible for the chemical and physical maintenance of the neurons. The nerve processes
can or can not be surrounded by a lipoprotein material called myelin. The cells responsible for the
production of  myelin are, the oligodendrocyte in the central nervous system, and the Schwann cell in
the peripheral nerves. The nerve fibers surrounded by a single layer of myelin are called nonmyelinated
fibers. Successive surrounding layers of myelin around the axon will produce a myelin sheath, and the
fibers are so called myelinated fibers.
    The distribution of nerve, glia cells and myelin can be associated macroscopically to the color of the
nervous tissue. The gray matter is formed by neurons, their processes and neuroglia. The nerve
processes in the gray matter do not present myelin. On the other hand, the white matter is composed by
the nerve processes with myelin sheaths and glia cells. The distribution of the gray and the white matter
in the nervous system is organized as followed: in the cerebral and cerebellar hemispheres, the gray
matter is found in an outer cortex and the white matter in a subcortical region; in the spinal cord, the
gray matter is concentrated in a central H-shaped region, that is surrounded by white matter.
    The cerebral cortex contains about 14 billion neurons, their processes and glia cells. Analyzing the
perikaryon shape it is possible to separate the cortex in 5 layers of nerve cells. These layer are not so
distinct as the ones observed in the cerebellar cortex. The gray matter in the cerebellum can be
segmented in 3 layers: an outer molecular layer, a Purkinje cell layer and a granule cell layer. The
distribution of the white and gray matter differs, however, in the spinal cord. In this part of the CNS, as
we presented above, the gray matter forms a H-shaped region, surrounded by white matter. The
H-shaped region contains the spinal horns, two ventral and two dorsal. These horns present big
multipolar neurons, that in the ventral horn are motor neurons and in the dorsal horn internuncial
neurons.