A. Nervous System tissue consists of:
1. neurons and their processes
- 2. glia: neuroglia of neuroectodermal origin and microglia (most probably
from bone marrow)
- 3. supporting elements, including meninges and blood vessels
B. Neurons -- Parenchymal Cells
Neurons are specialized for propagation of impulses along their surface
with transmission of information across synapses to establish a communication
network. The neuron consists of a cell body, a number of processes called
dendrites which are specialized for receiving information from other neurons
and transmitting it to the cell body, and a process called an axon which
is specialized to conduct impulses away from the cell body. The terminal
end of the axon is called the synaptic bouton or synaptic terminal and is
involved in neurotransmission.
- 1. There is a great deal of variability in the shape and size of neurons.
Neurons are specialized for specific types of information processing in
- 2. The cytoplasm of the cell body contains the same organelles found
in all cells. In addition, some neurons contain prominent clumps of granular
material called Nissl bodies, composed of rough endoplasmic
reticulum. There are 3 types of neurofibrils: microtubules, neurofilaments,
- 3. The synapse is the point of contact between the axonal ending of
one nerve cell and the dendrite or cell body of another nerve cell. Occasionally,
synapses occur between two dendrites or between two axons. The synapse
consists of 3 distinct components:
a. a presynaptic knob or terminal bouton (the axonal ending);
b. the synaptic cleft, a narrow extracellular space between the presynaptic
neuron and the post synaptic neuron across which neurotransmitter diffuses;
c. the postsynaptic membrane.
C. Glia -- Stromal or Supporting Cells
Glia, or neuroglia, are the specialized interstitial or stromal cells of
the CNS. There are four basic types of glial cells: astrocytes, oligodendrocytes,
microglia and ependymal cells.
- 1. Astrocytes have many cytoplasmic processes, foot-like extensions,
which terminate on blood vessels, neuronal cells bodies, and axons, and
surround synaptic terminals and clefts. Functions include biochemical and
physical support for neurons, insulation, guidance of migrating neurons
during development, and repair following injury. Astrocytes are commonly
subdivided into two subclasses: protoplasmic and fibrous. Fibrous astrocytes
contain many filaments in long thin processes and are found predominantly
in white matter. The protoplasmic form has frequently branched processes
and occurs mostly in gray matter.
- 2. Oligodendrocytes in the gray matter are called satellite
cells and function in fluid and respiratory exchange. Oligodendrocytes
in the white matter form myelin sheaths in the CNS (Schwann cells
form myelin sheaths in the PNS). Each oligodendrocyte supplies myelin sheath
segments to many axons while each Schwann cell supplies only one myelin
- 3. Microglia are small elongated cells -- the nervous system
representative of the mononuclear phagocytic (reticulo-endothelial) system.
Their normal function is not well understood but they participate in response
- 4. Ependymal cells are cuboidal ciliated cells lining the ventricles
and the central canal of the spinal cord.