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 different regions.

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, and microfilaments.

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 segment.

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 to injury.

4. Ependymal cells are cuboidal ciliated cells lining the ventricles and the central canal of the spinal cord.

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