Fig.20. Structure of Synapse.

The cytoplasm in the endings of the terminal contains numerous synaptic vesicles. Synaptic vesicles are small membrane-bound vesicles containing substances called neurotransmitters that are responsible for the transmission of the nerve impulse across the synapse. Examples of neurotransmitters are acetylcholine, norepinephrine, γ-aminobutyric acid (GABA), glutamic acid, dopamine, serotonin, glycine, endorphins and enkephalins.

These mediators are liberated at the presynaptic membrane by exocytosis and act on the postsynaptic membrane to initiate an excitatory (excitatory synapse) or inhibitory (inhibitory synapse) response (Fig.21). The membranes of synaptic vesicles that are incorporated into the presynaptic membranes undergo endocytosis and are reused to form new synaptic vesicles (Fig.22).

Fig.21.

Fig.22. Synaptic Transmission

NEUROGLIA

Several cell types found in the CNS in association with the neurons are classified as neuroglia (neuroglial, or glial, cells).

Neuroglia separate neurons, form myelin, and have trophic and phagocytic functions. Neurons cannot exist or develop without neuroglia.

There are 4 basic types of neuroglia, based on morphological and functional features (Fig.23):

• astrocytes

• oligodendrocytes

• microglia

• ependymal cells.

The astrocytes and oligodendroglia are large cells and are collectively known as Macroglia.

Fig.23.

It has been estimated that in the CNS there are 10 glial cells for each neuron. Since neuroglia are much smaller, however, they occupy only about half the total volume of nervous tissue.

Neuroglia differ from neurons:

• Neuroglia have no action potentials and cannot transmit nerve impulses;

• Neuroglia are able to divide (and are the source of tumors of the nervous system);

• Neuroglia do not form synapses;

• Neuroglia form the myelin sheathes of axons.

Astrocytes are the largest of the neuroglia, possessing numerous long processes. They have spherical, centrally located nuclei that stain lightly. Many of their processes have expanded pedicles at their ends that attach to the walls of blood capillaries. These pedicles, called the vascular feet, completely surround and ensheathe all vessels of the vascular network (Fig.24). Processes of astrocytes are also present at the periphery of the brain and spinal cord, forming a layer under the pia mater.

Fig.24.

Astrocytes provide some structural support for nervous tissue, and their extensions form a sealed barrier that protects the CNS.

After injury to the CNS, astrocytes proliferate at the site of injury and form a type of scar tissue.

In electron micrographs, astrocytes are identified by their light-staining, relatively organelle-free cytoplasm. Intermediate filaments, composed of glial fibrillar acidic protein, are abundant.

There are 2 types of astrocytes: protoplasmic, which are found in the gray matter of the brain and spinal cord; and fibrous, which are found chiefly in the white matter.

• Protoplasmic Astrocytes have abundant granular cytoplasm. Their processes have many branches, are shorter than those of fibrous astrocytes, and are relatively thick. Their processes envelop the surfaces of nerve cells, synaptic areas, and blood vessels.

• Fibrous Astrocytes have long, slender, smooth processes that branch infrequently. In special silver-stained preparations, their cytoplasm shows fibrillar material that is probably formed by the aggregation of intermediate filaments abundant in the cell bodies and processes of fibrous astrocytes.

Oligodendrocytes. Oligodendrocytes are much smaller than astrocytes, and their processes are less numerous and shorter than those present in other neuroglia. Their nuclei are smaller and stain more intensely than do the nuclei of astrocytes.

Oligodendrocytes are found in both gray and white matter. In gray matter, they are localized mainly close to perikaryons. In white matter, oligodendrocytes appear in rows among myelinated nerve fibers. The myelin sheath of CNS tissue is produced by the processes of oligodendrocytes.

In this aspect of their function, the oligodendrocytes are analogous to the Schwann cells of peripheral nerves. Unlike Schwann cells, oligodendroglial cells can participate in the myelination of more than one axon.

The cytoplasm of oligodendrocytes is electron dense and contains many mitochondria, a large Golgi complex, cisternae of rER, and numerous microtubules. Oligodendroglial cells are frequently found adjacent to neurons, and this association has given rise to the concept that oligodendrocytes have a symbiotic relation with neurons. Cytochemical studies have shown a metabolic dependency between neurons and satellite oligodendroglia.

Microglia. Microglia are phagocytic cells that represent the mononuclear phagocytic system in nervous tissue. Their cell bodies are small, dense and elongated. Their nuclei show highly condensed chromatin and en elongated shape along the axis of the cell body. Microglia have short processes covered by numerous small expansions, giving them a thorny appearance (Fig.28).

Fig.25.

Ependymal cells derive from the internal lining of the neural tube and retain their epithelial arrangement, while the other cells from the neural tube develop processes and give rise to neurons or neuroglia. Ependymal cells line the cavities of the brain and spinal cord and are bathed by the cerebrospinal fluid that fills these cavities. Most ependymal cells possess motile cilia that serve to produce movement of the cerebrospinal fluid.

NERVE FIBERS

Nerve fibers consist of axons enveloped by special sheaths. Groups of nerve fibers constitute the tracts of the brain, spinal cord, and peripheral nerves.

Most axons in adult nerve tissue are covered by single or multiple folds of a sheath cell. In peripheral nerve fibers, the sheath cell is the Schwann cell (or, the neurolemmocyte), and in central nerve fibers is the oligodendrocyte. Axons of small diameter are usually unmyelinated nerve fibers. Axonal conduction of the nerve impulse is faster in axons with larger diameters and thicker myelin sheaths.

Myelinated Fibers.

The first step in myelin formation is axon penetration of an existing groove of the Schwann cell cytoplasm. The edges of the groove come together to form a mesaxon, so that the plasma membranes of the 2 edges fuse together on their outer surface (Fig.26). Next, the mesaxon wraps itself around the axon several times; the number of turns determining the thickness of the myelin layer. The regular dark lines are called major dense lines and represent the line of fusion of cytoplasmic surfaces of Schwann cell membranes. The less regular lines are called intraperiod lines and are sites of close contact, but not fusion, of the extracellular surfaces of adjacent layers of Schwann cell membrane. After this process, both an internal and an external mesaxon can be seen. The layers of membranes of the Schwann cell unite and form myelin, a lipoprotein complex.

Fig.26.

Each axon is surrounded by myelin formed by a series of Schwann cells. The myelin sheath shows gaps along its path called the nodes of Ranvier or, the nodal interceptions (Fig.27). They represent the spaces between adjacent Schwann cells along the length of the axon.

Fig.27.

Interdigitating processes of Schwann cells partially cover the node. The distance between 2 nodes is called an internode and consists of one Schwann cell. The length of the internode varies according to the axonal diameters but it is constant along the length of a particular axon.

Under the light microscope, the myelin sheath shows cone-shaped clefts, or incisures, of Schmidt-Lanterman that are actually helical cytoplasmic tunnels from the outside of the sheath to the inside. They represent distensions within the myelin layers caused by the localized presence of Schwann cell cytoplasm (Fig.28). This cytoplasm and, consequently, the clefts move up and down the sheath.

Fig.28

There are no Schwann cells in the CNS; here, the myelin sheath is formed by the processes of the oligodendrocytes. Oligodendrocytes differ from Schwann cells in that different branches of one cell can envelop segments of several axons (Fig.29).

Fig.29

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