NERVOUS TISSUE

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8 NERVOUS TISSUE

DEVELOPMENT OF THE NERVOUS SYSTEM

The CNS develops from the primitive ectoderm (Figure 8-1 and Box 8-A). A simple epithelial disk—the neural plate—rapidly rolls into a hollow cylinder—the neural tube. This process is known as neurulation. During this process, a specialized portion of the neural plate—the neural crest—separates from both the neural tube and the overlying ectoderm. In later development, the neural crest forms the neurons of the peripheral ganglia and other components of the PNS. A defect in the closing of the neural tube causes different congenital malformations (see Box 8-B).

Neural crest cells remain separated from the neural tube and differentiate into (1) the sensory neurons of the dorsal root and cranial nerve ganglia and (2) the sympathetic and parasympathetic motor neurons of the autonomic ganglia.

Some of these cells invade developing visceral organs and form the parasympathetic and enteric ganglia and the chromaffin cells of the adrenal medulla.

The Schwann cells and satellite cells of the dorsal root ganglia also develop from neural crest cells. Schwann cells ensheathe and myelinate the peripheral nerve fibers, and the satellite cells encapsulate the neuronal cell bodies in the dorsal root ganglia.

The early neural tube consists of a pseudostratified columnar epithelium formed by three zones (Figure 8-2): (1) the ventricular zone—the zone where progenitor cells give rise to most cells of the nervous tissue (except microglial cells); (2) the intermediate zone—where neurons migrate toward the cortical plate and where excess neurons are destroyed by apoptosis; and (3) the cortical plate—the future gray matter of the cerebral cortex.

In the ventricular zone, germinal or ventricular cells proliferate rapidly during early development to give rise to ependymoblasts (remaining in the ventricular zone) and glioblasts and postmitotic neurons (migrating to the intermediate zone).

Immature neurons leave the ventricular zone, migrate to the intermediate zone, lose their capacity to undergo cell division, and differentiate into functional neurons. The neuronal migration mechanism and the consequences of abnormal migration are highlighted in Box 8-C.

During this differentiation process, a selection process—similar to that in the thymus for T cells (see Chapter 10, Immune-Lymphatic System)—results in either neuronal heterogeneity or death. Neurons that become postmitotic in the intermediate zone reach the outer layers of the cortical plate and continue their differentiation.

Once the production of immature neurons is complete, the germinal or ventricular cells produce glioblasts, which differentiate into astrocytes, oligodendrocytes, and ependymoblasts. Ependymoblasts give rise to ependymal cells, lining the ventricular cavities of the CNS, and choroid epithelial cells, which are components of the choroid plexus.

Later, astrocytes develop vascular end-feet attached to blood vessels of the CNS. Coincident with vascularization is the differentiation of microglia from monocytes. Microglia respond to injury and become active phagocytic cells.

In later development, glioblasts give rise to oligodendrocytes, marking the beginning of myelination in the CNS. In contrast to neurons, glioblasts and derived glial cells retain the ability to undergo cell division.

The number of neurons in the human brain is in the range of 109 to 1009. Up to 60% to 70% of these are present in the cerebral cortex. Most neurons are present at birth or shortly thereafter. As the brain continues to grow during the postnatal period, the number and complexity of interneuronal connections increase.

CELL TYPES: NEURONS AND GLIA

Neuron

The functional unit of the nervous system is a highly specialized, excitable cell, the nerve cell or neuron. Neurons usually consist of three principal components (Figures 8-3 and 8-4): (1) soma or cell body, (2) dendrites, and (3) axon.

The soma contains the nucleus and its surrounding cytoplasm (also called perikaryon; Greek peri, around; karyon, nucleus).

The dendrites are processes that arise as multiple treelike branches of the soma, forming a dendritic tree collectively. The entire surface of the dendritic branches is covered by small protrusions called dendritic spines. Dendritic spines establish numerous axonal synaptic connections, as we will see later (see Figure 8-7).

Neurons have a single axon originating from the soma at the axon hillock and ending in a terminal arborization, the telodendron. Each terminal branch of the telodendron has an enlarged ending, the synaptic terminal or synaptic bouton.

Note that although dendrites and axons branch extensively, axons branch at their distal end (the telodendron), whereas dendrites are multiple extensions of the soma or cell body.

The surface membrane of the soma and the dendritic tree are specialized for the reception and integration of information, whereas the axon is specialized for the transmission of information in the form of an action potential or a nerve impulse.

Synaptic terminals and synapses

The synaptic terminal (Figure 8-6) is specialized for the transmission of a chemical message in response to an action potential. The synapse is the junction between the presynaptic terminal of an axon and a postsynaptic membrane receptor surface, generally a dendrite.

The prefixes pre- and post- refer to the direction of synaptic transmission: (1) Presynaptic refers to the transmitting side (usually axonal). (2) Postsynaptic identifies the receiving side (usually dendritic or somatic, sometimes axonal). The presynaptic and postsynaptic membranes are separated by a space: the synaptic cleft. A dense material coats the inner surface of these membranes: the presynaptic and postsynaptic densities.

Presynaptic terminals contain a large number of membrane-bound vesicles (40 to 100 nm in diameter), the synaptic vesicles. Synaptic vesicles originate in the neuronal soma and are transported by molecular motor proteins along the axon (axonal transport) (Figure 8-7). Each vesicle contains a neurotransmitter. Presynaptic terminals contain mitochondria, components of the smooth endoplasmic reticulum, microtubules, and a few neurofilaments.

Synapses are classified by their location on the postsynaptic neuron (Figure 8-8) as follows:

Clinical significance: Axonal transport of rabies virus

The role of the axonal cytoskeleton and motor proteins (kinesin and cytoplasmic dynein; see Figure 8-7) was discussed in the Cytoskeleton section of Chapter 1, Epithelium. We emphasize once more the bidirectional transport of molecules along the axon: kinesin-mediated anterograde axonal transport of neurotransmitters—from the cell body toward the axon terminals, and the cytoplasmic dynein-mediated retrograde axonal transport of growth factors and recycling of axon terminal components—from the axon terminals to the cell body (see Box 8-D).

Axonal transport is important in the pathogenesis of neurologic infectious diseases. For example, the rabies virus introduced by the bite of a rabid animal replicates in the muscle tissue from as little as 2 to 16 weeks or longer. After binding to the acetylcholine receptor, the viral particles are mobilized by retrograde axonal transport to the cell body of neurons supplying the affected muscle. The rabies virus continues to replicate within infected neurons and after the shedding of the virions by budding, they are internalized by the terminals of adjacent neurons. Further dissemination of the rabies virus occurs in the CNS. From the CNS, the rabies virus is transported by anterograde axonal transport by the peripheral nerves to the salivary glands. The virus enters the saliva to be transmitted by the bite. Painful spasm of the throat muscles on swallowing accounts for hydrophobia (aversion to swallowing water).

The retrograde axonal transport to the CNS of tetanus toxin—a protease produced by the vegetative spore form of Clostridium tetani bacteria after entering at a wound site—blocks the release of inhibitory mediators at spinal synapses. Spasm contraction of the jaw muscles (known as trismus), exaggerated reflexes, and respiratory failure are characteristic clinical findings.

Oligodendrocytes and Schwann cells: Myelinization

Oligodendrocytes are smaller than astrocytes and their nuclei are irregular and densely stained. The cytoplasm contains an extensive Golgi apparatus, many mitochondria, and a large number of microtubules. One function of oligodendrocytes is axonal myelination.

Processes of oligodendrocytes envelop axons and form a sheathlike covering (Figure 8-10). The formation of this sheath is similar to that of Schwann cells in peripheral nerves.

Myelin sheaths extend from the initial segments of axons to their terminal branches. The segments of myelin formed by individual oligodendrocyte processes are internodes. The periodic gaps between the internodes are the nodes of Ranvier.

A single oligodendrocyte has many processes and may form 40 to 50 internodes. The nodes of Ranvier are naked segments of axon between the internodal segments of myelin. This region contains a high concentration of voltage-gated sodium channels, essential for the saltatory conduction of the action potential. During saltatory conduction in the myelinated axons, the action potential “jumps” from one node to the next.

During the formation of the myelin sheath, a cytoplasmic process of the oligodendrocyte wraps around the axon and, after one full turn, the external surface of the glial membrane makes contact with itself, forming the inner mesaxon (Figure 8-11).

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