Functional Groups of Neurons

Glial Cells

The numerically most common cells in the nervous system are, in fact, not the neurons, but the glial cells (also called glia or neuroglia). These cells do not participate directly in information processing and transmission; rather, they play an indispensable supportive role for the function of neurons. The three types of glial cells in the CNS are the astroglial cells (astrocytes), oligodendroglia (oligodendrocytes), and microglial cells.

Astrocytes are divided into two types: protoplasmic and fibrillary. In the intact nervous system, astrocytes are responsible for the maintenance of the internal environment (homeostasis), particularly with respect to ion concentrations. Fine astrocyte processes surround each synapse, sealing it off from its surroundings so that the neurotransmitter cannot escape from the synaptic cleft. When the central nervous system is injured, astrocytes are responsible for the formation of scar tissue (gliosis).

The oligodendrocytes form the myelin sheaths of the CNS (see above). The microglial cells are phagocytes that are activated in inflammatory and degenerative processes affecting the nervous system.

Development of the Nervous System

A detailed discussion of the development of the nervous system would be beyond the scope of this book and not directly relevant to its purpose. The physician should understand some of the basic principles of neural development, however, as developmental disturbances account for a large number of diseases affecting the nervous system.

The nervous system develops from the (initially) longitudinally oriented neural tube, which consists of a solid wall and a central fluid-filled cavity. The cranial portion of the neural tube grows more extensively than the rest to form three distinct brain vesicles, the rhombencephalon (hindbrain), the mesencephalon (midbrain), and the prosencephalon (forebrain). The prosencephalon, in turn, becomes further differentiated into a caudal part, the diencephalon, and the most cranial portion of the entire neural tube, the paired telencephalon (endbrain). The central cavity of the two telencephalic ventricles communicates with that of the diencephalon through the interventricular foramen (destined to become the foramen of Monro). The central cavity undergoes its greatest enlargement in the areas where the neural tube has its most pro-

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114 · 1 Elements of the Nervous System

nounced growth; thus, the lateral ventricles form in the two halves of the telencephalon, the third ventricle within the diencephalon, and the fourth ventricle in the brainstem. In those segments of the neural tube that grow to a relatively lesser extent, such as the mesencephalon, no ventricle is formed (in the fully developed organism, the cerebral aqueduct runs through the mesencephalon).

Over the course of vertebrate phylogeny, progressive enlargement of the telencephalon has caused it to overlie the brainstem and to rotate back on itself in semicircular fashion. This rotation is reflected in the structure of various components of the telencephalic gray matter, including the caudate nucleus and hippocampus; in the course of certain white matter tracts, such as the fornix;

and in the shape of the lateral ventricles, each of which is composed of a frontal horn, a central portion (atrium), and a temporal horn, as shown in Fig. 10.3, p. 407.

Cellular proliferation. Immature neurons (neuroblasts) proliferate in the ventricular zone of the neural tube, i.e., the zone neighboring its central cavity. It is a major aim of current research in neuroembryology to unveil the molecular mechanisms controlling neuronal proliferation.

Neuronal migration. Newly formed nerve cells leave the ventricular zone in which they arise, migrating along radially oriented glial fibers toward their definitive location in the cortical plate. Migratory processes are described in greater detail on pp. 350ff.

Growth of cellular processes. Once they have arrived at their destinations, the postmigratory neuroblasts begin to form dendrites and axons. One of the major questions in neurobiology today is how the newly sprouted axons find their way to their correct targets over what are, in some cases, very long distances. Important roles are played in this process by membrane-bound and soluble factors that are present in a concentration gradient, as well as by extracellular matrix proteins. There are ligand­receptor systems that exert both attractive and repulsive influences to steer the axon into the appropriate target area. These systems cannot be described in greater detail here.

Synaptogenesis. The axon terminals, having found their way to their targets, proceed to form synaptic contacts. Recent studies have shown that the formation of synapses, and of dendritic spines, is activity-dependent. Much evidence suggests that new synapses can be laid down throughout the lifespan of the individual, providing the basis of adaptive processes such as learning and memory.

Baehr, Duus' Topical Diagnosis in Neurology © 2005 Thieme

All rights reserved. Usage subject to terms and conditions of license.