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CHAPTER 2 General morphology of neurons and neuroglia s A N F 0 R D L * p A L A y I Departments of Anatomy and Neurobiology, v I c T 0 R I A c H A N - p A L A y I Harvard Medical School, Boston, Massachusetts CHAPTER CONTENTS be illuminated by doing so. Nevertheless nerve cells possess many features in common and it is to these Neuronal Shape as a Sign of Function qualities that the general discussions of this chapter Sizes of Neurons must be confined. Cytology of Neurons is Soma or cell body It important to note that the nervous system Nucleus derives from the outermost cellular layer of the em- Peri karyon bryo, the ectoderm, and that it retains into maturity Dendrites the morphological characteristics of an epithelium. Axon That is, it consists almost entirely of cells. Intercellu- Synapse Presynaptic ending lar space makes up only a very small proportion of its Postsynaptic membrane volume. Like some other epithelial organs it is in- Synaptic cleft vaded by blood vessels and connective tissue, but Synaptic vesicles unlike these organs, the central nervous system con- Dendritic spine tains relatively little connective tissue and the paren- Other varieties of synapse Neuron doctrine chyma is always separated from the blood vessels and Neuroglia perivascular connective tissue by an unbroken basal Astrocytes lamina, a delicate layer of fine collagen filaments Oligodendrocytes embedded in a glycoprotein matrix. Microglia Schwann cell In most epithelia the cells have a distinct polarity imposed by the fact that on one side they rest upon - connective tissue while on the other they present a free surface. In the central nervous system a vestige of this primordial orientation is represented by the Les neurones sont des appareils gkneruteurs et con- ducteurs de l‘onde nerveuse. Voila le fait fondamental. arrangement of the ependymal cells lining the ven- tricular cavities, and in a few places in which the Ramon y Cajal [(151), vol. I, p. 531 wall of the brain is unusually thin, such as in the floor of the third ventricle, the original epithelial THIS CHAPTER describes the form and internal struc- order can still be seen in the ependymal cells that ture of the cells composing the nervous system. These stretch from the ventricular surface to the basal lam- cells are divisible into two great classes: 1)the spe- ina on the external surface of the brain. But in most cific parenchymal elements, the nerve cells, or neu- parts of the nervous system, the parenchymal cells rons, and 2 ) the supporting elements, the neuroglial are not oriented with respect to the primitive land- cells. Both classes will be considered here. But as will marks; instead they are oriented with respect to one be seen below, these cells are so highly diversified in another. This difference in orientation expresses a shape, size, fine structure, and function that to define very important characteristic of the cells of the nerv- a typical cell in either class is to describe a chimera or ous system. In most epithelia the cells are independ- will 0’ the wisp. Nerve cells, especially, deserve to be ent morphological, trophic, and functional units. Al- analyzed according to their specific characteristics, though they are connected together by various junc- but they are manifestly too numerous to be consid- tional devices, their activities usually relate to ex- ered individually even if some useful principle could changes across the surface which they cover and in- volve laterally at most only a few of their immediate This work was supported by Public Health Service Grants neighbors. In contrast, the cells of the nervous sys- NS03659 and NS10536, Training Grant NS05591 from the Na- tional Institute of Neurological and Communicative Diseases and tem, although also independent morphological and Stroke, and an award from the William F. Milton Fund of Har- trophic units with similar junctional devices, are pro- vard University. foundly interdependent functionally; and their func- 5 6 HANDBOOK OF PHYSIOLOGY - THE NERVOUS SYSTEM I tional relations can extend for long distances through equipment of fully developed nerve cells, and, also complex circuits of interconnections. That these con- following Cajal, the specifically neural aspects of neu- nections are precisely ordered-not only grossly relat- ronal function have been traditionally parceled out ing one part of the nervous system with another but among them. For the nerve cell not only produces the also precisely disposed with respect to the geometry nerve impulse; it also conducts that impulse over of each cell-indicates how far-reaching the transfor- long or short distances, transmits it to other cells- mation of the original embryonic epithelium has other nerve, muscle, or gland cells-and is itself been. capable of being excited or inhibited by direct stimula- tion or by the activity of other nerve cells. It must possess the apparatus for generating, receiving, con- NEURONAL SHAPE AS A SIGN OF FUNCTION ducting, and transmitting the nerve impulse. To some extent these essential and characteristic func- Nerve cells are notable for the variety of form and tions are lodged in different parts of the nerve cell. In the extraordinary range of size that they display. No the first place, it must be indicated that the cell body, other cell type, not even the versatile macrophage, the seat of the nucleus, is specialized as the metabolic can compete with the nerve cell for sheer complexity and synthetic center of the cell. It provides the great of form. This very intricacy, the source of much diffi- majority of the proteins used in the processes of the culty for morphologists and biochemists, is inti- cell since the dendrites have very little and the axons mately bound up with the function of the nerve cell. have none of the apparatus necessary for protein Indeed, as Ramon y Cajal puts it, “the form of the synthesis. In addition, the dendrites and cell body cell, insofar as it is the expression of its relations appear to be specialized for receiving impulses, while (with other cells), is consequently one of its most the axon hillock and initial segment of the axon are important attributes” [(151), vol I, p. 531. specialized for generating the action potential, which It is the pursuit of these relations that rationalizes is conducted by the axon to its terminals. These in the effort of neuroanatomists to describe and classify turn are specialized for impulse transmission. The nerve cells in all parts of the nervous system of nerve cell therefore displays a dynamic polarity, first different animals-attempts that entail a progres- recognized by Ramon y Cajal (2501, which is reflected sively more minute examination of the form, size, in its complex form, its multiple dendrites funnelling and disposition of the cells, the arborization patterns information toward the single axon, the conducting of their processes, their terminal ramifications and and transmitting component. finest appendages, their internal fine structure, their In its original form the “law of dynamic polariza- ontogenesis, and their evolution. Such studies pro- tion” referred to the streaming of nervous energy ceed on the premise that there exists for the nerve through the dendrites and cell body into the axon, cell a fundamental, coherent, even ideal, form that is whence it was led away to be transferred to the discernible in histological preparations despite the dendrites or cell body of another cell. As an indicator seemingly endless permutations displayed in nature. of the direction of information flow through the nerve That ideal comprises a nucleated cell body that gives cell this law still holds true in a general way, espe- rise to two kinds of cytoplasmic processes, termed cially for those nerve cells that project their axons to dendrites and axons. Each cell can have many den- distant points. It does not deal with the microanat- drites, but should have only one axon. While the omy and microphysiology underlying the polarity. great majority of nerve cells in vertebrates conform to Actually, it is the asymmetry of the synapse that this paradigm, most nerve cells in invertebrates do results in the apparent functional polarity of the not, since dendrites and axons in these animals are nerve cell. On closer view, it will be seen that in usually diflicult to distinguish by their form alone. In many nerve cells the traditional morphological subdi- vertebrates also there are nerve cells lacking a single vision into dendrites, cell body, and axon does not characteristic process that can be called an axon, and correspond to the expected restrictions of function, there are even some well-known types in which the and, indeed, in many instances even the accepted identification of proper dendrites is a subject of much morphological distinctions and subdivisions cannot disagreement. But these very deviations from the be upheld at the level of fine structure. For these norm substantiate its existence in the minds of neu- reasons the law of dynamic polarization must be un- roanatomists at least, for they are seen as exceptions, derstood as a propaedeutic principle, which applies as curiosities, that will probably be explained by a particularly to the idealized nerve cell, modeled after detailed functional analysis of the particular cells such large cells as the motor neuron, but which involved. We shall revert to this theme later when should not be conceived of as a constraint either on the significance of the synaptic junction has been the flexibility of the living nerve cell or on our ability considered. to comprehend its mode of construction and opera- The three major parts of the nerve cell, perikaryon tion. or cell body, dendrites. and axon, have been regarded The idea that the form of the nerve cell expresses since the early studies of Ramon y Cajal as standard its function extends, however, to a more recondite CHAPER 2: GENERAL MORPHOLOGY OF NEURONS AND NEUROGLIA 7 principle than the simple law of dynamic polariza- tion, which is, after all, an oversimplification.

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