Home >> Anatomy Of The Brain >> Arteries The Cerebral Circulation to Or Sensory Paths Afferent >> Its Gray and White_P1

Its Gray and White Matter

cells, neurone, neurones, cell, neural, nerve, cytoplasm, substance, cell-body and tube

Page: 1 2 3 4 5 6 7 8 9 10 | Next

ITS GRAY AND WHITE MATTER We have noticed that nerve tissue in a mass is made up of gray and white substance. The gray substance contains the bodies of the nerve cells and certain nerve processes, which for the most part are non-medullated; the white substance con tains the medullated nerve processes (axones and collaterals). The body and all the processes of a nerve cell constitute a neurone. These neuronic elements are supported by epiblastic and mesoblastic connective tissues and nourished by a very rich blood supply.

Neurones.—The one essential and highly specialized ele ment of the nervous system is the neurone, which, in countless thousands, makes up the functionating part of the system. The neurones are developed from the columnar cells of the neural plate. The early evolution of that plate into neural tube and neural crests is coincident with a cleavage in the life history of the cells; the cells of the neural tube develop multi polar neurones; those of the neural crests form bipolar neurones (see note).

NOTE.-It has been taught hitherto that the neural crest is the source•of multipolar neurones in the sympathetic ganglia; but the recent investiga tions of A. Kuntz and others make it very probable that all the sympathetic multipolars are carried out from the neural tube along with the developing efferent fibers (see Jour. Comp. Neurol., 1914, etc.).

Thus very early in embryonic life, by the fifteenth day, the anlagen of the two great classes of neurones are laid down.

{ I. Multipolar First type Second type II. Bipolar Fusiform Pyriform As soon as the anlagen are laid down there may be observed, near the lumen of the neural tube, large spherical cells with clear cytoplasm and with nuclei possessing mitotic figures. These spherical cells are the "germ cells" of His. By their mitotic division the germ cells produce indifferent daughter cells, which later differentiate into spongioblasts and neuro blasts. The former develop the ependyma cells and these in turn the neuroglia cells. The neuroblasts by division and differentiation become neurones. From the deep end of the neuroblast (the end which originally was next the mesoblast) a pseudopod is thrown out which develops into the axone of the cell. Later in the life of the cell one or more pseudopods are projected from the superficial end of the neuroblast; they form the dendrites. All functionating neurones are composed of a cell-body, an axone and one or more dendrites. The bipolar neurones have but one dendrite; the multipolars have two or more dendrites.

I. Multipolar Neurones.—Multipolar neurones are found in the brain, spinal cord and sympathetic ganglia. They are derived from the neural tube (Figs. 65-68).

The cell-body of a multipolar neurone may be pyramidal, fusiform, stellate or pitcher-form in shape. Its size varies from 2-135p. The cell-body is made up of a mass of cytoplasm with its limiting cell-wall and a large spherical nucleus. It is contained in a pericellular lymph space. The nucleus is central in position in a normal neurone. Its most visible content is a nucleolus, sometimes more than one. Among several other things, it contains chromatin imbedded in a fluid ground sub stance. The cytoplasm surrounds the nucleus quite uniformly,

except in a degenerated neurone. A homogeneous liquid ground substance, called neuroplasm, and a number of histologic con stituents make up the cytoplasm (E. V. Cowdry, Amer. Jour. Anat., Vols. 15 and 16). The first cytoplasmic constituents to ap pear are small granular or rod-like bodies called mitochondria. They are present in the chick within the first 24 hours of in cubation, even before the first somite is developed. The granules measure from a and the rods or filaments 2-4 u in length. The mitochondria are universal cell constitu ents in the tissues of all vertebrates; they are also known to be present in several invertebrates. They have a concentric ar rangement in the cytoplasm and a parallel arrangement in the medullated fibers. Meves' iron hematoxylin stain, or Cajal's silver nitrate, or the Janus green intravitam shows them satis factorily. The neurofibrillce (Fig. 67) are second to appear in the cytoplasm. They are very delicate threads. They form a reticulum in the cell-body and continue as parallel fibrils in the axone and dendrites. They are coterminous with the neurone. At about the fortieth hour of incubation they appear in the chick, when 15 somites are present. It is claimed by some authors that the neurofibrillw are the conductive elements in the neurone; but perhaps the neuroplasm is equally impor tant. A little later in cell life (third day in the duck, Marcora) a system of anastomosing canals can be detected in the cyto plasm. These canals of Holmgren open into the pericellu]ar lymph space which surrounds the neurone. They probably convey food into the cell-body and waste products out of it. The next prominent cytoplasmic constituent to develop is the tigroid or Nissl substance, which in the chick is found in the sixth day of incubation (Marcora). It stains deeply by the Nissl Method. In small cells (under 3o p,) it is uniformly distributed, but is aggregated into granules, rod-like and cone shape masses in large cell-bodies (over 3o ,z); the formation of discrete masses appears to be due to coagulation. It is prob ably a food substance as it is gradually exhausted by continued stimulation of the neurone. More or less pigment is frequently found in neurones, especially in the base of the dendrites. It is said to be more abundant in the aged and to be rarely found in the young cell, except in a few special situations. Cowdry also speaks of lipoid globules occurring in clumps in the cell body and along its processes. They have a reciprocal relation to the mitochondria and may be derived from them. He does not consider them an evidence of degeneration. The last cell content to be mentioned might have been given first, viz., the organ of cell division, composed of the archiplasm sphere and its centrosome. This has been demonstrated in the nerve cells of many lower animals and in cells of the cerebral cortex, spinal ganglia, etc., in man. Fully developed nerve cells are not known to undergo division and this organ may perform some other function in the mature neurone; it may determine the discharge of nerve currents.

Page: 1 2 3 4 5 6 7 8 9 10 | Next