The Structural Units
As would naturally be expected, the ectoderm, that is, the embryonic tissue which presents primary contact with the environment, gives rise to the principal cell units destined to form the nervous system. These ectodermal cells eventually depart very far from their original epithelial compactness, becoming either true nerve cells (neurons), or non-nervous structures (neuroglia cells), which have a secondary supportive role.
Still other cells, from the embryonic mesoderm, may also serve the all-important neurons in the form either of nutritive blood or as protective coverings.
The neuron theory, which is the most commonly accepted working hypothesis of biologists concerned with the nervous system, holds to two propositions: first, that the nervous system proper is entirely made up of neurons; and second, that transmission of nerve impulses from one neuron to another is by means of make-and-break contact without protoplasmic fusion.
The point of contact between neurons is called a synapse. The entire working nervous system in man involves several hundred million synapses.
After differentiation from their generalized spherical embryonic form, neurons come to vary enormously in size, shape, thickness, length, and manner of their insulation. The cell body immediately surrounding the nucleus sprouts out into various processes, the so-called “nerve fibers,” which may become enormously attenuated and lengthened (Fig. 593), although always remaining an essential part of the cell and under immediate control of the nucleus. Thus the neuron assumes a form well adapted to its primary function of transmission at the same time that it is specializing in sensitivity, a function which is simply one of the general endowments common to all cells.
By “joining hands” in synapses the long, drawn-out neurons form continuous living bridges and networks that connect points of stimulation with points of reaction. In short they are conductors which, like telegraph wires, permit the passage of messages while remaining stationary themselves.
The more recently developed neurohumeral theory points out that transmission from neuron to neuron, in some instances at least, may depend not upon direct contact, but be due to the hormonic production of “neurohumors” that make a bridge at the junction of one neuron with another over which impulses may pass.
The extended fibers of certain nerve cells in man may reach an actual length of three feet from nerve cord to toe tip, an astonishing span for any single animal cell to attain. The end of such a fiber is thus frequently much nearer to the nucleus of some neighboring cell with which it may come into synaptic touch than with its own nucleus.
Until the neuroblasts, or embryonic neurons, have sprouted out their cell processes sufficiently to make contact possible, the transmission of impulses cannot be even temporarily established. These pathways are thus gradually set up and any break in the network of links interrupts the operation of the whole system to some degree. One explanation of unconsciousness and sleep is that for the time being the integrating synapses between some of the neurons are at least partially interrupted.
The capacity for conduction through valve-like synapses results in polarity, or one-way traffic, within the individual neurons. Impulses enter along certain fibers or processes and depart by others, a relation which is never reversed. The fibers forming incurrent pathways, with respect to the nucleus and cell body, are known as dendrites, because they frequently present abundant treelike arborizations. The excurrent pathway, on the other hand, is through a single special fiber, called the neurite or axon, which is usually larger and much longer than the more numerous dendrites and not so much given to branching. Whenever branching does occur in a neurite, the twigs, or collaterals, except at the tip of the neurite, characteristically leave the main fiber at right angles instead of by acute angles as in the case of dendrites (Fig. 593).
Neurons may be bipolar or multipolar, according to the number of fibers present. The more primitive bipolar type, with one centripetal dendrite and one centrifugal neurite, is found in fishes, and also in the dorsal ganglia of higher vertebrates. Multipolar neurons, on the other hand, have several dendrites and a single neurite. The apparently unipolar type in dorsal ganglia, results when the proximal ends of a dendrite and a neurite appear to emerge together from the cell body. In reality, however, they are brought around side by side as a result of the mechanics of growth and are quite independent morphologically from each other, although taking on secondarily a unipolar appearance (Fig. 594).
When a neurite ends in a muscle fiber or a gland, it may enlarge into a flattened end plate or bulb, or expand into a tiny brush, or end simply like a thread. The distal tips of neurites and dendrites, at the points of synapsis, form the weakest links in the chain of neuronic elements.
Nerve Fibers and Their Sheaths
Nerve fibers fall into four categories with respect to the degree and manner of sheathing, as follows:
(1) Naked fibers without sheaths, occurring in the “gray substance” of the central nervous system.
(2) Fibers with a protective neurolemma, or Schwann’s sheath, characteristic of invertebrate “nerves” generally; in the nerves of amphioxus and the cyclostomes as well as the olfactory nerves of other vertebrates; and at the distal ends of the spinal nerves. Fibers of this type are known as Remak’s fibers.
(3) Fibers with an insulating sheath of fatty or lipoid substance, the medullary, or myelin, sheath, occurring in the “white substance” of the central nervous system.
(4) Fibers with both sheaths present, the medullary sheath within Schwann’s sheath. This type occurs in nerves generally, except at the ends.
The neurolemma, composed of ectodermal cells derived from the original medullary tube, is a very thin, living sheath while the myelin sheath is a thicker, secreted layer which is white in color in fresh tissue. Probably the neurolemma secretes the myelin material, but only in the presence of the nerve fiber. Nervous tissue in which myelinated fibers predominate is white in color while cell-bodies and unmyelinated fibers, either naked or Remak’s fibers, being composed of living protoplasm, form “gray matter.”
In the case of fibers covered by both sheaths, the medullary sheath, but not the neurolemma, is interrupted at frequent intervals along the fiber, at the nodes of Ranvier (Fig. 595). Each internodal (“between the nodes”) region is associated with a single neurolemma cell containing a conspicuous nucleus. When the myelin sheath alone is present it is not segmented but forms instead a continuous tubular covering over the fiber.
All these types of fibers and their sheaths are diagrammatically represented in Figure 595.
Microscopic examination of a neuron after proper staining reveals structural details of great complexity.
Neurofibrils, like myofibrils in a muscle cell, extend through the cell from end to end. They also surround the nucleus. In all probability they are highly elaborated parts of the cytoplasm directly concerned with the transmission of nerve impulses. Around the nucleus and in the dendrites, but not in the neurite, is found the so-called tigroid substance, or Nissl bodies, which apparently plays some part in the metabolism of the neuron, since it varies in amount in accordance with the amount of work done.
Another cytoplasmic structure of problematic function, which is particularly characteristic of neurons, is the Golgi apparatus, an anastomosing network near the nucleus. In spite of the triumphs of histological discovery, much still remains to be found out. Our present knowledge of the marvelous units of the nervous system might be compared to what it would be possible to learn of short wave transmission by examining the picture of a cross section through a house that has a radio in it. With increasingly refined technic what new discoveries has the neurologist of the future in store!