Psychology - Robert S. Woodworth (trending books to read TXT) 📗

called the "nerve current", and is electrical and chemical in nature.

Fig. 2.--(From Martin's "Human Body.") General view of the nervous system, showing brain, cord, and nerves.

All nerve connections, like the great majority of telephone connections, are effected through the centers, called "centrals" in {29} the case of the telephone. Telephone A is connected directly with the central, telephone B likewise, and A and B are indirectly connected, through the central switchboard. That is the way it is in the nervous system, with "nerve center" substituted for "central", and "sense organ" and "muscle or gland" for "telephones A and B."

Fig. 3.--Location of the cord, cerebrum and cerebellum. The brain stem continues the cord upward into the skull cavity. (Figure text: cerebrum, cerebellum, cord, tongue)

The advantage of the centralized system is that it is a system, affording connections between any part and any other, and unifying the whole complex organism.

The nerve centers are located in the brain and spinal cord. The brain lies in the skull and the cord extends from the brain down through a tube in the middle of the {30} backbone. Of the brain many parts can be named, but for the present it is enough to divide it into the "brain stem", a continuation of the spinal cord up along the base of the skull cavity, and the two great outgrowths of the brain stem, called "cerebrum" and "cerebellum". The spinal cord and brain stem contain the lower or reflex centers, while the cerebellum, and especially the cerebrum, contain the "higher centers". The lower centers are directly connected by nerves with the sense organs, glands and muscles, while the higher centers have direct connections with the lower and only through them with the sense organs, glands and muscles. In other words, the sensory nerves run into the cord or brain stem, and the motor nerves run out of these same, while interconnecting nerve strands extend between the lower centers in the cord and brain stem and the higher centers in the cerebrum and cerebellum.

The spinal cord contains the reflex centers for the limbs and part of the trunk, and is connected by sensory and motor nerves with the limbs and trunk. The brain stem contains the reflex centers for the head and also for part of the interior of the trunk, including the heart and lungs, and is connected with them by sensory and motor nerves. The nerve center that takes part in the flexion reflex of the foot is situated in the lower part of the cord, that for the similar reflex of the hand lies in the upper part of the cord, that for breathing lies in the lower or rear part of the brain stem, and that for winking lies further forward in the brain stem.

Big movements, such as the combined action of all four legs of an animal in walking, require cord and brain stem to work together, and throw into relief what is really true even of simpler reflexes, namely that a reflex is a coordinated movement, in the sense that different muscles cooperate in its execution.

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Internal Construction of the Nerves and Nerve Centers

We shall understand nerve action better if we know something of the way in which the nervous system is built. A nerve is not to be thought of as a unit, nor are the brain and cord to be thought of as mere masses of some peculiar substance.

Fig. 4.--A motor nerve cell from the spinal cord, highly magnified. (Figure text: dendrites, cell body, axon, termination of axon in muscle)

A nerve is a bundle of many slender insulated threads, just as a telephone cable, running along the street, {32} is a bundle of many separate wires which are the real units of telephonic communication. A nerve center, like the switchboard in a telephone central, consists of many parts and connections.

The whole nervous system is essentially composed of neurones. A neurone is a nerve cell with its branches. Most nerve cells have two kinds of branches, called the axon and the dendrites.

The nerve cell is a microscopic speck of living matter. Its dendrites are short tree-like branches, while its axon is often several inches or even feet in length. The axon is the "slender thread", just spoken of as analogous to the single telephone wire. A nerve is composed of axons. [Footnote: The axon is always protected or insulated by a sheath, and axon and sheath, taken together, are often called a "nerve fiber".] The "white matter" of the brain and cord is composed of axons. Axons afford the means of communication between the nerve centers and the muscles and sense organs, and between one nerve center and another.

The axons which make up the motor nerves are branches of nerve cells situated in the cord and brain stem; they extend from the reflex center for any muscle out to and into that muscle and make very close connection with the muscle substance. A nerve current, starting from the nerve cells in the reflex center, runs rapidly along the axons to the muscle and arouses it to activity.

The axons which make up the optic nerve, or nerve of sight, are branches of nerve cells in the eye, and extend into the brain stem. Light striking the eye starts nerve currents, which run along these axons into the brain stem. Similarly, the axons of the nerve of smell are branches of cells in the nose.

The remainder of the sensory axons are branches of nerve cells that lie in little bunches close alongside the cord or {33} brain stem. These cells have no dendrites, but their axon, dividing, reaches in one direction out to a sense organ and in the other direction into the cord or brain stem, and thus connects the sense organ with its "lower center".

Fig. 5.--Sensory and motor axons, and their nerve cells. The arrows indicate the direction of conduction. (Figure text: eye, brain stem, skin, cord, muscle)

Where an axon terminates, it broadens out into a thin plate, or breaks up into a tuft of very fine branches ( the "end-brush"), and by this means makes close contact with the muscle, the sense organ, or the neurone with which it connects.

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The Synapse

Now let us consider the mode of connection between one neurone and another in a nerve center. The axon of one neurone, through its end-brush, is in close contact with the dendrites of another neurone. There is contact, but no actual growing-together; the two neurones remain distinct, and this contact or junction of two neurones is called a "synapse". The synapse, then, is not a thing, but simply a junction between two neurones.

Fig. 6.--The synapse between the two neurones lies just above the arrow.

The junction is good enough so that one of the two neurones, if itself active, can arouse the other to activity. The end-brush, when a nerve current reaches it from its own nerve cell, arouses the dendrites of the other neurone, and thus starts a nerve current running along those dendrites to their nerve cell and thence out along its axon.

Now here is a curious and significant fact: the dendrites are receiving organs, not transmitting; they pick up messages from the end-brushes across the synapse, but send out no messages to those end-brushes. Communication across a synapse is always in one direction, from end-brush to dendrites.

This, then, is the way in which a reflex is carried out, the pupillary reflex, for example. Light entering the eye starts a nerve current in the axons of the optic nerve; these axons terminate in the brain stem, where their end-brushes arouse the dendrites of motor nerve cells, and the axons of these {35} cells, extending out to the muscle of the pupil, cause it to contract, and narrow the pupil.

Or again, this is the way in which one nerve center arouses another to activity. The axons of the cells in the first center (or some of them) extend out of this center and through the white matter to the second center, where they terminate, their end-brushes forming synapses with the cells of the second center. Let the first center be thrown into activity, and immediately, through this connection, it arouses the second.

Fig. 7.--Different forms of synapse found in the cerebellum, "a" is one of the large motor cells of the cerebellum (a "Purkinje cell"), with its dendrites above and its axon below; and "b," "c" and "d" show three forms of synapse made by other neurones with this Purkinje cell. In "b," the arrow indicates a "climbing axon," winding about the main limbs of the Purkinje cell. In "c," the arrow points to a "basket"--an end-brush enveloping the cell body; while "d" shows what might be called a "telegraph-wire synapse." Imagine "d" superimposed upon "a": the axon of "d" rises among the fine dendrites of "a," and then runs horizontally through them; and there are many, many such axons strung among the dendrites. Thus the Purkinje cell is stimulated at three points: cell body, trunks of the dendrites, and twigs of the dendrites.

The "gray matter" comprises the nerve centers, lower and higher. It is made up of nerve cells and their dendrites, of the beginnings of axons issuing from these cells and of the terminations of incoming axons. The white matter, as was said before, consists of axons. An axon issues from the {36} gray matter at one point, traverses the white matter for a longer or shorter distance, and finally turns into the gray matter at another point, and thus nerve connection is maintained between these two points.

There are lots of nerve cells, billions of them. That ought to be plenty, and yet--well, perhaps sometimes they are not well developed, or their synapses are not close enough to make good connections.

Fig. 8.--A two-neurone reflex arc. (Figure text: stimulus, skin, sensory axon, bit of the spinal cord, motor axon, muscle)

Examined under the microscope, the nerve cell is seen to contain, besides the "nucleus" which is present in every living cell and is essential for maintaining its vitality and special characteristics, certain peculiar granules which appear to be stores of fuel to be consumed in the activity of the cell, and numerous very fine fibrils coursing through the cell and out into the axon and dendrites.

The reflex arc can now be described more precisely than before. Beginning in a sense organ, it extends along a sensory axon (really along a team of axons acting side by side) to its end-brush in a lower center, where it crosses a synapse and enters the dendrites of a motor neurone and so {37} reaches the cell body and axon of this neurone, which last extends out to the muscle (or gland). The simplest reflex arc consists then of a sensory neurone and a motor neurone, meeting at a synapse in a lower or reflex center. This would be a two-neurone arc.

Fig. 9.--A three-neurone arc, concerned in respiration. This also illustrates how one nerve center influences another. (Figure text: white matter, gray matter, lung, respiratory center in the brain stem, diaphragm, motor center in cord for the diaphragm)

Very often, and possibly always, the reflex arc really consists of three neurones, a "central" neurone intervening between the sensory and motor neurones and being connected through synapses with each. The central neurone plays an important rôle in coördination.

COÖRDINATION

The internal structure of nerve centers helps us see how coördinated movement is produced. The question is, how {38} several muscles are made to work together harmoniously, and also how it is possible that a pin prick, directly affecting just a few sensory axons, causes a big movement of many muscles. Well, we find the sensory axon, as it enters the cord, sending off a number of side branches, each of which terminates in an end-brush in synaptic connection with the dendrites of a motor nerve cell.

Fig. 10.--Coördination brought about