CHAPTER X

THE ACTION OF THE NERVOUS SYSTEM

§ 25. The Functions of the Neurones

The neurone or nerve cell, besides possessing the powers of absorption, growth, etc., common to all the cells of the body, has three special duties or functions:(1) It is especially sensitive to or influenced by what happens to it or to parts of it.

(2) It conducts or transmits; i.e., it so acts that a stimulation or disturbance or activity at one end of it results in a stimulation or disturbance or activity at the other.

(3) It is especially modifiable; i.e., its action at any time depends upon its previous actions. In the human nervous system this third function is probably restricted in the main to the cells in certain parts of the central nervous system.

Sensitivity. The first function, sensitivity or impressibility, needs no explanation. All matter is influenced by what happens to it; all living matter is especially so; and the nerve cells are the parts of living animals which carry this trait to the extreme. If we compare a man's body to a building, calling the steel frame-work his skeleton and the furnaces and power-station his digestive organs and lungs, the nervous system would include with other things the thermometers, heat regulators, electric buttons, door bells, valve openers,-the parts of the build

ing in short which are specially designed to respond to influences of the environment.

Conductivity. Just how the nerve cell conducts or transmits is not known. But the fact itself is sure. As a copper wire at one end of which an electric current is excited is so influenced that the current appears at the other end; as the air so acts that a vibration in any part spreads to other parts,—so the neurone when stimulated at one end acts so as to produce a corresponding activity at the other end; and so as to produce, under certain conditions, activity in the neurones in connection or synapsis with it.

The activity or disturbance which is transmitted is called the nervous impulse. When such an impulse is started in a nerve cell we say that the nerve cell is stimulated and call the agency by which the impulse is begotten the stimulus. Also when the nerve cell transmits the impulse to some body cells or other nerve cells we say that it stimulates them. Just as an electric current might pass along one wire, thence to another and along it to a third, so the nervous impulse passes from neurone to neurone when these are in functional connection.

The conduction or transmission is commonly over a series of nerve cells. For instance when the stimulus of pain at the finger's end makes us rub the injured spot, the impulse does not go from the skin to the muscles via a single cell (or set of cells), but traverses at least three, one set running from the skin to the spinal cord, one from the spinal cord to the muscles and one or more sets in the spinal cord connecting these. The transmission is, in any given neurone, usually, if not always, in the same direction; namely, toward the extremities of the neuraxon. A cell carrying impulses from the brain to a muscle does not, so to speak, carry return messages. For that another

wire is used. A nerve cell may receive impulses from several nerve cells. It may and commonly does transmit its impulses to many nerve cells.

As might be expected, the two functions of sensitivity and conductivity are aided by such an arrangement of the nerve cells that stimuli are received at important points in the body and conducted to appropriate muscles. The constituents of the nervous system are not arranged at random. They are not like the chance tangle of a billion little threads which would receive stimuli hit or miss and conduct them nowhere in particular, but are, like the wires of the telephone system of a city or the railroads of a country, definitely placed lines of transmission between important points, and are so arranged at central offices as to permit a great number of useful connections. For instance, the neurones receiving the stimuli of light in the retina have definite connections with the neurones that carry impulses to the muscles that open and close the eyes, also to the muscles that move the eyes in turning to and converging upon and focussing for objects, also to the muscles that move the head.

Modifiability. The analogy with a telephone system fails when we come to the third function, characteristic of many of the nerve cells1 of the human brain, their power of modification by use. Unlike any system of wires or machines, the human nervous system possesses the power, at least in many of its parts, to be so altered by whatever happens to it as to enable the body on the next occasion to meet the situation more successfully. The nervous system which causes at a child's early sight of the fire the reaction of reaching for it, becomes a

1This paragraph refers only to the cells of the so-called 'higher' parts of the nervous system. Of the cells in the spinal cord and peripheral system it is almost surely not true. Of just what groups of cells it is true is not known.

nervous system that causes in later trials the reaction avoidance. The connection between the impression a the one motor discharge has been weakened and an opposite one formed. The neurones learn, so to speak, to form, break and modify their inter-connections. If a telephone system possessed within itself the power by which the connections between the wires to certain houses would, with successful use, become more and more easily made while other connections would become increasingly more difficult, the analogy would be complete. But of course no mere machine has such a power to modify its workings, to make or break connections in accordance with the frequency of their use and the desira bility or discomfort of the results to which they lead. This function of the neurones will be more fully described after the facts concerning the arrangement of the neurones in a system have been presented.

§ 26. The Arrangement of the Neurones

The Three Chief Groups.-The cells of the nervous system as a whole may best be divided into three classes. There are first, neurones which are stimulated by heat, light and other physical or chemical forces and discharge into other neurones; second, neurones which are stimulated by other neurones and themselves stimulate muscles; third, neurones which are stimulated by and themselves stimulate other neurones. In other words, there are cells directly sensitive to the environment, cells directly active. in causing muscular contraction and cells acting as intermediaries between the former and the latter. The first are called Afferent or Centripetal neurones because they bring stimuli toward the brain and spinal cord (ad-fero= bring to); the second are called Efferent or Centrifugal

neurones because they carry stimuli away from the brain and spinal cord toward the muscles. The third are called Associative or Connecting neurones.

Cells of class I are also called Sensory neurones or cells; cells of class 2, Motor neurones or cells. The term motor cell is also used to include cells which, though themselves not directly connected with muscles, lead out from the brain and connect in the spinal cord with cells which are so connected. Afferent, centripetal and sensory are used of nerves when the cells composing the nerve bring stimuli from the different parts of the body. to the brain and spinal cord; efferent, centrifugal or motor nerves meaning of course nerves of an opposite function.

Sensory Neurones. From almost every part of the body there originate afferent neurones to transmit stimuli arising there to the brain and spinal cord. Not only in the eyes, ears, nose, mouth and skin, but also in the muscles, articular surfaces, connective tissues, along the digestive tract, and throughout the sensitive areas of the body, there are neurone-endings capable of being set in action by the proper stimuli at the point where they are located.

Motor Neurones. To all the muscles,-to the muscles which control the peristalsis of the intestine and the contraction of the blood vessels, as well as to those that more obviously move our limbs,-run efferent neurones, stimuli from which influence the amount and duration of muscular action.

The nervous system thus furnishes a most elaborate mechanism for receiving stimuli at almost all points in the body, and, in the case of light, heat and smell, of stimuli from distant objects, and for controlling the action of the body in the most minute details. If we