The General Mechanism of Response

Suppose, now, that we attempt a general physical analysis of the mechanism or apparatus which modern physiology offers in at least partial explanation of human and higher animal behavior. This apparatus, which we may designate as the response mechanism, is essentially that of the nervous system, although it of course involves in its processes all portions of the body and particularly the muscles. Response is ordinarily said to be initiated by objects or stimuli in the environment of the organism. However, in many cases, particularly where we are dealing with human behavior, it is very difficult to identify the objects in question, unless perhaps we look for a clue in the accompanying introspective consciousness. The activities of living beings would not be so characteristically spontaneous if it were always evident that their causes lay in the environment.

Nevertheless, the most helpful approach to a study of these more difficult cases is by way of an understanding of the more typical kind of response, which begins with an object. This object must act upon a sense-organ by means of a force or energy, which is called the stimulus, if it is to call forth a reaction from the organism. Thus, in the case of visual response, the object emits or reflects light which enters the eye and stimulates the retina. The latter is an example of a receptor mechanism, in which the stimulus initiates an excitation of the nervous system. The receptor process, in turn, is followed by a transference of the excitation to a so-called afferent nerve fibre, along which the disturbance is propagated to a nerve center, such as those of the spinal cord or brain. The processes which transpire at the nerve center determine along what path the excitation will be conducted in an outgoing direction to reach the muscular system. In other words, the central process is of critical importance in determining the character of the reaction or behavior in any given instance. For this reason, it is sometimes called the adjustor process. Its mechanism always involves one or more junction points between individual nerve fibres, such points being known as synapses.

After leaving the central, synaptic, region the disturbance passes along an afferent or outgoing nerve, and is transferred, through a mechanism known as the end-plate, usually into a muscle. Muscles are the most typical examples of the class of effectors, which also include glands and electrical organs (in certain fishes). The arrival of the excitation at the effector is marked by the characteristic reaction, which controls the relation of the organism to its environment. This may be regarded as the last stage of the response activity, and be designated as the effect. When we study response in the superficial manner which characterizes behaviorism, or the everyday observations of the behavior of men or animals, we usually recognize only the first and the last links in this chain of events, and very frequently we miss the first link.

The mechanism of response, as above outlined, belongs to a class of processes known as propagation or conduction. A disturbance is set up a certain point in space at a certain instant, and this gives rise to a series of further disturbances at successively different points in space and instants in time. Other examples of processes of this sort are to be found in the propagation of radiant energy, or of sound, in space; or the conduction of fluids through pipes, or of electricity through wires. They are controlled by a number of fairly simple general principles. Firstly, the path of the conduction depends upon the structure of the conducting medium. Secondly, the nature of the process at any point in this path is a function of the nature of the conducting medium at the same point, as well as of the character of the activity at the immediately preceding point. The propagation of the disturbance is energized, or pushed by an agency which corresponds to pressure or voltage, but the process is impeded by another factor exemplified by viscosity or resistance. The conduction may be regarded as an outcome of the struggle, or balance between these two determining factors. In the case of electrical conduction this relationship is expressed mathematically in the formula known as Ohm'slaw, according to which the current strength is proportional to the voltage and inversely proportional to the resistance. At a later point in our discussion we shall consider in considerable detail the analogy between the laws of electrical and of nervous conduction.