GENERAL ZOOLOGY 



graded responses in adjuster and motor neurons may be reinforced by the 

 arrival of successive impulses on the same or different sensory neurons until 

 the threshold for firing an all-or-none impulse is reached. When this hap- 

 pens, the pattern of response to the stimulus begins to take shape as the con- 

 duction pathways are selected. It is important to understand that the nerve- 

 cell bodies in the central nervous system are kept in a constant state of excita- 

 tion, the central excitatory state, by the arrival of successive impulses from the 

 many sense organs. If the volleys of impulses are frequent enough, the graded 

 responses will build up in a neuron until they trigger the firing of all-or-none 

 impulses in its axonic process. The central nervous system is functionally 

 characterized by its readiness to respond. 



The mechanism of the reflex arc obviously makes possible the highest de- 

 gree of coordination in what is known as the behavior of the animal. Sher- 

 rington has generalized the facts of nervous coordination in his principle of the 

 final common path. Each afferent neuron is a special pathway by which im- 

 pulses from its particular receptor enter the central nervous system. Within 

 the central nervous system the impulse can travel over varied paths formed 

 by synapses between adjustor neurons and, theoretically, can produce a reac- 

 tion in any of the effectors. The efferent neurons, over which impulses travel 

 from the central nervous system to the effectors, differ from the afferent 

 neurons in that they are not private paths for particular impulses. It is a 

 commonplace that many different kinds of stimuli can produce the same re- 

 action or effect. Consider, for example, the many and varied stimuli to which 

 man responds by walking. The efferent neuron is, therefore, a final common 

 path over which impulses established by stimulation of receptors all over the 

 body can be discharged at a particular effector. By means of the adjustor 

 neurons of the central nervous system, connections are made possible between 

 all the special pathways that lead from receptive areas and these final com- 

 mon pathways to effector regions. The conduction of impulses according to 

 this principle of the final common path establishes a mechanism for the com- 

 plicated and varied responses that characterize nervous coordination. By 

 means of this mechanism the animal can behave as a unit in its reactions to 

 the changing conditions of its external environment and also maintain its 

 internal environment within a narrow range of physiological variation. 



Series of reflexes, or their occurrence in sequence, are well understood in 

 some situations and are a very important factor in reflex coordination. The 

 procedure by which a frog obtains its food involves a sequence or chain of 

 reflexes. The visual stimulus produced by a moving insect is followed by pro- 

 trusion of the tongue. If the insect is captured, its contact with the roof of 

 the mouth cavity initiates the swallowing reflexes, which occur in sequence. 



In the examples considered so far, the response to the stimulus has been 

 studied with respect to the usual external conditions that produce the effect. 

 Pavlov, a Russian physiologist, discovered that it is possible to produce what 

 he termed conditioned reflexes. For instance, the flow of saliva is a reflex 

 action stimulated normally by the sight of food. Under experimental condi- 



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