210 The Vertebrate Organ Systems 



glands which respond to the change which has been noted by the 

 receptor. This change is mediated either by the brain or spinal cord, 

 and the response may be either conscious or unconscious. 



Some of the simplest, innate reflexes such as the knee jerk probably 

 involve only a few reflex arcs, and the original stimulus passes through 

 association neurons in the cord and does not go up the nerve tracts to 

 the brain until after the response has been made. This is an example 

 of an inherited or unconditioned reflex. The learned or conditioned 

 reflex is based on the inherited reflexes, but involves many more arcs 

 and pathways. An example of a modified reflex was that first studied 

 by the Russian physiologist, Pavlov. He noted that when food was 

 presented to a dog the salivary glands started secreting. He next 

 experimented by ringing a bell at the same time that the food was pre- 

 sented. After some time, the dog began to salivate when the bell 

 sounded even though food was not presented. Pavlov called this the 

 conditioned reflex. In this case, an entirely different stimulus evoked 

 an unrelated response, and it could be said that learning had taken 

 place. 



In making this response, the dog had to modify the pathways over 

 which the impulses bringing about salivary secretion traveled; this 

 involved many synaptic connections between neurons. At first the 

 impulses are much delayed over the new pathways, but as learning 

 proceeds, the pathways become more easily traversed and gradually 

 the new response is automatic. This has been suggested as the basis 

 of the learning process, that is, existing synaptic connections are modi- 

 fied. Clearly, constant repetition is the key to the learning of either 

 a process or an abstract technique. 



As is well known, all conscious activities are centered in the cere- 

 brum ; many common activities such as walking, swimming, and dancing 

 begin as cerebral matters. Once they are thoroughly learned, the activity 

 is apparently relegated to other centers. Cerebral action is needed to 

 begin the reflex chain ; for example, one consciously starts to walk and 

 then quickly forgets about it. After some illnesses which affect the 

 nervous system, such as poliomyelitis, new nerve pathways for these 

 activities must be established. 



Brain Waves. — All the activity of the nervous system is essentially 

 electrical in nature and can be detected by means of the proper record- 

 ing apparatus. Inasmuch as there is such a tremendous number of 

 neurons present in the brain, these electrical changes occur at all times. 



