302 - Multicellular Animals, Especially Man 



The main receptor-conductor-effector path- 

 way of a salivation reflex is shown in Figure 

 16-17. All flow of saliva is abolished if the 

 motor nerves to the glands are severed or if 

 the brain is damaged in the region of the 

 salivation centers. This experiment proves 

 (hat the entire output of the glands is con- 

 trolled by the nervous system; that is, no 

 hormone is involved. 



The salivation reflex was used by the well- 

 known Russian physiologist, Pavlov, in his 

 early studies on reflexes generally. Pavlov 

 worked on dogs in which the ducts of the 

 salivary glands were brought to the outer 

 surface of the cheeks, thus permitting him 

 to observe the flow of saliva whenever a suit- 

 able stimulus was applied. These studies led 

 Pavlov to draw a distinction between the 



primary receptors are mainly the taste buds 

 (p. 421), although the tactile receptors of the 

 oral mucosa play a minor role. This type of 

 reflex, which can be elicited in the absence 

 of any previous training or conditioning, is 

 called an unconditioned reflex. But Pavlov 

 showed that in trained, or conditioned, ani- 

 mals, other types of stimulation — or more 

 precisely other receptors — can be substituted 

 for the primary receptors. Thus in older dogs 

 or children, the sight or smell of food — or 

 sounds associated with food — will cause the 

 "mouth to water." These conditioned re- 

 flexes result from a frequent association be- 

 tween stimuli affecting the primary receptors 

 and stimuli acting upon other (secondary) 

 receptors of the body. After a period of con- 

 ditioning, salivation is obtained merely by 



two main kinds of reflexes: (1) conditioned sounding a bell that previously was rung 



reflexes and (2) unconditioned reflexes. 



If food, or anything associated with food, 

 is shown to a very young puppy, or child, 

 no salivation occurs, but the saliva promptly 

 flows if anything — even a tasteless object — is 

 placed directly in the mouth. This shows 

 that the primary receptors of die salivation 

 reflex are localized in the mouth. These 



each time a puppy was fed, or by showing 

 any object that has been displayed at each 

 feeding during the conditioning period. Thus 

 in man and other animals, the salivation of 

 an older individual represents a dual reflex. 

 Ii is partly an unconditioned reflex, elicited 

 by the food that is actually in the mouth; 

 and partly a conditioned reflex, resulting 



TONGUE 



UPPER PART OF BRAIN 



RECEPTORS 

 (TASTE BUDS) 



LINGUAL NERVE 



GANGLION 

 -SALIVARY GLAND 



Fig. 16-17. Diagram showing one of the pathways of the salivation reflex. Such a 

 typical reflex involves a series of nerve cells, which convey impulses from the re- 

 ceptors, via the central nervous system, to the effectors. 



