590 VERTEBRATE LIFE AND ORGANIZATION 



Figure 29.9. Diagrams of important types of neuronal interrelationships. A, A 

 divergent pathway; B, a convergent pathway; C, a multiple chain circuit; D, a closed 

 chain circuit. 



would not induce salivation by itself. Salivation was reflexly stimulated 

 by the sight or smell of food. The bell was rung each time the dog was 

 fed and the dog gradually learned to associate the bell with food. 

 Eventually ringing the bell without presenting food would initiate 

 salivation and a conditioned reflex had been established. Many of our 

 responses, including subconscious responses to stimuli when driving a 

 car, are conditioned reflexes that have developed as a result of our spe- 

 cific training. An inexperienced driver must consciously think of what 

 to do if his car starts to skid, but an experienced driver reflexly re- 

 sponds to the feel of a car that is beginning to skid. 



Reflexes in the spinal cord and brain form the basis of a great many 

 of our responses, but there are other neuronal interrelations that are 

 important for an understanding of the activities of the nervous system. 

 Most pathways within the nervous system involve many neurons, not 

 just two or three as in the simpler reflexes, and this permits a variety of 

 complex interrelations. A great many pathways are divergent (Fig. 

 29.9 A). The axon of a neuron may branch many times, synapse with a 

 number of different neurons, and these in turn may branch further. 

 Such an arrangement permits a single impulse to exert an effect over 

 a wide area; a single impulse may ultimately activate a thousand or 

 more neurons. Many other pathways are convergent (Fig. 29.9 B); 

 neurons coming from many different areas converge upon a single 

 neuron or group of neurons. The convergence of neurons upon centers 

 in the brain and upon the cell bodies of efferent neurons are examples 

 of this type of pathway. It has been estimated that the efferent neurons 

 receive impulses that originate from fifteen or twenty different sources. 

 The response of the last neuron in a convergent pathway is the result 

 of the interaction of a variety of excitatory and inhibitory influences. 

 Convergent pathways are important in forming the structural basis for 

 the integrative activity of the nervous system. 



Many neuronal circuits, including those diagrammed in Figure 

 29.9 A and B, involve the passage of impulses only as long as the first 



