SENSE ORGANS AND NERVOUS COORDINATION 



589 



You then become aware of the stimulus and may voluntarily decide to 

 do something about it, perhaps withdraw your whole arm or turn off 

 the stove. If so, impulses will pass out from the brain along efferent 

 internuncial neurons to the appropriate efferent neurons. 



Many other kinds of reflexes occur in the spinal cord and in parts 

 of the brain in addition to the three-neuron reflex discussed above. The 

 familiar knee jerk is a two-neuron reflex; the afferent neuron synapses 

 directly with the efferent neuron, and no internuncial neurons are in- 

 volved. Reflexes often involve several regions of the body. If a drop of 

 acid is placed on the flank skin of a frog, both hind legs will converge 

 on this spot and alternately flex and extend in an attempt to scrape off 

 the acid. This will happen even if the entire brain has been destroyed. 

 Complex, coordinated reflexes of this type are possible because inter- 

 nuncial neurons extend from the afferent neurons through the cord to 

 many different efferent neurons. 



Reflexes of the types described are present in all individuals as soon 

 as the neuronal pathways have developed. These are inherited or inborn 

 reflexes, and they are not dependent upon the training that the indi- 

 vidual receives. Other reflexes, known as conditioned reflexes, develop 

 as a result of specific training. Conditioned reflexes were first demon- 

 strated by Pavlov, the Russian physiologist who also performed experi- 

 ments on the control of gastric secretion. In a classic experiment, Pavlov 

 fed a dog and simultaneously rang a bell. The bell, of course, had 

 nothing to do with salivation, and at the beginning of the experiment 



"Internuncial 

 neuron 



Cerebral corbejc 



0^ ( C^:^ internuncial 

 Jt,.^^ V^T^ neuron. 



Th.cda.mus 



Cerebellum 



Afferent neuron 



Temperature 

 receptor 

 (skin) 



Hand muscle- 



Afferent internuncied 

 neurons 



"Efferent neurons 

 -Arm muscle 



Figure 29.8. The types of neurons that make up the nervous system. An afferent 

 neuron, an internuncial neuron, and an efferent neuron are involved in the spinal 

 reflex described in the text. 



