ACTION 



Responses of Higher Animals-. The Receptors - 429 



POTENTIAL 



WEAKER STIMULUS 



RESTING POTENTIAL 



ACTION 



POTENTIAL 



STRONGER STIMULUS- 



RESTING POTENTIAL 



Fig. 23-11. Diagram of an action potential recorded in a nerve near connection with a stretch receptor 

 (muscle spindle, Fig. 23-1). The stronger the stimulus the greater is the generator potential and the greater 

 the frequency of the spikes in the volley of action potential discharges. Presumably the generator potential 

 originates in the receptor (spindle) and is transmitted to the associated nerve fibers, causing them to dis- 

 charge impulses. (Based on the records of Bernhard Katz, University College, London.) 



On the chemical side, the taste buds and 

 olfactory cells respond very directly to the 

 chemical action of impinging molecules; and 

 probably the thermoreceptors are activated 

 by chemical changes induced in the cells by 

 heat and cold. The pain receptors respond 

 to very strong stimuli of any kind, which in- 

 dicates that excitation depends upon chemi- 

 cal substances released from damaged cells. 

 And finally, photochemical reactions induced 

 by light impinging on the retina constitute 

 the true excitatory mechanism in the rods 

 and cones. 



The highly specialized sensory structures 

 of complex animals all seem to have been 

 evolved from the relatively simple mechani- 

 cal and chemical receptors of lower organ- 

 isms. The senses have great significance in 

 survival, since the animal must rely upon its 



senses in the search for food and the avoid- 

 ance of danger. No organism, so far as we 

 know, has developed a sensitivity to mag- 

 netic fields, or to radio or cosmic waves, al- 

 though certain bees seem to be sensitive to 

 ultraviolet and infrared light, and bats are 

 able to guide their flight in total darkness 

 because they possess a sensitivity to high- 

 frequency vibrations that are supersonic to 

 other animals. And despite the fact that all 

 cells are readily excitable by electric cur- 

 rents, few, if any organisms have developed 

 specialized receptors for this type of stimula- 

 tion. Animals have evolved without any di- 

 rect appreciation of these other forces of the 

 environment, probably because such forces, 

 having little relation to survival, have not 

 played a significant role in natural selection 

 (Chap. 28). 



