Ulh Responsiveness in Single Cells 



THE CHANGING conditions within and 

 around each living body constantly act as 

 stimuli that excite the organism to perform 

 a variety of responses. The dog searches for 

 food when stimulated by the hunger con- 

 tractions of his empty stomach; if over- 

 heated by the sun he may lie down in a 

 shady spot. Responsiveness includes the sum 

 total of all an organism's reactions to stimuli. 

 Our present aim is to analyze some of the 

 factors that determine the responses of rela- 

 tively simple organisms. 



DISCONTINUOUS (ABRUPT) VS. 

 CONTINUOUS (SLOW) RESPONSES 



The most characteristic responses of organ- 

 isms are of very short duration. They flare 

 up and quickly subside after the stimulation 

 ceases — as when one shies from a tossed stone 

 or sneezes from some dust. The magnitude of 

 such discontinuous responses is out of all 

 proportion to the strength of the stimula- 

 tion. Some very slight stimulus — of sight, 

 touch, sound, etc. — precipitates a very ener- 

 getic reaction. 



In abrupt or discontinuous responses the 

 stimulus does not provide the energy ex- 

 pended during the reaction. Like the pres- 



sure on the trigger of a rifle, the stimulus 

 merely precipitates the liberation of a sud- 

 den burst of energy, and this burst subsides 

 as soon as the responsive act has been per- 

 formed. Accordingly, discontinuous responses 

 are also called "explosive responses," and 

 the stimulus is said to display a "trigger 

 action." 



In other cases, however, a very gradual and 

 enduring change of condition (such as light, 

 temperature, atmospheric pressure) leads to 

 an equally gradual and enduring change in 

 the general form and activity of the organ- 

 ism. A continuous exposure to the sun, for 

 example, produces a gradual browning of 

 the skin, or a lengthy sojourn at high alti- 

 tude leads to a gradual enrichment of the 

 hemoglobin of the blood. In such continu- 

 ous responses, the quantity and quality of 

 the bodily changes depend in large measure 

 upon the quantity and quality of the stimu- 

 lating factors. In the broadest sense such 

 gradual changes in the organism may be 

 called responses, and the environmental fac- 

 tors may be referred to as stimuli. But for 

 the present, the discussion will include only 

 responses of the discontinuous or "explosive" 

 type, in which the stimulus exerts a "trigger- 

 like" action. 



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