Responses of Higher Animals: The Receptors - 427 



of the receptors, but solely upon how the 

 excitations are routed through the nervous 

 system. Only such excitations as are trans- 

 mitted to the cerebral cortex of the brain 

 succeed in generating any conscious percep- 

 tions; this phase of responsiveness will be 

 considered in Chapter 25. 



Indirectly, however, the sense organs do 

 determine the quality of sensations, because 

 each sense organ is connected to a specific 

 part of the nervous system by a specific set 

 of sensory nerve fibers. The rods and cones, 

 for example, generate sensations of light 

 rather than of sound, because the rods and 

 cones relay their excitations to the visual 

 area of the brain via the optic nerves rather 

 than to the auditory area via the auditory 

 nerves. The nerve fibers and their connec- 

 tions, therefore, determine directly the qual- 

 ity of the sensation, and the stimulation of a 

 certain group of nerve fibers gives rise to the 

 certain kind of sensation; this is true even if 

 these nerve fibers are stimulated artificially 

 by electrical excitation, rather than naturally 

 by impulses from one of the sense organs. 



Likewise the localization of sensations is a 

 function of the brain; but again the position 

 of the stimulated receptors is an important 

 determining factor. Some sensations, such as 

 pain, are referred to a part of the body it- 

 self; but others, such as the warmth of a 

 stove, are referred to the external environ- 

 ment. In either case, however, the location 

 of the stimulated receptors is very important 

 in determining where the sensation is local- 

 ized by the mind. A cold floor stimulates the 

 thermoreceptors of the soles of the feet, and 

 consequently the coolness is judged to come 

 from the floor; or if one burns oneself, the 

 pain is referred to the injured organ, where 

 the stimulated pain receptors are localized. 

 In the case of pain originating in some of the 

 internal organs, however, the mind is not 

 very accurate in its judgments; quite fre- 

 quently internal pain is referred to some 

 specific external part of the body. 



Judgments as to the intensity of a sensa- 

 tion are conditioned largely by the behavior 



of the stimulated sense organ. Ordinarily 

 when a receptor is stimulated, it discharges 

 not one, but a volley of excitations into the 

 sensory nerves. The stronger the stimulus, 

 the more prolonged is the volley from the 

 receptor; but still more important, the fre- 

 quency of excitations is greater if the stimu- 

 lus is stronger. Thus a very dim light may 

 stimulate the photoreceptors of the eye to 

 discharge excitations at the slow rate of about 

 10 per second, as compared to more than 200 

 per second when the light is bright. Further- 

 more, stronger stimuli usually succeed in 

 stimulating a greater number of receptor cells 

 in the sense organ, and thus a greater num- 

 ber of sensory nerve fibers will carry excita- 

 tion volleys toward the brain. All in all, 

 therefore, the brain is influenced by several 

 factors in mediating responses to stimuli of 

 different intensities. With stronger stimuli, 

 greater numbers of nerve cells become in- 

 volved in spreading the excitation volleys, 

 and the stronger the stimulus, the greater is 

 the duration and frequency in the individual 

 volleys. 



BASIC MECHANISMS OF RECEPTION AND 

 TRANSMISSION 



A receptor becomes excited by a stimulus 

 and it relays the excitation to one or more 

 nerve cells. Basically these processes appear 

 to involve: (1) a conversion, or transduction, 

 of energy derived from the stimulus to elec- 

 trical energy and (2) a utilization of the 

 electrical energy for triggering excitations in 

 associated neurons. These processes are ex- 

 tremely difficult to analyze. However, some 

 progress in this direction has recently been 

 made, under the leadership of H. K. Hart- 

 line, of Johns Hopkins University, Bernhard 

 Katz, of University College, London, Donald 

 Kennedy, of Stanford University, and a num- 

 ber of other workers. Exceedingly fine glass 

 capillary ultramicroelectrodes, with tip di- 

 ameters of less than half a micron, have been 

 devised. These can be inserted without ap- 

 parent injury into the cytoplasm of a sensory 



