msm 



XXVI 



SENSORY RECEPTORS 



(Readings: G. von Bekesy, "The Ear," Sci. Am. 197, No. 2, 66-78, Aug. 1957, 

 Reprint No. 44. G. Wald, "Eye and Camera," Sci. Am. 183, No. 2, 32^0, 

 Aug. 1950, Reprint No. 46. W. Loewenstein, "Biological Transducers," Sci. 

 Am. 203, No. 2, 98-108, Aug. 1960, Reprint No. 70. S. P. T., pp. 195-208. 

 Weisz, pp. 480-495. Villee, pp. 373-386.) 



All that we know we lean] through our sense 

 organs. They are our ultimate instruments for 

 exploring the environment. It is of the highest 

 importance that we understand what kind of 

 instruments they are, what they can do, and 

 where they fail. 



The anatomical unit of every receptor system 

 is the single receptor cell or end-organ, particu- 

 larly sensitive to one kind of stimulus, and 

 giving rise to one quality of sensation in the 

 brain. The effect of the stimulus upon such an 

 end-organ is a depolarization ("generator poten- 

 tial"), in most cases long-lasting compared with 

 the depolarizations that stimuli excite in nerve 

 or muscle fibers — which in turn causes the firing 

 of the attached nerve fiber. In some instances, 

 as for example in touch spots, there may be 

 only one or two all-or-nothing discharges in the 

 nerve fiber in response to each stimulus. In 

 most receptor systems, however, the depolariza- 

 tion of the end-organ lasts relatively long, and 

 results in a long burst of all-or-nothing responses 

 in the attached nerve fiber, which may cease 

 after a time though the stimulus continues (e.g.. 



smell), or may go on as long as the stimulus 

 lasts, as in vision. 



TOUCH 



The skin contains a wide variety of end- 

 organs, specific for pain, heat, cold, pressure, 

 and touch. Touch receptors are of two kinds : 

 bulbous arrangements of cells enclosing the 

 naked terminal twigs of a sensory nerve fiber 

 (Pacinian and Meissner corpuscles), or the 

 widely branching terminal arborization of such 

 a nerve fiber around the basal bulb or "root" 

 of a hair ("hair crown"). With very small 

 stimuli one can map out the locations of the 

 sharply localized points at which stimulation 

 conveys any one of the skin sensations. 



We shall map out the touch spots in various 

 areas of skin in this way, using a bristle as 

 stimulus. This will demonstrate a general con- 

 dition of all receptor systems — that the receptors 

 form a discontinuous mosaic of isolated sensitive 

 points, relatively coarse in the case of touch, and 



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