ELECTRICAL ACTIVITY OF 

 A SENSE ORGAN: THE LIMULUS EYE 



(Readings: Weisz, pp. 485-490. S. P. T., pp. 188-201. L. J. Milne, "Electrical 

 Events in Vision," Sci. Am. 195, No. 6, 113-122, Dec. 1956. W. R. Loewenstein, 

 "Biological Transducers," Sci. Am. 203, No. 2, 98-108, Aug. 1960, Reprint 

 No. 70. W. H. Miller, F. RatlifF, and H. K. Hartline, "How Cells Receive 

 Stimuli," Sci. Am. 205, No. 3, 222-238, Sept. 1961, Reprint No. 99.) 



During the past two weeks we have studied the 

 electrical responses in peripheral nerve axons 

 and the spontaneous activity of the heart. Today 

 we will examine the electrical activity of a re- 

 ceptor and its attached nerve. The receptor we 

 have chosen is the eye of the horseshoe crab or 

 king crab, Limulus polyphemus. Though this 

 animal is called a crab, it is not a crustacean, but 

 an arachnid, closely related to the spiders. Many 

 of the characteristics of its eye are shared by all 

 types of eye, and indeed by all other types of 

 sensory receptor. 



The sensory receptors are the outposts of the 

 nervous system. Their business is to translate 

 various types of stimuli into meaningful patterns 

 of nerve impulses. The stimulus is always some 

 exchange of energy or material with the environ- 

 ment. This may be light (photoreceptors), heat 

 (hot and cold receptors), mechanical (touch, 

 pressure, sound), or chemical substances (taste, 

 smell, common chemical sense). The receptors 

 translate all such stimuli into relatively slow, 

 local electrical potentials, that depolarize the 



associated nerve fibers, causing them to fire 

 trains of all-or-nothing impulses. These are con- 

 ducted to other portions of the nervous system, 

 and sometimes eventually out again to excite 

 muscles and glands. The more intense the stim- 

 ulus, the larger the depolarization of the re- 

 ceptor, and the higher the frequency and greater 

 the number of impulses in the associated nerve. 



That is, the response of the receptor cell to the 

 external stimulus is not all or nothing, but small 

 or large, depending upon the intensity of the 

 stimulus; and this graded potential in the re- 

 ceptor is then translated into frequency and 

 number of all-or-nothing discharges in the asso- 

 ciated nerve fibers. Such slow receptor poten- 

 tials, called in general generator potentials, have 

 special names in the different receptors. In an 

 eye such as that of Limulus they are called retinal 

 potentials, and the records of them are called 

 electroretinograms (ERG's). 



How a receptor transduces ("leads over") 

 stimuli of all kinds into electrical activity is not 

 known; but in photoreceptors we do know 



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