108 ELECTRICAL ACTIVITY OF A SENSE ORGAN 



Exercise XX 



focus the light on the eye. Touch one wick elec- 

 trode to the front of the eye, and sling the optic 

 nerve over the other wick electrode. Be sure that 

 the wick touches nothing but the nerve. On 

 stimulating the eye, you should see small nerve 

 impulses superimposed on the ERG. Remember 

 that the eye has probably been light-adapted 

 during your manipulations, so if the responses 

 seem small, wait a few minutes for them to grow 

 larger. (Getting good reponses from a prepara- 

 tion like this may take some fussing. If your 

 responses are small, or none is visible, try re- 

 adjusting the nerve on the electrode. It is usually 

 advantageous to have the electrode close to, 

 though not touching, the eye. If the nerve is too 

 wet, the responses may be shorted out by the sea 

 water; and the nerve should be dried with a bit 

 of cotton. On the other hand, if the nerve is too 

 dry, it will not make suitable contact, and should 

 be moistened. So if your responses are not ideal, 

 keep fiddling and don't get discouraged. Your 

 instructor may also have suggestions.) 



Examine the relationship between the in- 

 tensity of the light, the height of the ERG, and 

 the relative number of impulses in the optic 

 nerve. Examine also the responses to short and 

 long flashes at one intensity. Describe your 

 observations and draw conclusions. 



If you wish to study the nerve impulses alone, 

 you can filter out the ERG by turning the low- 



frequency dial on the amplifier to the 80-cycle 

 setting. This makes the amplifier unresponsive 

 to signals that have a time course longer than 

 1/80 sec. Examine the eff'ect of a long flash of 

 light on the train of impulses. What changes in 

 frequency of impulses do you see? At what 

 point, relative to the onset of stimulus and the 

 shape of the ERG, does the nerve response reach 

 the highest frequency? Does the response stop 

 completely after the stimulus has been on for a 

 time? What would you conclude of the animal's 

 sensations? 



It seems to be a general rule that receptors re- 

 spond most strongly to change, rather than to 

 steady stimulation. Demonstrate this for your- 

 self by flickering the light to the Limulus eye by 

 rapidly moving the cardboard back and forth 

 through the light beam. 



It is possible to separate out single fibers from 

 the optic nerve of Limulus. If you have time at 

 the end of the experiment, try teasing out small 

 bundles of fibers with glass needles and fine 

 forceps. Move both electrodes to the back 

 of the eye, and sling one such nerve bundle 

 across both wicks. If you are lucky, you may 

 be able to separate out a bundle that contains 

 only one or a few active fibers. This is not an 

 easy thing to do, and several tries may be 

 necessary. 



EQUIPMENT 



Per 2-4 students 



electronic recording equipment 



copper cage (2 ft square) 



pair of wick electrodes (see notes on electronic 



equipment) 



wooden block, 3" X 3" 



2-3 nails 



3 neutral density filters (0.5, 1.0, 2.0) (partly exposed 



films will do) 



condensing lens 



microscope lamp, wired for d-c 



piece of cardboard, 8" X 10" 



2 blocks of plasticene, 2" X 2" 



2 glass needles 



flashlight with red cellophane filter 



limulus, 2" to 4" across carapace 



thread 



razor blades 



