106 ELECTRICAL ACTIVITY OF A SENSE ORGAN 



Exercise XX 



You must understand that though light is 

 needed to inactivate the pigment, it regenerates 

 equally well in light or darkness. In a steady 

 light the pigment is continuously inactivated and 

 continuously regenerated, so that a balance is 

 struck between these processes, in which some 

 pigment is constantly present, permitting vision 

 to continue. In the dark, only regeneration 

 occurs, bringing the visual pigment back up to 

 its maximum concentration and returning the 

 eye to its maximum sensitivity. 



In today's experiment you will examine the 

 retinal generator potential (ERG), dark-adapta- 

 tion as measured by the ERG, the relation of the 

 ERG to action potentials in the optic nerve, and 

 the patterns of nerve impulses in the optic nerve. 



Equipment 



The light stimulus is provided by a micro- 

 scope lamp, and its intensity is controlled with 

 neutral filters inserted in the beam. The light 

 will be focused on the eye with a condensing lens, 

 and the duration of the stimulus controlled by 

 raising and lowering a piece of cardboard that 

 shuts off the beam. Before beginning the experi- 

 ment, look over this setup and try it out. 



Since exposed wick electrodes will be used, the 

 preparation must be shielded. For this it is set 

 up inside the copper cage. Make sure that the 

 wick electrodes are connected with the binding- 

 post terminals on the side of the copper cage. 

 The input cable from the preamplifier is then 

 connected to these binding posts. 



EXPERIMENTAL PROCEDURE 



Note. In this first part of the experiment, use 

 as little light as possible so as not to light-adapt 

 your preparation strongly. The Limulus eye is 

 not stimulated by deep red light, since its visual 

 pigment does not absorb the long wavelengths of 

 the spectrum. Red lamps will be available, and 

 can be used freely without affecting the prepara- 

 tion. 



Plan to work in groups of two to four. Place 

 the horseshoe crab on the block of wood in the 



shielded cage, and fasten it down with nails 

 through the edges of the shell. Putting nails 

 through the shell causes no more pain than cut- 

 ting your fingernails. 



Identify the prominent faceted eyes, so-called 

 compound eyes. With a fresh razor blade, gently 

 scrape the horny surface of the eye (again a pain- 

 less operation). This removes the highly water- 

 resistant waxy substance that helps to make the 

 eye waterpoof. Don't scrape too long or too 

 hard; it is better to do too little than too much. 

 Then with the tip of a sharp scalpel, dig a tiny 

 hole through the shell directly back of the eye, 

 just large enough to admit the tip of a wick elec- 

 trode. 



Set the animal in position in the shielded cage, 

 and focus the light beam on its eye, using very 

 dim light, and exposing it only for short inter- 

 vals. The cotton wicks used as electrodes will 

 have been soaked with sea water, so as to con- 

 duct the electric current. Place one such wick on 

 the cornea of the eye, and insert the tip of the 

 other through the small hole behind the eye. 



Convenient settings on the recording instru- 

 ment are: 



Preamplifier magnification = 1000 



Indicator amplification = 0.05 volts/division 



Waveform duration = 1000 msec (1 sec) 



The trace should be steady and should not ex- 

 hibit waves due to interference from the 60-cycle 

 power lines. If you do have hash on the screen, 

 readjust the electrodes to make a better contact, 

 If the hash persists, you may have to scrape the 

 surface of the eye a little more, but consult the 

 instructor first. 



Stimulate the eye with a dim, brief flash of 

 light (through density 2.0 filter), and observe 

 the response. How long does the response last 

 compared with the stimulus? How does it com- 

 pare in duration with a nerve action potential? 

 Let the animal dark-adapt for a few minutes, and 

 stimulate the eye again. If the response has 

 grown, let the animal continue to dark-adapt 

 until the responses have become constant. This 

 may take up to 15 minutes or longer. 



