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HANDBOOK OF PHVSIOLOGV 



NEUROPHYSIOLOGY I 



tration. Simultaneously the mechanism would attain 

 its maximum sensitivity to light energy. In continuous 

 illumination the system should reach an cquiliijrium 

 such that the rate at which the pigment is altered by 

 absorbed energy is equal to the rate at which the 

 pigment is produced. The time required to reach 

 maximum concentration in the dark has been found 

 to be several times as great as that required to reach 

 an equilibrium in continuous illumination. The 

 former is a measure of dark adaptation and the latter 

 of light adaptation. 



Since radiant energy arrives a quantum at a time 

 and, according to Einstein's law of photochemical 

 equivalence, is absorbed only at the rate of one quan- 

 tum per molecule affected, this initial step in photo- 

 sensitivity has a statistical character. At low intensities 

 of light, so few molecules may capture a photon in a 

 given time that the organism ignores the scattered 

 events. At a slightly higher intensity of stimulation, 

 the frequency of capture would rise. If the lower 

 limits for response to light are explored with a test 

 flash of constant duration, some definite intensity 

 level can be found at which a sensation of light is 

 obtained 50 per cent of the time. At a slightly lower 

 intensity, the response is obtained perhaps 30 per 

 cent of the time. At a slightly higher intensity, per- 

 haps 80 per cent of test flashes elicit a response. Both 

 subjective and objective measurements of this kind 

 show a range in 'frequency of seeing.' Some value, 

 such as 50 per cent, can be defined as threshold. 



Variation in response at threshold may be entirely 

 attributable to variations in the quantum content 

 of test flashes. Whether one molecule of pigment 

 modified in a brief time (such as o. i sec.) is enough to 

 trigger the entire photosensitive mechanism is still 

 unsettled (10, 77, 97, 213). Different nervous systems 

 may require several molecules of pigment to be al- 

 tered almost simultaneously. In any case it is clear 

 that photosensitivity has an efficiency approaching 

 the theoretical limit of one quantum and one mole- 

 cule. 



Relatively few pigments are so unstable that a 

 single photon can produce a chemical change. A 

 photon simply lacks the amount of energy required 

 to start most chemical reactions. From this it might 

 be expected that photons with the largest content of 

 energy would be most important in photosensitivity. 

 In the wavelength band visible to the human eye, 

 that giving the sensation of violet consists of photons 

 with about double the energy of those in the red. 

 Ultraviolet includes photons with an energy content 

 double that of photons in the violet; but the seem- 



ingly transparent media of terrestrial vertebrate eyes 

 ab.sorb the ultraviolet before it reaches the photo- 

 sensitive retina. Aquatic organisms are shielded from 

 ultraviolet by the water around them. Except under 

 laljoratory conditions, only the terrestrial arthropods 

 (such as insects) appear to be stimulated visually by 

 wavelengths shorter than 400 m/x. 



The photosensiti\e pigments extracted from in- 

 vertebrate and vertebrate eyes (152, 153, 276, 277) 

 appear consistent in having their eflTective maximum 

 of absorption between 400 and 700 m/z — well within 

 the spectrum visible to man (fig. i). Indirect evidence 

 is available to indicate that the corresponding pig- 

 ment or pigments in insects may be more affected by 

 the ultraviolet components of sunshine than by energy 

 absorbed at a secondary absorption maximum in the 

 human range. Hence it is apparent that the chemical 

 adaptations which permit photosensitivity in aquatic 

 life and terrestrial vertebrates are related less to the 

 energy content of the photons than to the wave- 

 lengths of radiant energy which penetrate most 

 deeply into seas (480 mju) and lakes (560 m/ii). Sensi- 

 tivity to ultraviolet seems to have come secondarily 

 as a gain when some arthropods became both ter- 

 restrial and diurnal. 



For extraction of photosensitive pigments in suf- 

 ficient quantity for spectrophotometric analysis, con- 

 siderable masses of photosensitive tissue are needed. 

 So far this requirement has limited direct study to the 

 large eyes of squids (20, 21, 65, 150, 229) and the 

 stalked eyes of euphausiid crustaceans (143) which 

 can be cut from hundreds of specimens taken with 

 plankton nets. Most other invertebrates are either 

 too small or too difficult to catch in adequate num- 

 bers for a biochemical approach. In consequence 

 other avenues of investigation have been necessary 

 for studying their photosensitivity. 



The most valid approach is beset with technological 

 difficulties. It consists of inserting microelectrodes 

 into photosensitive cells and recording electrical events 

 which follow stimulation of the cells by light. These 

 changes in electrical potential clearly demonstrate 

 the peripheral origin of nervous activity in visual 

 systems (90) and suggest that depolarization of the 

 photosensitive cell is responsible for initiating nerve 

 impulses in its associated nerve fiber (177). 



With some invertebrate eyes it is possible to study 

 impulses in surviving nerve fillers emerging from 

 photosensitive cells (88, 89, 281, 286). Far easier and 

 more widely applicable is the less informative pro- 

 cedure of applying an electrode to the corneal surface 

 of an intact eye and examining the gross potential 



