60 The Physiology of Sense Organs 



by light energy of various wavelengths. Although both the 

 amplitude and duration of the major part of this depolarization 

 are functions of light intensity, a large peak is prominent at the 

 beginning of such records (especially when the stimulus intensity 

 is great), and this phase of the electrical response decays at a rate 

 which is independent of the stimulus (fig. 26). At the present 

 time there is no clue as to the specific nature of t his init ial, rapidly- 

 declining phase of the receptor potential. iThe situation is 

 complicated by the fact that, in some related preparations, this 

 part of the response can attain great amplitudes — often over- 

 shooting the zero level of potential — while its duration can become 

 quite brief. This response thus has some of the characteristics 

 of a regenerative spike, although it is apparently not propagated 

 along the nerve. A report of a n»ii=propagated electrically- 

 excitable component of the receptor potential in the Limulus eye 

 has, in fact, recently appeared. ^^ It is possible that the response 

 illustrated in figure 26 has a similar physiological basis. Further 

 studies may establish these rapid components as an entirely new 

 response mechanism peculiar to visual receptors. 



Large gaps exist in our knowledge of the excitatory train of 

 events which occur in photoreceptor cells following the absorption 

 of light. In all photoreceptors the initial event involves the 

 utilization of specific wavelengths to isomerize a ph otolabile 

 pigment; it is generally thought that the chemical properties of s 

 pigments in different sensory cells determine which of the available /. 

 energy spectrum will be most effective in the process of photo-( 

 isomerization, and this is now known to be the physical basis for\ 

 color vision.'^ Pignuts from different animal groups differ iny 

 1;he minor details of chemical structure; however, a protein- 

 carotenoid molecular complex appears to be invariably tnvolved, 

 whether the animal is a crayfish or a cow, and thus many of the 

 specific chemical details in the detection of light can be generalized. 



In man, the visual pigment subserving night-time, or scotopic, 

 vision is located in the outer segments of the retii ^ sen sory cells 

 known as rods. The pigment^complement of these cells, rhodopsin, 

 is a complex of a specific protein and a carotenoid — a derivative 

 of vitamin A. ^The protein, opsin, and the carotenoid, neo-b 

 retinal, can exist in a conjugated state only in the dark. Electro- 

 magnetic energy, especially in the wavelength band of 400 - 600 /i, 



