GENERAL ZOOLOGY 



Rhodopsin \ 



Bleached by light 



Scotopsin 



+ 



Blue light 

 Neo-b retinene "^ AM -trans retinene 



Alcohol dehydrogenase + DPN 



Neo-b vitamin A " AH- trans vitamin A 



Blood " Pigment epithelium of retina 



Fig. 4.16. Diagram of the rhodopsin system in the retina of land vertebrates. The visual pig- 

 ment rhodopsin is formed bv the combination of scotopsin, a protein, and retinene, an oxidized 

 form of vitamin A. The molecule known as vitamin A exists in a number of diflTerent physical 

 shapes or isomers, each of which can be oxidized to a retinene molecule of the same shape. Only 

 the form known as neo-b retinene can combine with scotopsin to form rhodopsin. When rhodop- 

 sin absorbs light, establishing a visual impulse, and the pigment bleaches, the resulting retinene 

 is the all-trans form. This may be transformed or isomerized, in the presence of blue light, to 

 neo-b retinene or reduced to all-trans vitamin A. Again, this may be isomerized to neo-b vita- 

 min A or lost to the pigment epithelium of the retina (cf. Fig. 4.14) or to the blood. The 

 reversible transformation between vitamin A and retinene is dependent on the enzyme alcohol 

 dehydrogenase and coenzyme DPN, which contains nicotinamide, one of the B vitamins (p. 33). 

 (Adapted from G. Wald, American Journal of Ophthalmology, vol. 40, 1955.) 



Receptors located in the inner ear are of two types: those that respond to 

 changes in position and make possible the maintenance of equilibrium, and 

 those that are sensitive to sound waves and facilitate hearing (Fig. 4.4). 

 During the evolution of vertebrates, these receptors have become localized in 

 different parts of the inner ear. In higher vertebrates, the semicircular ducts ^ 

 and ampullae of the ear, with their associated receptors, function in maintain- 

 ing equilibrium (Fig. 4.17). The cochlea in these vertebrates contains struc- 

 tures which make possible response to sound waves or vibrations. Sound 

 waves set up vibrations in the tympanic membrane or eardrum. These 

 vibrations are transmitted across the middle ear by way of the auditory 

 ossicles to the so-called round window of the cochlea and produce waves 

 in its basilar membrane (Fig. 4.18). This membrane varies greatly in stiff- 

 ness along its length, and sounds of different frequencies excite specific regions 

 of it; the highest frequencies set up maximum vibrations in the stiffest portion 

 which is near the window. These localized movements in the basilar mem- 

 brane bring the surfaces of the hair cells against the tectorial membrane in 

 particular organs of Corti and establish impulses that are conducted to the 



'The term semicircular canals refers to the spaces in the skull occupied by the semi- 

 circular ducts. 



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