VISUAL SYSTEM: STATE OF THE ART 51 



Table 3. Rod and cone photoreceptors (various sources). 



Rod 



Cone 



1. Outer segment long and slender. 



2. Plasma membrane of outer segment 

 encloses disks, continuity of mem- 

 brane limited to base of outer seg- 

 ment. 



3. Periodicity of disk membrane 

 185-220 A. 



4. Disks replaced by basal formation 

 and apical shedding. 



5. Disks often lobulated. 



6. Diameter of inner and outer seg- 

 ments similar. 



7. Visual pigment contains "rod 

 opsin." 



8. Outer segment connective long, 

 slender, and derived from flagellum 

 by vesicle formation. 



9. Oil droplets, polysaccharides 

 absent, mitochondria few. 



11. May lengthen in light. 



12. Output terminal "rod spherule" 

 with several synaptic units. 



13. Cell destroyed by Iodoacetate. 



1. Outer segment tapered, coni- 

 cal. 



2. Plasma membrane continuous 

 with disks along entire length 

 of outer segment, disks open 

 to ventricular space. 



3. Periodicity: 220-225 A. 



4. Disks probably replaced by 

 in situ exchange of materials. 

 New disks not formed. 



5. Disks not lobulated. 



6. Inner segment larger. 



7. "Cone opsin" present. 



8. Connective shorter; derived 

 by formation of flattened 

 microvilli. 



9. May store oil droplets, poly- 

 saccharides in inner segment, 

 mitochondria densely packed. 



11. May contract in light. 



12. Output terminal "cone 

 pedicle"; 12-25 synaptic 

 units. 



13. Cell insensitive to 

 Iodoacetate. 



14. Index of refraction is 1.41. 



14. Index of refraction is 1.38. 



