THE EYE: LAST LECTURE: SOME REVIEW, SOME NEW

1. THE EYE: LAST LECTURE: SOME REVIEW, SOME NEW

3. Fig. 17-6, p. 560

4. Light Transduction Path of Light Cornea Aqueous humor Pupil Lens Vitreous humor All of the above ALLOW LIGHT TO PASS THROUGH AND ALL REFRACT IT ( EXCEPT PUPILS). Retina (photoreceptor layer) Photopigmentrhodopsin splits into opsin and retinol ULTIMATELY………in the ganglian cells….Chemical gated sodium channels open Membrane Depolarization Action potential is started

5. Optic chiasma - optic nerves partially cross (right side of the field of each eye combining and going to the lateral geniculate on the right, those from the left to the left) • Medial nerve fibers of each eye in the “criss-cross” form the Optic chiasma.

7. Pathway of Nerve Impulse Photoreceptors TO Bipolar Cells to 1. Ganglian cells which have axons that are the Optic nerve then to 2. Optic chiasm which goes to the 3. . superior colliculus and then to the 4.thalamus ( Lateral Geniculate Nucleus of the Thalamus) Final Destination: . Visual cortex in Occipital lobe #1 is the ganglian cells as this is these are where the first AP is generated!!!!

8. Now, We Want the DETAILS of PHOTORECEPTION …

9. This is the area of the retina where we have the clearest, most distinct vision • Here we find a small depression called the fovea centralis ( CONES are present here—NO RODS!)

10. The NERVOUS TUNIC and PHOTORECEPTION The Innermost Nervous Sensory Tunic : • Contains the PHOTORECEPTORS • This is where TRANSDUCTION TAKES PLACE • We will review the cells leading to this process • Remember CODING takes place in the cerebrum

13. Rods and Cones • These are photoreceptor cells; they are NOT considered to be neurons but derived from stem cells that produce ependymal cells of the brain. • They absorb light and initiate visual signaling • Each has a special outer segment specialized to absorb light , microtubules, mitochondria, nucleus. • Each synapses with a retinal layer next to it.

14. RODS • The outer segment of a rod contains a STACK OF MEMBRANOUS DISKS • Each DISK has globular proteins which consists of: • 1. Rhodopsin: also called visual purple ( 2 parts: opsin plus retinal, derived from Vitamin A) • 2. Opsin: is embedded in the disc membrane s of the rod’s outermost segment. • RODS CANNOT distinguish color!!

15. CONES • The pigment here is called PHOTOPSIN ( IODOPSIN) • RESPONSIBLE FOR COLOR VISION: ONLY IN BRIGHT LIGHT

17. REMEMBER G-PROTEIN RECEPTORS? • These exert their effect INDIRECTLY through a G-PROTEIN which acts as a middleman to activate (usually) a membrane –bound ion channel or enzyme. • As a result, one or more intracellular chemical signals, commonly called SECOND MESSENGERS, are generated and instigate internal cellular events. EX: cAMP and ionic Calcium

18. Fig. 17-13, p. 567

19. Fig. 17-13a, p. 567

20. Both RODs and CONEs contain light sensitive pigment that decompose when exposed to light. • The pigment in RODs is called Rhodopsin • (Visual Purple) Rhodopsin decomposes to opsin plus retinal . Retinal is synthesized from Vitamin A

21. Fig. 17-13b, p. 567

22. Fig. 17-14, part 1, p. 568

23. TURN TO PAGE 564 IN YOUR TEXT.

24. Fig. 17-14, part 2, p. 568

25. Fig. 17-14, part 3, p. 568

26. ENZYMATIC CASCADE DUE TO PHOTOEXCITED RHODOPSIN • PhotoexcitedRhodopsin triggers an enzymatic cascade process resulting in the hydrolysis of GMP • TRANSDUCIN ACTIVATIVATES Photodiesterase (PDE)PDE BREAKS DOWN OR HYDROLYZES cGMP • This in turn closes cation-specific channels within the rod cell membrane which are naturally open to influx of Na+ in the dark  Hyperpolarization • Due to the effect of hyperpolarization, the inner synaptic body sends a nerve signal to BIPOLAR NEURONS

27. Turn To Page 565 in your TEXTBOOK

28. Fig. 17-14, part 5, p. 568

29. Fig. 17-14, part 6, p. 568

30. Fig. 17-15, p. 569