Midterm Marks posted by next Monday Today - Vision Structure / anatomy of eyes

1. Midterm Marks posted by next Monday Today - Vision • Structure / anatomy of eyes • Photochemistry of pigment molecules • Transduction of light energy to electrical signals

2. Eyes • Photoreceptor organs • At least 10 different ‘eyes’ have evolved • Range from simple ‘eyespots’ with a few photoreceptors to very complex with thousands of receptors

3. Vertebrate eye • single lens gathers light • and focuses it on many • receptor cells • Compound Eye • Many units called ommatidium • each with it own lens

4. Single Ommatidium Light Lens Photoreceptor cell Rhabdomere Extension of photoreceptor cell that contains visual pigment Photoreceptor axon

5. Vertebrate eye Lens Light Optic Nerve

6. The retina Rod Cone • Has two types of photoreceptors • Rod • High sensitivity • Low resolution • Black & White vision • Cones • High resolution • Lower sensitivity • Colour vision Outer Segment Inner Segment Synaptic Terminal

7. Electrical Properties of Vertebrate Photoreceptors • Compared to other neurons, resting Vm is more positive (~ -20mV) • With light exposure, Vm hyperpolarizes! 0 mV rest Vm Light on

8. Why hyperpolarize in response to light? • In the dark, • PNa  PK (outer segment) • Vm therefore between ENa and EK • In response to light, • PNa is reduced (outer segment), PK > PNa • therefore, Vm  EK, hyperpolarizes

9. Outer Segment Na+ Inner Segment Dark Current Synaptic Ending Na/K pump Steady release of neurotransmitter

10. Visual Pigments membrane folds of the outer segment Eg. Cone Outer Segment • Membrane folds: • called disks • contain pigment • Increase surface area

11. Visual Pigment molecules • calledRhodopsin • Retinal (Vitamin A derivative) + Opsin (G-protein coupled receptor) • In humans 4 types of Opsin molecules • One type of rod • 3 types of cones • These specify which wavelength of light the receptor responds to

12. ‘Light’

14. Colour Blindness Everyone should see number 12 ‘normal’ see number 8 Red-green deficiency see number 3 Total colour blind see no number Colour blindness effects ~7-10% males, <1% females

16. Colour vision – not so simple!

17. Photochemistry of Pigment molecules Activated form Light Rhodopsin all-trans-Retinol (Vitamin A) Opsin + 11-cis-Retinal all-trans-Retinal isomerase Opsin 11-cis-Retinal Light converts 11-cis-Retinal to all-trans-Retinal

19. Outer Segment Na+ Inner Segment Dark Current Synaptic Ending Na/K pump Steady release of neurotransmitter

20. GMP cGMP Light phosphodiesterase Disk Membrane PDE Rhodopsin Transducin (G-Protein) Dark Current Channel Plasma membrane Na+

21. Phototransduction cascade Light activates rhodopsin activates the G-protein Transducin activates a phosphodiesterase enzyme (PDE) converts cGMP  GMP  cGMP closes ion channel, (the dark current channel) Hyperpolarizes the photoreceptor

22. Dark-current channel • Open in the dark • Closes in response to light • Nucleotide-gated channel (opened by cGMP) • Permeable to Na+ • Keeps photoreceptor Vm more positive than most neurons  Steady release of neurotransmitter

23. Photoreceptor With Light In the Dark Depolarized Hyperpolarized Steady release of of neurotransmitter Neurotransmitter release is reduced Inhibition is relieved Inhibitory synapse Hyperpolarized Depolarizes Bipolar cell  transmitter release  transmitter release Excitatory synapse APs APs Ganglion cell To Optic Nerve 

24. Summary • Retina has two types of photoreceptors • Vertebrate Photoreceptors have ‘dark current’ • Light converts rhodopsin from cis to trans configuration • Activates G-protein, which closes dark current channel by regulating cGMP • Photoreceptor hyperpolarizes, reducing neurotransmitter release • Relieves inhibition of bipolar cell • Increases excitatory synaptic transmission to ganglion cell,  action potentials