1. Unit Ten: The Nervous System: B. Special Senses
Chapter 50: The Eye: II. Receptor and
Neural Function of the Retina
Guyton and Hall, Textbook of Medical Physiology, 12th edition

2. Anatomy and Physiology of the Retina
Layers of the Retina-functional components
arranged in layers from the outside to the inside
Pigmented layer
Layer of rods and cones
Outer nuclear layer containing the cell bodies of the
rods and cones
Outer plexiform layer
Inner nuclear layer
Inner plexiform layer
Ganglionic layer
Layer of optic nerve fibers
Inner limiting membrane

3. Anatomy and Physiology of the Retina
Layers of the Retina
Fig Layers of the retina

4. Anatomy and Physiology of the Retina
Fovea- minute area in the center of the retina
(1 sq mm) capable of acute vision; contains
only cones
Rods and Cones- the major functional segments
of either a rod or cone are:
The outer segment
The inner segment
The nucleus
The synaptic body

5. Anatomy and Physiology of the Retina
Fig Schematic drawing of the functional parts
of the rods and cones

6. Anatomy and Physiology of the Retina
Rods and Cones
Light sensitive photochemicals are found in the
outer segment
In rods, it is rhodopsin
In cones, it is one of three color pigments which
function exactly like rhodopsin

7. Anatomy and Physiology of the Retina
Rods and Cones
In the outer segments of both rods and cones are
large numbers of discs (as many as 1000 per rod or
cone)
Pigments are conjugated proteins incorporated
into the membranes of the discs
Inner segment contains the usual organelles and
cytoplasm

8. Anatomy and Physiology of the Retina
Rods and Cones
Synaptic body connects with the neuronal cells,
the horizontal and bipolar cells
Pigment Layer of the Retina
Melanin prevents light refraction throughout
the eyeball
b. Stores large quantities of vitamin A

9. Anatomy and Physiology of the Retina
Pigment Layer of the Retina
Vitamin A is an important precursor of the
photosensitive chemicals of rods and cones

10. Anatomy and Physiology of the Retina
Fig Membranuous structures of t he outer segments of a
rod and cone

11. Anatomy and Physiology of the Retina
Blood Supply of the Retina
Central retinal artery enters with the optic nerve
Branches to supply the entire retinal surface
Outermost layer is adherent to the choroid which
is also a highly vascular area

12. Photochemistry of Vision
Rhodopsin-Retinal Visual Cycle
Fig Rhodopsin-retinal visual cycle in the rod

13. Photochemistry of Vision
Rhodopsin-Retinal Visual Cycle-The Decomposition
by Light Energy
When light energy is absorbed by rhodopsin, the
rhodopsin begins to decompose;
The cause of this is photoactivation of electrons in
the retinal portion of rhodopsin, which converts
cis into a trans form and cannot bind to the active
site on the protein.
c. This leads to unstable intermediates

14. Photochemistry of Vision
Reformation of Rhodopsin
First step is re-convert to cis form of retinal
Requires energy and is catalyzed by retinal isomerase
Once formed it binds to the protein and is stable

15. Photochemistry of Vision
Role of Vitamin A
Second pathway converts the trans-retinal to
trans-retinol (one form of vitamin A)
The trans-retinol is then converted to cis-retinal
Vitamin A is present in the pigment layer of the
retina and in the cytoplasm of rods
d. Excess retinal is converted to vitamin A

16. Photochemistry of Vision
Excitation of the Rod When Rhodopsin is Activated
by Light
The rod receptor potential is hyperpolarizing, not
depolarizing
When rhodopsin decomposes, it decreases the
rod membrane conductance for sodium ions
in the outer segment of the rod
This causes hyperpolarization of the entire rod
membrane

17. Photochemistry of Vision
Fig Movement of sodium and potassium ions through the inner
and outer segments of the rod

18. Photochemistry of Vision
Fig Phototransduction in the outer segment of the photoreceptor membrane

19. Photochemistry of Vision
Duration of the Receptor Potential and Log Relation
of the Receptor Potential to Light Intensity
Receptor potential occurs in 0.3 seconds and
lasts for about 1 second in the rods
In the cones it occurs four times as fast
Receptor potential is approx. proportional to the
logarithm of the light intensity which allows the
eye to discriminate light intensities through a range
many thousand times as great as would be otherwise

20. Photochemistry of Vision
Mechanism by Which Rhodopsin Decomposition
Decreases Membrane Sodium Conductance
(Excitation Cascade)
Photon activates an electron in the cis-retinal portion
of rhodopsin and leads to the formation of
metarhodopsin
Activated rhodopsin acts as an enzyme to activate
many molecules of transducin
Activated transducin activates many mcles of
phosphodiesterase

21. Photochemistry of Vision
Mechanism by Which Rhodopsin Decomposition
Decreases Membrane Sodium Conductance
(Excitation Cascade)
Activated phosphodiesterase hydrolyzes cGMP which
allows the sodium channels to close
e. Within a second, rhopdopsin kinase inactivates
metarhodopsin and reversion back to the normal
state with open sodium channels

22. Photochemistry of Vision
Photochemistry of Color Vision by the Cones
Only one of three types of color pigments is present
in each of the different cones
Color pigments are blue, green, and red sensitive
pigments

23. Photochemistry of Vision
Fig Light absorption by the pigment of the rods and the three color receptive cones

24. Photochemistry of Vision
Automatic Regulation of Retinal Sensitivity
Light Adaptation- in bright light the
concentrations of photosensitive chemicals are
reduced
Dark Adaptation- in darkness, the retinal and
opsins are converted back into the light
sensitive pigments

25. Photochemistry of Vision
Fig Dark adaptation, demonstrating he relation of cone adaptation to rod adaptation

26. Photochemistry of Vision
Other Mechanisms of Light and Dark Adaptation
Change in pupillary size
Neural adaptation

27. Tricolor Mechanism of Color Detection
Color Vision
Tricolor Mechanism of Color Detection
Spectral sensitivities of the three types of cones
Interpretation of color in the Nervous System
Fig Demonstration of the degree of stimulation of the different color sensitive cones
by monochromatic lights of four colors: blue, green, yellow, and orange

28. Color Vision
Perception of White Light- equal stimulation of
the red, green, and blue cones gives the sensation of seeing white
Color Blindness- when a single group of cones is
missing, the person is unable to distinguish
some colors from others
Red-green
Blue weakness

29. Neural Function of the Retina
Fig Neural organization of the retina; peripheral
area to the left, foveal area to the right

30. Neural Function of the Retina
Neural Circuitry of the Retina
Photoreceptors transmit signals to the outer plexiform layer where they synapse with bipolar cells and horizaontal cells
Horizontal cells which transmit signals horizontally in the outer plexiform layer from the rods and cones to bipolar cells
Bipolar cells which transmit signals vertically to the inner plexiform layer, where they synapse with ganglion cells and amacrine cells

31. Neural Function of the Retina
Neural Circuitry of the Retina
Amacrine cells transmit signals either directly from bipolar cells to ganglion cells or horizontally from axons of the bipolar cells to dendrites of the ganglion cells or other amacrine cells
Ganglion cells which transmit output signals from the retina through the optic nerve into the brain

32. Neural Function of the Retina
Visual Pathway from the Cones to the Ganglion Cells Functions Differently from the Rod Pathway
(Fig ) Visual pathway from the fovea has three neurons in a direct pathway: cones, bipolar cells, and ganglion cells
For rod vision there are four neurons in the direct pathway: rods, bipolar cells, amacrine cells, and ganglion cells

33. Neural Function of the Retina
Neurotransmitters
Rods and cones release glutamate
Amacrine cells release: GABA, glucine, dopamine,
acetylcholine, and indolamine; all of which are
inhibitory
Transmission of Most Signals Occurs in the
Retinal Neurons by Electrtonic Conduction, Not
by Aps- direct flow of electric current in the
neuronal cytoplasm and nerve axons from the point of excitation all the way to the output synapses

34. Neural Function of the Retina
Lateral Inhibition- enhances visual contrast and is a function of the horizontal cells
Fig Excitation and inhibition of a retinal area caused by
a beam of light

35. Neural Function of the Retina
Excitation and Inhibition- two sets of bipolar
cells provide opposing and inhibitory signals in the visual pathway
Depolarizing bipolar cells
Hyperpolarizing bipolar cells

36. Neural Function of the Retina
Amacrine Cells and Their Functions- 30 types
identified and the functions of 6 have been
characterized
Part of the direct pathway for rod vision
Responds strongly at the onset
Responds to changes in illumination
Movement of a spot across the retina

37. Neural Function of the Retina
Ganglion Cells and Optic Nerve Fibers
100 million rods, 3 million cones, and 1.6 million
ganglion cells (60 rods and 2 cones converge on
an individual ganglion cell)
Central fovea has 35,000 cones and no rods
Greater sensitivity of the peripheral retina to weak
light
d. Rods are x more sensitive to light than cones; 200 rods converge on a fiber in the periphery

38. Neural Function of the Retina
Excitation of the Ganglion Cells
Spontaneous continuous APs in the ganglion cells
Transmission of changes in light intensity- the
off-on response
Fig Responses of a ganglion to light

39. Neural Function of the Retina
Transmission of Signals Depicting Contrasts in the Visual Scene: The Role of Lateral Inhibition
Fig

40. Neural Function of the Retina
Transmission of Color Signals by the Ganglion Cells
Single ganglion may be stimulated by several cones or by only a few
Some cells may be stimulated by one type but inhibited by another
Importance of color contrast mechanisms is that the retina itself begins to differentiate colors