1. The retina contains light-sensitive rods and cones that detect light and transmit signals through neurons to the brain. 2. Light exposure causes the photopigment rhodopsin in rods to decompose, hyperpolarizing the rods and decreasing the "dark current" of sodium ions flowing into them. 3. The fovea contains only cones, maximizing visual acuity, while the peripheral retina contains more rods for low-light vision.

1. Receptor and Neural Function Of the Retina
Dr Mohamed Hassan

2. Retina
• light sensitive portion of the eye.
• contains cones for color vision.
• contains rods for night vision.
• contains neural architecture.
• light must pass through the neural elements to strike
the light sensitive rods and cones.

4. The Fovea
• a small area at the center of the retina about 1 sq millimeter.
• the center of this area “the central fovea” contains only cones.
• these cones have a special structure.
• aid in detecting detail.
• in the central fovea the neuronal cells and blood vessels are displaced
to each side so that the light can strike the cones directly.
• this is the area of greatest visual acuity.

6. Rods and Cones
• high sensitivity; specialized
for night vision
• more photopigment
• high amplification; single
photon detection
• slow response, long
integration time
• more sensitive to scattered
light
• lower sensitivity;
specialized for day
vision
• less photopigment
• less amplification
• fast response, short
integration time
• more sensitive to direct
axial rays

7. • low acuity; highly
convergent retinal pathways,
not present in central fovea
• achromatic; one type of rod
pigment
• high acuity; less convergent
retinal pathways,
concentrated in central fovea
• chromatic; three types of
cones, each with a different
pigment that is sensitive to a
different part of the visible
spectrum

8. Structure of the Rods and Cones

9. Pigment Layer of Retina
• pigment layer of the retina is very important.
• contains the black pigment melanin.
• prevents light reflection in the globe of the eye.
• without the pigment there would be diffuse scattering of light
rather than the normal contrast between dark and light.
• this is what happens in albinos.
– poor visual acuity because of the scattering of light.

10. Photochemistry of Vision
• rods and cones contain chemicals that decompose on exposure to light.
• this excites the nerve fibers leading from the eye.
• the membranes of the outer-segment of the rods contain rhodopsin or
visual purple.
• rhodopsin is a combination of a protein called scotopsin and a pigment,
retinal.
• the retinal is in the cis configuration.
• only the cis configuration can bind with scotopsin to form rhodopsin.

11. Light and Rhodopsin
• when light is absorbed by rhodopsin it immediately begins to
decompose.
• decomposition is the result of photoactivation of electrons in the
retinal portion of rhodopsin which leads to a change from the cis
form of the retinal to the trans form of the molecule.
– trans retinal has the same chemical structure but is a straight molecule
rather than an angulated molecule.
– this configuration does not fit with the binding site on the scotopsin and
the retinal begins to split away.
– in the process of splitting away a number of intermediary compound are
formed.

14. Mechanism for Light to Decrease Sodium Conductance
• cGMP is responsible for keeping Na+ channel in the outer segment of the rods open.
• light activated rhodopsin (metarhodopsin II) activates a G-protein, transducin.
• transducin activates cGMP phosphodiesterase which destroys cGMP.
• Rhodopsin kinase deactivates the activated rhodopsin (which began the cascade) and
cGMP is regenerated re-opening the Na+ channels.

15. Role of Vitamin A
• vitamin A is the precursor of all-trans-retinal, the
pigment portion of rhodopsin.
• lack of vitamin A causes a decrease in retinal.
• this results in a decreased production of rhodopsin and
a lower sensitivity of the retina to light or night
blindness.

16. The Rod Receptor Potential
• normally about -40 mV.
• normally the outer segment of the rod is very
permeable to Na+ ions.
• in the dark an inward current (the dark current)
carried by the Na+ ions flows into the outer
segment of the rod.
• the current flows out of the cell, through the efflux
of K+, ions in the inner segment of the rod.

17. In the dark an inward current (the dark current) carried by the
Na+ ions flows into the outer segment of the rod.

18. Rod Receptor Potential (Cont.)
• when rhodopsin decomposes it causes a hyperpolarization of
the rod by decreasing Na+ permeability of the outer segment.
• the Na+ pump in the inner segment keeps pumping Na+ out of
the cell causing the membrane potential to become more
negative (hyperpolarization).
• the greater the amount of light the greater the electronegativity.

19. The Dark Current
When rhodopsin decomposes in response to light it causes a
hyperpolarization of the rod by decreasing Na+ permeability of
the outer segment.

20. Mechanism for Light to Decrease Sodium Conductance
1. Light activated
rhodopsin activates
transducin
2. Transducin activates
cGMP
phosphodiesterase
which destroys cGMP
3. cGMP levels decrease
and sodium channels
close
4. The photoreceptors
hyperpolarize

21. Dark and Light Adaptation
• in light conditions most of the rhodopsin has been reduced to retinal so
the level of photosensitive chemicals is low.
• in dark conditions retinal is converted back to rhodopsin.
• therefore, the sensitivity of the retinal automatically adjusts to the light
level.
• opening and closing of the pupil also contributes to adaptation because it
can adjust the amount entering the eye.

22. Color Vision
• color vision is the result of activation of cones.
• 3 types of cones.
– blue cone.
– green cone.
– red cone.
• the pigment portion of the photosensitive molecule is the same as in the
rods, the protein portion is different for the pigment molecule in each of the
cones.
• makes each cone receptive to a particular wavelength of light.

24. Color Blindness
• lack of a particular type of cone.
• genetic disorder passed along on the X chromosome.
• occurs almost exclusively in males.
• about 8% of women are color blindness carriers.
• most color blindness results from lack of the red or green cones.
• lack of a red cone, protanope.
• lack of a green cone, deuteranope.

27. Neural Organization of the Retina
Direction of
light

28. Signal Transmission in the Retina
• transmission of signals in the retina is by electrotonic
conduction.
• allows graded response proportional to light intensity.
• the only cells that have action potentials are ganglion cells.
–send signals all the way to the brain.

29. Lateral Inhibition to Enhance Visual Contrast
• horizontal cells connect laterally between the rods and
cones and the bipolar cells.
• output of horizontal cells is always inhibitory.
• prevents the lateral spread of light excitation on the retina.
• have an excitatory center and an inhibitory surround.
• essential for transmitting contrast borders in the visual
image.

30. Lateral inhibition, the
function of horizontal
cells

31. Rods, Cones and Ganglion Cells
• each retina has 100 million rods and 3 million cones and 1.6 million
ganglion cells.
• 60 rods and 2 cones for each ganglion cell.
• at the central fovea there are no rods and the ratio of cones to
ganglion cells is 1:1.
• may explain the high degree of visual acuity in the central retina.

32. Three Types of Ganglion Cells
• W cells (40 %) receive most of their excitation from rod cells.
• sensitive to direction movement in the visual field.
• X cells (55%) small receptive field, discrete retinal locations, may be responsible
for the transmission of the visual image itself, always receives input from at least
one cone, may be responsible for color transmission.
• Y cells (5%) large receptive field respond to instantaneous changes in the visual
field.

33. Excitation of Ganglion Cells
• spontaneously active with continuous action
potentials.
• visual signals are superimposed on this background.
• many excited by changes in light intensity.
• respond to contrast borders, this is the way the pattern
of the scene is transmitted to the brain.

34. Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
Visual Pathways to the Brain
• optic nerve.
– axons of ganglion cells of the retina.
• optic chiasm.
– all fibers from the nasal halves of the retina cross to the
opposite side and join fibers from the opposite temporal
retina to form the optic tracks.
• synapse in the dorsal lateral geniculate nucleus
(LGN).
• from LGN to primary visual cortex by way of the
optic radiation.

35. Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
Visual pathways to the brain
Figure 51-1

36. Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
Retinal Projections to Subcortical Regions
• suprachiasmatic nucleus of the hypothalamus.
– control of circadian rhythms ??
• pretectal nuclei.
– pupillary light reflex.
– accommodation of the lens.
• superior colliculus
– rapid directional movement of both eyes.
• ventral lateral geniculate
– control of bodies behavioral functions ??

37. Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
Function of the Dorsal Lateral Geniculate
• Two principle functions.
– relay of information to primary visual cortex.
– “gate control” of information to primary visual cortex.

38. Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
Lateral Geniculate Nucleus
Layers 2, 3, and 5 receive input from
ipsilateral eye
Layers 1, 4, and 6 receive input from
contralateral eye
Half of the fibers in each optic tract
are derived from one eye, and half
from the other eye.
Signals from the two eyes are kept apart
in the LGN.
Kandel, Schwartz and Jessell
4th edition 2000, McGraw Hill
fig 27-6

39. Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
“Gate” Function of the LGN
• LGN receives input from corticofugal fibers
originating in the primary visual cortex.
• input from reticular areas of the midbrain.
• both inputs are inhibitory and can turn off
transmission of the signal in select areas of the
LGN.
• both inhibitory inputs presumably control the
visual input that is allowed to pass to the
cortex.

40. Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
Input from Y type
ganglion cells
Magnocellular pathway
Input from X type
ganglion cells
Parvocellular path
- no color information
- poor point -to- point
transmission because
of limited number of
Y cells.
- transmit color
information.
- accurate
point -to- point
transmission.
Additional Organization
of the LGN
Kandel, Schwartz and Jessell
4th edition 2000, McGraw Hill
fig 26-4

41. Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
Primary Visual Cortex
• located in the occipital lobe in the calcarine
fissure region.
• large representation in visual cortex for the
macula (region for highest visual acuity).
• layered structure like other cortical areas.
• columnar organization as well.
• receives the primary visual input.

42. Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
Location of visual areas in the cortex
Figure 51-2

43. Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
Secondary Visual Areas
• visual association cortex.
• responsible for analyzing the visual
information.
• area for 3 dimensional position, gross form,
and motion.
• area for color analysis.

44. Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
Figure 51-4
Connections in the
Visual Cortex

45. Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
Fields of Vision
• nasal field located on the temporal side of the
retina.
• temporal field located on the nasal side of the
retina.
• interruption of the visual pathway at different
points leads to very specific visual field defects.
• “blind spot” located about 15 degrees lateral to the
central point of vision.
• no rods or cones in this area, called the optic disc,
exit point for axons of the ganglion cells.

46. Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
Medial and lateral
recti move eyes side
to side
Superior and inferior
recti move eyes up
and down
Superior and inferior
obliques rotate the
eyes
Eye movements are controlled by
3 separate pairs of muscles.
Figure 51-7

47. Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
Neural Pathways for Control
of Eye Movement
• fixation movements of the eyes controlled by two
neuronal mechanisms, voluntary and involuntary.
– voluntary fixation movements controlled by an area in
the premotor cortex.
– involuntary fixation mechanism causes eyes to “lock”
on object of attention found with the voluntary fixation
mechanism.
– controlled by secondary visual areas of the occipital
cortex.
– results from negative feedback mechanism controlled at
the level of the superior colliculus that prevents objects
of attention from leaving the foveal portion of the
retina.

48. Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
Neural pathways for control of eye movement
Figure 51-8

49. Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
Saccadic Eye Movements
• when the visual scene is moving (turning the
head), the eyes fix on one highlight after
another in the visual field jumping at a rate of
2 -to- 3 jumps/sec, these jumps are called
saccades, and the movements are called
opticokinetic movements.

50. Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
The Autonomic Nerves to the Eyes
• the eye is innervated by both parasympathetic and
sympathetic neurons.
• parasympathetic fibers arise in the Edinger-
Westphal nucleus, pass in the 3rd cranial nerve to
the ciliary ganglion.
– postganglionic fibers excite the ciliary muscle and
sphincter of the iris.
• sympathetic fibers originate in the
intermediolateral horn cells of the superior
cervical ganglion.
– postganglionic fibers spread along the corotid artery
and eventually innervate the radial fibers of the iris.

51. Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
Autonomic pathways to the eye
Figure 51-11

52. Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
Control of Accommodation
(Focusing the Eyes)
• results from contraction or relaxation of
the ciliary muscle.
• exact mechanism is not known.

53. Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
Control of Pupillary Diameter
• miosis: decreasing of pupillary aperture due to
stimulation of parasympathetic nerves that
excite the pupillary sphincter muscle.
• mydriasis: dilation of pupillary aperture due to
stimulation of sympathetic nerves that excite
the radial fibers of the iris.

54. Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
Pupillary Light Reflex
• when the amount of light entering the eyes
increases, the pupils constrict.
• functions to help the eye adapt extremely
rapidly to changing light conditions.
• light excites fibers going to pretectal nuclei.
• from pretectal nuclei fibers pass to Edinger-
Westphal nucleus and back through
parasympathetic nerves to constrict iris
sphincter.