Details about phototransduction along with biochemical changes are explained.

1. PHYSIOLOGY OF RETINA

2. Retina Structure
• Light sensitive layer is made of photo-
receptors: rods (120 millions) and cones (7
millions) which absorb the light.
• Plexiform Layer: nerve cells that process the
signals generated by rods and cones and
relay them to the optical nerve.
• Choroid: carries mayor blood vessels to
nourish the retina and absorb the light so
that it will not be reflected back (dark pupil!)

3. Rods and Cones
• Covers an area of 5 cm2. A baseball a mile
away gives an image covering one cone.
• Cones: for more precise vision, need strong
light. help to see colors. Mostly distributed
in the center of the retina (fovea).
• Rods: for peripheral and night vision.
Sensitive to light. Mostly distributed away
from fovea.

5. Sensitivity
• Cones: slow, fine grain, like color film.
– Need high level of light (photopic condition, day)
– High density, high resolution.
• Rods: fast, coarse grain, black & white film
– Low level of light (scotopic condition, at night)
No color is obvious.
• Adaptation: Changing of retina sensitivity.

6. Singal Processing
• Trace the signal through
the retina:
• The retina is a seven-
layered structure involved
in signal transduction.
– Light enters from the GCL
side first, and must
penetrate all cell types
before reaching the rods
and cones.
– The outer segments of the
rods and cones transduce
the light and send the
signal through the cell
bodies of the ONL and out
to their axons.
OPL
Horizontal cells
DENDRITES OF
BIPOLAR CELLS
OUTER SEGMENTS OF RODS & CONES
Signal through the
cell bodies ONL
LIGHT
GCL Penetrate all cells to
reach rods & cones

7. • In the OPL photoreceptor axons contact the
dendrites of bipolar cells and horizontal
cells.
• Horizontal cells are interneurons which aid
in signal processing
• The bipolar cells in the INL process input
from photoreceptors and horizontal cells,
and transmit the signal to their axons.

8. • In the IPL, bipolar
axons contact
ganglion cell
dendrites and
amacrine cells,
another class of
interneurons.
• The ganglion cells of
the GCL send their
axons through the
OFL to the optic disk
to make up the optic
nerve.
• They travel all the way
to the lateral
geniculate nucleus.
OPTIC NERVE
OPTIC CHIASMA LGB
GCL
OFL Optic disc and
optic nerve
IPL
Bipolar axons contact
GANGLION CELL
DENDRITES &
AMACRINE CELLS

9. Fovea
• The fovea defines the center of the retina, and
is the region of highest visual acuity. The fovea
is directed towards whatever object you wish
to study most closely.
• In the fovea there are almost exclusively
cones, and they are at their highest density.

10. Processing Time
• Latency: it takes a bit time for the cells in
retina to respond to a flash of light.
• Persistence of response: the response does
not stop at the instant the flash stops.
– 1/25 second at low intensity, 1/50 second at high
intensity.
– The persistence allows as to see moving things
clearly.

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

12. 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
Phototransduction cascade

13. • 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

14. In the Dark
Steady release
of
neurotransmitt
er
Inhibitory synapse
Hyperpolarized
With Light
Neurotransmitte
r release is
reduced
Inhibition is relieved
Depolarizes
Bipolar cell
Ganglion cell
To Optic Nerve 
Excitatory synapse
 transmitter
release
 transmitter release
Photoreceptor
APs APs
Depolarized Hyperpolarized

15. ERG
• The clinical ERG is the recording of
the electrical potential waveform
generated by the total (pre-
ganglionic) retina in response to a
diffuse light stimulus.
• Performed in dark adaptation
• Reference electrode is attached to
forehead

16. • Negative ‘a wave’ – activities of
rods & cones.
• Positive (composite)‘b-wave’ –
from Muller cells in the bipolar
region(inner retinal layers).
• c-wave – retinal metabolism
(RPE).
• Peak amplitudes and latencies as
well as waveform shape are
considered in the interpretation
of the ERG.
• Monitors preganglionic retinal
activity so optic atrophy – N ERG.
• ERG - mass retinal response; an
isolated lesion of the macula
would not be expected to affect
this test as it contains only 7% of
total retinal cone population.

17. The Multifocal ERG
• Produces topographical maps of retinal
function
• Stimulus is scaled for variation in
photoreceptor density across the retina; at
fovea where receptor density is high
smaller stimulus element is used than in
periphery
• The information can be summarised in
form of a 3-D plot, resembling hill of vision
• Use : Any disorder that affects retinal
function.

18. CLINICAL APPLICATIONS
1. Vitamin A deficiency & xerosis
2. Retinitis pigmentosa & allied diseases e.g.
(a) Congenital Night Blindness
(b) Oguchi's Disease
(c) Retinitis punctate albicans
3. Prognosis in Cataract
4. Prognosis in Glaucoma
5. Detachment of retina
6. Systemic & retinal vascular conditions
7. Macular diseases
8. Malingering