Midterm Marks posted by next Monday
Today - Vision
• Structure / anatomy of eyes
• Photochemistry of pigment molecules
• Transduction of light energy to electrical
signals
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
Compound Eye
• Many units called ommatidium
each with it own lens
Vertebrate eye
• single lens gathers light
and focuses it on many
receptor cells
Single Ommatidium
Light
Lens
Photoreceptor cell
Rhabdomere
Extension of photoreceptor cell
that contains visual pigment
Photoreceptor axon
Vertebrate eye
Lens
Optic Nerve
Light
The retina
Rod
•
Has two types of
photoreceptors
Cone
Outer Segment
1. Rod
–
–
–
High sensitivity
Low resolution
Black & White vision
Inner
Segment
2. Cones
–
–
–
High resolution
Lower sensitivity
Colour vision
Synaptic
Terminal
Electrical Properties of Vertebrate
Photoreceptors
• Compared to other neurons, resting Vm is
more positive (~ -20mV)
• With light exposure, Vm hyperpolarizes!
Vm
0 mV
rest
Light on
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
Outer
Segment
Na+
Inner
Segment
Synaptic
Ending
Dark Current
Na/K pump
Steady release of neurotransmitter
• Visual Pigments membrane folds of the
outer segment
Eg. Cone Outer Segment
Membrane folds:
• called disks
• contain pigment
• Increase surface area
Visual Pigment molecules
• called Rhodopsin
• Retinal (Vitamin A derivative) + Opsin (Gprotein 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
‘Light’
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
Colour vision – not so simple!
Photochemistry of Pigment molecules
Activated form
Rhodopsin
Light
Opsin + 11-cis-Retinal
Opsin
all-trans-Retinal
all-trans-Retinol
(Vitamin A)
isomerase
11-cis-Retinal
Light converts 11-cis-Retinal to all-trans-Retinal
Outer
Segment
Na+
Inner
Segment
Synaptic
Ending
Dark Current
Na/K pump
Steady release of neurotransmitter
Light
phosphodiesterase
Disk Membrane
PDE
Rhodopsin
Transducin
(G-Protein)
GMP
cGMP
Plasma membrane
Na+
Dark Current
Channel
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
• 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
Photoreceptor
With Light
In the Dark
Depolarized
Hyperpolarized
Steady release of
of
neurotransmitter
Inhibitory synapse
Hyperpolarized
Bipolar cell
 transmitter release
Neurotransmitter
release is reduced
Inhibition is relieved
Depolarizes
 transmitter release
Excitatory synapse
APs
Ganglion cell
APs
To Optic Nerve 
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