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 ommatidiumeach with it own lens
Vertebrate eye• single lens gathers lightand focuses it on many receptor cells
Single Ommatidium
Photoreceptor cell
RhabdomereExtension of photoreceptor cellthat contains visual pigment
Lens
Photoreceptor axon
Light
Vertebrate eye
Lens
LightOptic Nerve
The retina
• Has two types of photoreceptors
1. Rod– High sensitivity– Low resolution– Black & White vision
2. Cones– High resolution– Lower sensitivity– Colour vision
Rod Cone
Outer Segment
InnerSegment
SynapticTerminal
Electrical Properties of Vertebrate Photoreceptors
• Compared to other neurons, resting Vm is more positive (~ -20mV)
• With light exposure, Vm hyperpolarizes!
Light on
rest0 mV
Vm
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
OuterSegment
InnerSegment
SynapticEnding
Na+
Dark Current
Steady release of neurotransmitter
Na/K pump
• Visual Pigments membrane folds of the outer segment
Eg. Cone Outer SegmentMembrane folds:• called disks• contain pigment• Increase surface area
Visual Pigment molecules
• called Rhodopsin• 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
‘Light’
Colour Blindness
Everyone should see number 12 ‘normal’ see number 8Red-green deficiency see number 3Total colour blind see no number
Colour blindness effects ~7-10% males, <1% females
Colour vision – not so simple!
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)
OuterSegment
InnerSegment
SynapticEnding
Na+
Dark Current
Steady release of neurotransmitter
Na/K pump
PDERhodopsin
Transducin(G-Protein) Dark Current
Channel
Light
Plasma membrane
Disk Membrane
Na+
cGMP
GMP
phosphodiesterase
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
• 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
In the Dark
Steady release of of neurotransmitter
Inhibitory synapse
Hyperpolarized
With Light
Neurotransmitter release is reduced
Inhibition is relieved
Depolarizes
Bipolar cell
Ganglion cellTo Optic Nerve
Excitatory synapse transmitter release transmitter release
Photoreceptor
APs APs
Depolarized Hyperpolarized
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
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