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