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 Photoreceptor cell Rhabdomere Extension of photoreceptor cell that contains visual pigment Lens Photoreceptor axon Light

Vertebrate eye Lens Light Optic 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 Inner Segment Synaptic Terminal

Electrical Properties of Vertebrate Photoreceptors Compared to other neurons, resting Vm is more positive (~ -20mV) With light exposure, Vm hyperpolarizes! Light on rest 0 mV Vm

Why hyperpolarize in response to light? In the dark, –P Na  P K (outer segment) –Vm therefore between E Na and E K In response to light, –P Na is reduced (outer segment), P K > P Na –therefore, Vm  E K, hyperpolarizes

Outer Segment Inner Segment Synaptic Ending Na+ Dark Current Steady release of neurotransmitter Na/K pump

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 (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 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!

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)

Outer Segment Inner Segment Synaptic Ending Na+ Dark Current Steady release of neurotransmitter Na/K pump

PDE Rhodopsin 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 cell To 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