Transcript Lect14
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