VISUAL TRANSDUCTION.pptx

VISUAL TRANSDUCTION  Overview of Questions How do our eyes respond to light? Why do our eyes have two different sets of receptors – rods and cones? Fig.1 The electromagnetic spectrum, showing the wide range of energy in the environment and the small range within this spectrum, called visib...

VISUAL TRANSDUCTION  Overview of Questions How do our eyes respond to light? Why do our eyes have two different sets of receptors – rods and cones? Fig.1 The electromagnetic spectrum, showing the wide range of energy in the environment and the small range within this spectrum, called visible light, that we can see. Light is the Stimulus for Vision Electromagnetic spectrum – Energy is described by wavelength. – Spectrum ranges from short wavelength gamma rays to long wavelength radio waves. – Visible spectrum for humans ranges from 400 to 700 nanometers. – Most perceived light is reflected light of surfaces. Fig.2 An image of the cup is focused on the retina, which lines the back of the eye. The close-up of the retina on the right shows the receptors and other neurons that make up the retina.  Focusing Images on the Retina The cornea, which is fixed, accounts for about 80% of focusing. The lens, which adjusts shape for object distance, accounts for the other 20%. – Accommodation results when ciliary muscles are tightened which causes the lens to thicken. Light rays pass through the lens more sharply and focus near objects on retina.  Eye Structures Fig. 3 Cross section of the vertebrate eye Note how an object in the visual field produces an inverted image on the retina.  Light goes through other layers of neurons before being transduced at the rod and cone photoreceptors Photoreceptors send the light information to the bipolar cells, which send it to the ganglion cells, and the information exits the retina to the brain  Blind Spot Blind spot - place where optic nerve leaves the eye – We don’t see it because: one eye covers the blind spot of the other. it is located at edge of the visual field. the brain “fills in” the spot. Fig.4 There are no receptors at the place where the optic nerve leaves the eye. This enables the receptor’s ganglion cell fibers to flow into the optic nerve. The absence of receptors in this area creates the blind spot.  Visual Coding and Retinal Receptors The Eye and Its Connections to the Brain Pupil-opening in the center of the eye that allows light to pass through Lens-focuses the light on the retina Retina-back surface of the eye that contains the photoreceptors The Fovea-point of central focus on the retina  Diseases that Affect the Retina Macular degeneration – Fovea and small surrounding area are destroyed – Creates a “blind spot” on retina – Most common in older individuals – “Wet” causes vision loss due to abnormal blood vessel growth – “Dry” results from atrophy of the retinal pigment, which causes vision loss through loss of photoreceptors  Awareness. Most stimuli that are received, transduced and coded are then perceived. E.g, when smelling a flower, scent molecules strike olfactory receptors in the nose (reception). This produces a chemical reaction that depolarizes the resting potentials of the olfactory receptors, they fire (transduction) and this information is passed via the olfactory nerve to the olfactory bulb  The Visual System. Light enters the eye through an opening in the centre of the iris called the pupil. The light is focused by the cornea and lens and projected onto the retina - the light sensitive cells that line the rear of the eyeball. Light from the top left of the visual scene strikes the bottom right of the retina and vice versa so the visual image on the retina is upside down and reversed. The centre of the retina is called the macula and this is the most sensitive part of the retina used for resolving fine detail. The most precise region of visual analysis  Photoreceptors A photoreceptor cell is a specialized type of neuron found in the retina that is capable of photo transduction. they convert light (visible electromagnetic radiation) into signals that can stimulate biological processes.there are two types of Rods are highly sensitive to light, and thus are good for dim light or night vision. They are able to capture more light, but they do not respond well to moving stimuli because their response time is slow. Cones are not as sensitive to dim light, and are able to respond quickly, and therefore transduce moving stimuli. They are used more in daytime vision, and also are able to detect color because three classes of cones express three different How much light does it take? Prior physical evidence showed that it takes one photon to isomerize a pigment molecule. Purpose of Hecht experiment - to determine how many pigment molecules need to be isomerized for a person to see Fig. 5 The observer in Hecht et al.’s (1942) experiment could see a spot of light containing 100 photons. Of these, 50 photons reached the retina, and 7 photons were absorbed by visual pigment molecules.  Experiment by Hecht Results showed: –a person can see a light if seven rod receptors are activated simultaneously. –a rod receptor can be activated by the isomerization of just one visual pigment molecule.  Rods and Cones Differences between rods and cones ◦ Shape  Rods - large and cylindrical  Cones - small and tapered ◦ Distribution on retina  Fovea consists solely of cones.  Peripheral retina has both rods and cones. ◦ More rods than cones in periphery. ◦ Number - about 120 million rods and 5 million cones  How do our eyes convert light into neural signal? Transduction First take a look at receptors we find in the retina.  Visual Pigment Regeneration Process needed for transduction: – Retinal molecule changes shape – Opsin molecule separates – The retina shows pigment bleaching. – Retinal and opsin must recombine to respond to light. – Cone pigment regenerates in six minutes. – Rod pigment takes over 30 minutes to regenerate. Rods and Cones Fig. 6 Fig. 7 (a) Rod receptor showing discs in the outer segment. (b) Close-up of one disc showing one visual pigment molecule in the membrane. (c) Close-up showing how the protein opsin in one visual pigment molecule crosses the disc membrane seven times. The light-sensitive retinal molecule is attached to the opsin at the place indicated. Each disk contains:- 1- photopigment (rhodopsin) 2- G protein transducin 3-CGMP phosphodiesterase enzyme  Transduction of Light into Nerve Impulses Rod Receptor have outer segments, which contain: – Visual pigment molecules, which have two components: Opsin - a large protein Retinal - a light sensitive molecule Visual transduction occurs when the retinal absorbs one photon. – Retinal changes it shape, called isomerization. Fig. 8 Model of a visual pigment molecule. The horizontal part of the model shows a tiny portion of the huge opsin molecule near where the retinal is attached. The smaller molecule on top of the opsin is the light-sensitive retinal. The model on the left shows the retinal molecule’s shape before it absorbs light. The model on the right shows the retinal molecule’s shape after it absorbs light. This change in shape is one of the steps that results in the generation of an electrical response in the receptor.  Transduction in Rods Absorption of light causes rhodopsin to break into opsin and retinal. Enzymes break down cyclic GMP. Fewer sodium channels remain open. Cell hyperpolarizes in light (it’s depolarized in the dark). Photoreceptors produce graded potentials.  Visual Transduction: The Conversion of Physical Energy to Neural Energy Photopigments are located in the membrane of the outer segment of rods and cones Each pigment consists of an opsin (a protein) and retinal (vitamin A derivative) In the dark, membrane NA+/CA++ channels are open -> glutamate is released which depolarizes the membrane (K+ channels are also open: net effect results in depolarization) Light splits the opsin and retinal apart-> Activates transducin (G protein)-> Activates phosphodiesterase-> Reduces cGMP -> closes NA+/CA++ channels (K+ remain open) The net effect of light is to hyperpolarize the retinal Fig. 9 Phototransduction in rod photoreceptors Fig. 10 Rhodopsin (from rods) is a combination of retinal, the light-absorbing molecule, and a large protein called opsin. The opsins are closely related in structure to the G-protein- coupled Fig. 11 (A)  Phototransduction - dark current AMPLIFICATION! One rhodopsin molecule absorbs one photon 500 transducin molecules are activated 500 phosphodiesterase molecules are activated 105 cyclic GMP molecules are hydrolyzed 250 cation channels close 106-107 Na+ ions per second are prevented from entering the cell for a  Phototransduction - dark currentLight hyperpolarizes the photoreceptor With light stimulation, the cGMP channels are closed. Pathway: light rhodopsin rhodopsin transducin transducin phosphodiesterase phosphodiesterase cGMP cGMP cGMP-gated channel  Mechanisms of Receptor Regulation D D D D P P P P P P Arresti Arresti (2) Phosphorylation n n α β Clathri (3) Arrestin (4) Clustering in α α γ n binding clathrin-coated (7) Recycling pits (1) Agonist binding and G protein (5) Endocytosis activation Endosomes D P P (6) Dissociation of agonist: Arrestin (8) Traffic to Dephosphorylation lysosomes Sorting between cycling Lysosomes and lysosomal pathways  Light Transduction DARK LIGHT trans-retinal transformed to cis- cis-retinal transformed to trans-retina retinal trans-retinal and opsin dissociate now active opsin activates transducin cis-retinal and opsin form rhodopsin rhodopsin activates guanylate transducin activates PDE cyclase (GC) PDE breaks down cGMP to 5’-GMP GC increases the synthesis of cGMP 5’GMP closes Na+ channels cGMP opens Na+ channels rod cell hyperpolarizes rod cell depolarizes reduces the release of glutamate  Rhodopsin Cascade Rhodopsin molecule LIGHT Rod cell disc Inside Rod cell Outside 1 photon of light can block the entry of 1,000,000 Na + ions