Photoreceptors and Phototransduction PDF

Neurophysiology PH5208 Photoreceptors and Phototransduction Dr. Moira Jenkins Vision • Light that enters each eye will stimulate photoreceptors arrayed along the surface of the retina, which transduce photoenergy into the nervous signals that give rise to our sense of “seeing things” • “Phototran...

Neurophysiology PH5208 Photoreceptors and Phototransduction Dr. Moira Jenkins Vision • Light that enters each eye will stimulate photoreceptors arrayed along the surface of the retina, which transduce photoenergy into the nervous signals that give rise to our sense of “seeing things” • “Phototransduction” is the conversion of light energy that strikes the surface of the retina into nervous signals for transmission into the brain How the Eye Works retina cornea pupil lens Light passes through cornea, pupil, and lens Image projected on the retina (upside-down!) because light rays cross Visual Space- 2 Eyes Visual Fields We see all the objects within our visual space that either emit or reflect light that enters into our eyes, 180’ arc of visual space. Projected onto the retina (retinal fields) Visual Space Binocular and Monocular Regions • Each eye “sees” is the light that enters that eye from a certain span of visual space – that span of visual space that is “seen” by each eye encompasses its “visual field” • Overlap between the right and left visual fields → binocular region of visual space – objects within the binocular region of visual space, are seen by both eyes, 60” from midline • the remaining “outer” 30 comprise the monocular regions of the right and left hemifields – objects within these regions are seen by only one eye Neuroscience Fig. 12.3 Image on the Retina The image projected onto the retina is inverted and reversed Retinotopic map The position of objects across the arc of visual space are preserved in their projection on the retina “retinotopic mapping” Knowing how each pixel maps to location in visual space enables the brain to determine the correct location of each object whose image is projected upon the retina This is preserved in it’s relay to the cortex Image on the Retina The cornea and lens refract light entering each eye to form a “focused image” of each object within visual space upon the surface of the retina This focal point is at the fovea Myopia/nearsighted- image in front of retina Hyperopia/farsighted- image behind retina Visual Neural Pathway Specific visual field deficits arise from lesions at specific locations along the optic anatomy pathway The right hemisphere of the visual cortex receives input stimulated by light sources from the left hemifield of visual space, and vice versa Retinal Cell Columns • The cells of the retina are arranged in columns, creates microcirciuts • Vision (our capacity to perceive shapes and objects by sight) is not based upon detection of the presence versus the absence of light • The hardwired organization of the cells and receptive fields in the retinal microcircuits create the ability to detect contrasts in illumination and color • Arrangement also allows amplification of the stimulus (NOT faithful transmission) Kandel & Schwartz Fig. 25-4 [Pintos by Bev Doolittle] Each microcircuit exhibits a distinct pattern of synaptic connectivity. The retina is a tremendously complex image processor, containing numerous cell types that form microcircuits encoding different aspects of the visual scene. “The eye speaks to the brain in a language already highly organized and interpreted, instead of transmitting some more or less accurate copy of the distribution of light on the receptors” – Lettvin et al. in What the frog’s eye tells the frog’s brain, Chapter 7 in The Mind: Biological Approaches to its Functions (1968, reprinted from 1959) Specific cell types are localized to specific layers-highly organized structure to the retina • pigment epithelium • photoreceptor layer • neural network layer bipolar cells horizontal cells amacrine cells • ganglion cell layer see also: Rhoades & Bell Fig. 4.11 Kandel & Schwartz Fig. 26-2 Cells of the Retina The cellular structure of the retina presents a columnar arrangement: photoreceptors → bipolar cell → ganglion cell photoreceptors: rods & cones • transduce light energy into a neurochemical signal that is transmitted to a bipolar cell via release of a neurotransmitter, GLUTAMATE bipolar cells: • serve as a relay for signaling from photoreceptors to a ganglion cell • bipolar cells release neurotransmitter to a ganglion cell in response to signaling from the photoreceptors horizontal and amacrine cells: modulate bipolar ganglion cells: Neuroscience Fig. 11-5 • function as “projection neurons” that generate nervous signals (trains of action potentials) for transmission into the brain via their individual nerve fibers which are carried in the optic nerve … Cells of the Retina- Muller cells Similar to glial cells • Phagocytize • Provide nutrition • Provide structure • Direct light to cones http://www.the-scientist.com/?articles.view/articleNo/40996/title/Guiding-Light/ How does the retinal cell column work? photoreceptors transduce the light energy … but ganglion cells generate the nervous signaling that is sent into the brain … • signaling from bipolar cells to ganglion cells occurs in response to signals (neurotransmitter) received from photoreceptors → photoreceptors therefore “control” bipolar cell stimulation of the ganglion cells • ganglion cells are responsively stimulated by the signaling (neurotransmitter) they receive from bipolar cells → the bipolar cell therefore “controls” the nervous signaling sent to the brain from ganglion cells Column -Vertical flow of information photoreceptors: • release neurotransmitter (glutamate) to bipolar cells → the release of transmitter varies inverse to the intensity of illumination bipolar cells: • serve as an intermediate relay (like an interneuron) between photoreceptors and ganglion cells → their postsynaptic response to transmitter released from the photoreceptor may be either an EPSP (OFF-type) or an IPSP (ON-type) → 2 types of bipolar cells OFF bipolar cell, ON bipolar cell, act opposite ganglion cells: • generate nervous signaling (trains of action potentials) for transmission from the retina to the thalamus via the optic nerve → their signaling is controlled by the transmitter released from the bipolar cell – the postsynaptic response of a ganglion cell to glutamate released from bipolar cells is always excitatory (EPSP) Cells of the Retina-Photoreceptors • Photons, the elementary particles of light, are converted into electrical signals • This is called phototransduction • Occurs in photoreceptors- rods and cones • Outer segment in pigmented epithelial cells • Epithelial cells absorb scatter light • 125 million photoreceptors/eye • Photon stimulates a pigment within the Rod or Cone ( 3 different types of cones, different pigments) • Involves many steps→ Phototransduction cascade Retina- Distribution of Photoreceptors • More rods than cones in the retina • However, at the fovea-all cones Fovea Photoreceptors- Rods • SCOTOTOPIC Vision (night vision) • Rods sense low levels of light (dim, dark light), they are monochromatic (no color) • Different shades of gray • Poor acuity • Rhodopsin pigment Photoreceptors- Cones • • • • • • • • PHOTOTOPIC Vision (color vision) Cones sense COLOR, chromatic, bright light, visual acuity Photopsin/opsin pigment (3 different pigments, each cone has one type) 3 different types which are stimulated/excited by light at 3 different wavelengths Red (L) – receptive to long wavelength Green (M)-receptive to medium wavelength Blue (S)-receptive to short wavelength Every color perceived is combination, trichromatic Color-Wavelengths that can be perceived • Visible light is electromagnetic radiation within a certain band of wavelengths, consisting of streams of photons Phototransduction: photoreceptors sense and respond to both the intensity and the wavelength of light energy Rods Cones High sensitivity, specialized for night vision Lower sensitivity; specialized for daylight vision More photopigment per cell than cones; captures more light Less photopigment per cell than rods High amplification; single photon detection Less amplification per cell than rods Low temporal resolution; slow response, long integration time High temporal resolution; fast response, short integration time More sensitive to scattered light Most sensitive to direct axial stimulation Saturate in daylight Saturate only in intense light Low acuity; highly convergent pathways; absent in central fovea region High acuity; less convergence of retinal pathways, especially in fovea Achromatic: only one type of photopigment in rods Chromatic: 3 types of cones, each with a pigment sensitive to a different part of the visible spectrum of light Phototransduction Cascade • Photoreceptors have a membrane potential that is depolarized in the dark • Due to open sodium ion channels • cGMP opens the channels • Nonstimulated (dark), cGMP is high in the cytosol) • Neurotransmitter is released, glutamate Photons- stimulate pigments in photoreceptors • • • • Rods have rhodopsin pigment in stacked discs of the outer membrane Rhodopsin consists of opsin (7-transmembrane protein) and retinal (derivative of Vitamin A) Photon stimulation of retinal causes a confirmational change from cis form to all trans form The rhodopsin pigment changes from purple to yellow, ka: bleaching Phototransduction Cascade • Rhodospin is similar to metabotropic G-protein • Change in retinal form causes a confirmational change in opsin →activated form meta rhodopsin • This activates G-protein Transducin • Transducin activates enzyme in the membrane, PDE phosphodiesterase • PDE hydrolyzes cGMP-→ 5’GMP • Reduction in cGMP→ Na ion channels close, inhibits/hyperpolarizes the rod cell • Less/no release of neurotransmitter to Bipolar cell cGMP regulates sodium channels in the photoreceptor membrane cGMP interacts with Na+ channels, causing those channels to remain open Neuroscience Fig. 11-9 consequently … → in low intensity light (“dark”), cGMP is plentiful and Na+ channels are open → in high intensity light (“bright light”), cGMP is reduced and Na+ channels close Phototransduction Cascade The membrane potential of photoreceptors is continually determined by the prevailing intensity of light in the dark (no light): • the membrane is “depolarized” to approximately -40 mV due to the “dark current” Na+ entry through open channels the membrane in bright light: • the membrane is “hyperpolarized” to approximately -65 mV due to the loss of Na+ conductance caused by closure of the channels → under varying light conditions: Rhoades & Bell Fig. 4.13 • with decreasing illumination, the photoreceptor membrane will depolarize, more NT • with increasing illumination, the photoreceptor membrane will hyperpolarize, less NT • What happens next? → Bipolar cells