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Ch. 15 Special Senses: Vision Slides mostly Marieb requires ATP. Pigment regeneration: Light absorption by rhodopsin triggers a rapid series of steps in which retinal changes shape (11-cis to all- trans) and eventually releases from opsin. Pigment bleaching: 11-cis-retinal, derived from vitamin A, is combined with opsin to form rhodopsin. Pigment synthesis:1 2H+ 2H+ All-trans- retinal All-trans-retinal Rhodopsin Dark 3 2 11-cis-retinal Vitamin A Oxidation Reduction Opsin and Light 11-cis-retinal O 2013 Pearson Education, Inc. Figure 15.17 Events of phototransduction. Slide 1 Recall from Chapter 3 that G protein signaling mechanisms are like a molecular relay race. Retinal absorbs light and changes shape. Visual pigment activates. Light (1st messenger) Receptor G protein Enzyme2nd messenger Visual pigment 1 Light 11-cis-retinal Transducin (a G protein) All-trans-retinal 23 Visual pigment activates transducin (G protein). Transducin activates phosphodiesteras e (PDE). 4 5 PDE converts cGMP into GMP, causing cGMP levels to fall. As cGMP levels fall, cGMP-gated cation channels close, resulting in hyperpolarization. cGMP-gated cation channel open in dark cGMP-gated cation channel closed in light Phosphodiesterase (PDE) 2013 Pearson Education, Inc. Phototransduction In Cones Similar as process in rods Cones far less sensitive to light Takes higher-intensity light to activate cones 2013 Pearson Education, Inc. Light Transduction Reactions Light-activated rhodopsin activates G protein transducin Transducin activates PDE, which breaks down cyclic GMP (cGMP) In dark, cGMP holds channels of outer segment open Na+ and Ca2+ depolarize cell In light cGMP breaks down, channels close, cell hyperpolarizes Hyperpolarization is signal! 2013 Pearson Education, Inc. Information Processing In The Retina Photoreceptors and bipolar cells only generate graded potentials (EPSPs and IPSPs) When light hyperpolarizes photoreceptor cells Stop releasing inhibitory neurotransmitter glutamate Bipolar cells (no longer inhibited) depolarize, release neurotransmitter onto ganglion cells Ganglion cells generate APs transmitted in optic nerve to brain 2013 Pearson Education, Inc. Figure 15.18 Signal transmission in the retina (1 of 2). Slide 1 In the dark cGMP-gated channels open, allowing cation influx. Photoreceptor depolarizes. 1 Voltage-gated Ca2+ channels open in synaptic terminals. Neurotransmitter is released continuously. Neurotransmitter causes IPSPs in bipolar cell. Hyperpolarization results. Hyperpolarization closes voltage-gated Ca2+ channels, inhibiting neurotransmitter release. No EPSPs occur in ganglion cell. No action potentials occur along the optic nerve. Photoreceptor cell (rod) Bipolar Cell Ganglion cell Ca2+ 40 mV40 mV 2 3 4 5 6 7 Ca2+ Na+ 2013 Pearson Education, Inc. Figure 15.18 Signal transmission in the retina. (2 of 2). Slide 8 70 mV No neurotransmitter is released. Depolarization opens voltage-gated Ca2+ channels; neurotransmitter is released. EPSPs occur in ganglion cell. Action potentials propagate along the optic nerve. cGMP-gated channels close, so cation influx stops. Photoreceptor hyperpolarizes. Lack of IPSPs in bipolar cell results in depolarization. Voltage-gated Ca2+ channels close in synaptic terminals. 1 Photoreceptor cell (rod) Bipolar Cell Ganglion cell In the light Light Ca2+ 70 mV 2 3 4 5 6 7 Below, we look at a tiny column of retina. The outer segment of the rod, closest to the back of the eye and farthest from the incoming light, is at the top. Light 2013 Pearson Education, Inc. Visual Pathway To The Brain Axons of retinal ganglion cells form optic nerve Half of the fibers (medial half) of each optic nerve cross over at optic chiasm; optic tracts exit Most optic tract fibers go to lateral geniculate nucleus of thalamus Fibers from thalamic (LGN) neurons form optic radiation and project to primary visual cortex in occipital lobes Other optic tract fibers go to superior colliculi in midbrain (initiating visual reflexes) A few ganglion cells contain melanopsin and project to other brain areas Regulate pupil diameter, daily rhythms 2013 Pearson Education, Inc. Figure 15.19 Visual pathway to the brain and visual fields, inferior view. Right eye only Both eyes Fixation point Right eye Supra- chiasmatic nucleus Pretectal nucleus Lateral geniculate nucleus of thalamus Superior colliculus The visual fields of the two eyes overlap considerably. Note that fibers from the lateral portion of each retinal field do not cross at the optic chiasma. Occipital lobe (primary visual cortex) Left eye Left eye only Optic nerve Optic chiasma Optic tract Lateral geniculate nucleus Superior colliculus (sectioned) Uncrossed (ipsilateral) fiber Crossed (contralateral) fiber Optic radiation Corpus callosum Photograph of human brain, with the right side dissected to reveal internal structures. 2013 Pearson Education, Inc. Depth Perception Eyes see world from slightly different angles Depth perception (three-dimensional vision) results from detection of the small differences between R & L eye images Requires input from both eyes 2013 Pearson Education, Inc. Visual Processing Retinal cells Color, brightness, edge detection (by amacrine and horizontal cells) Lateral geniculate nuclei of thalamus Process for depth perception, cone input emphasized, contrast sharpened Primary visual cortex (striate cortex) Neurons detect edges, object orientation, movement Provide form, color, motion inputs to visual association areas (prestriate cortex) 2013 Pearson Education, Inc. Cortical Proces