QUESTIONS Review for Quiz 6 1 What is lateral inhibition What is its function Interconnections between cells of the visual system alter the apparent brightness of visual objects and also enhance boundaries by means of a process known as lateral inhibition The visibility of objects is also enhanced if they are moving at biologically relevant speeds 2 What are the functions of the cornea lens and ciliary muscles The cornea and lens focus light Ciliary muscles adjust the focus by changing the shape of the lens Refraction is the bending of light that is done by the cornea and forms the image 3 What are the bipolar cells and ganglion cells What makes up the optic nerve Bipolar cells receive input from rods cones synapse on ganglion cells whose axons form the optic nerve Visual processing commences with the cells of the retina These cells include the photoreceptors rods and cones as well as bipolar cells and ganglion cells Horizontal and amacrine cells provide lateral connections across the retina The axons of ganglion cells carry information out of the eye to the brain 4 What is the optic chiasm and what happens there Most visual information reaches the brain via the following route optic nerves optic chiasm optic tract LGN optic radiations visual cortex Some retinal fibers also project to the superior colliculus to the hypothalamus especially to the suprachiasmatic nucleus and to the pulvinar in the thalamus Ganglion cell axons form the optic nerve The optic chiasm is where the two optic nerves leaving the eye meet In humans half of the axons from each eye cross to the other side After passing the optic chiasm the axons are called the optic tract 5 Describe the path of light from the cornea to the place where it activates a photoreceptor 6 What are horizontal and amacrine cells Horizontal cells contacts photoreceptors and bipolar cells HPB Amacrine cells contact bipolar and ganglion cells ABG 7 Which cells in the retina generate only graded potentials and which generate action potentials ganglion cells fire action All cell types except ganglion cells generate only graded potentials potentials Bipolar cells process and report light stimulation to ganglion cells Additional processing is performed by horizontal and amacrine cells Ganglion cells produce action potentials along their axons to the brain The information from many photoreceptors converges on each ganglion cell that travel 8 What are the photopic and scotopic systems and when is each active The photopic visual system which relies on cones requires brighter light but is sensitive to wavelength PCBSW The scotopic visual system works well in dim light is relatively insensitive to wavelength and relies on rods SRDIW 9 What is the relationship of RETINAL and opsin in rhodopsin What does light do The transduction of photic energy into neural impulses begins when photons are captured by molecules of photopigment When activated by light rhodopsin the rod pigment dissociates into RETINAL and opsin catalyzing an intracellular second messenger cascade that results in the closing of many sodium channels light activates the rods pigment this becomes retinal and ospin these catalyze a second messanger which results in closing of many sodium channels retinal and ospin are proteins similar to metatropic receptors The resultant graded hyperpolarization leads to a decrease in glutamate release from tonic levels signaling a change in illumination 10 What are 11 cis RETINAL and all trans RETINAL In the dark retinal is bent at carbon 11 11 cis retinal it fits into a pocket in the big opsin protein When light strikes it the long hydrocarbon chain straightens all trans retinal can no longer fit in the pocket The opsin changes shape to expose an enzymatic site and initiates a series of reactions that ultimately close a sodium channel 11 What type of electrophysiological response occurs in the rods cones when phototransduction occurs An ocular dominance column is a column of cells perpendicular to the surface with greater synaptic input from one eye As an electrode advances from the surface to a deeper layer all cells have the same preference for input from one eye 12 Describe the responses of on center and off center bipolar and ganglion cells Bipolar cells release glutamate which always depolarizes ganglion cells Off center bipolar when light cells excite off center ganglion cells is turned off On center bipolar cells excite on center ganglion cells when light is turned on 13 What transmitter is released by all rods cones What are its effects on on center off center bipolar cells What transmitter is released by bipolar cells What is its effect on ganglion cells Glutamate is released by rods and cones On center bipolar cells excite on center ganglion cells when light is turned on Off center bipolar cells excite off center ganglion cells bipolar and ganglion Bipolar cells release glutamate this depolarizes ganglion cells 14 What is the relationship between on center bipolar cells and on center ganglion cells How about off center bipolar and off center ganglion cells Bipolar and ganglion cells have concentric receptive fields Illumination of the center of an on center off surround cell results in an increase in firing and illumination of the surround results in a decrease in firing The reverse pattern holds for off center on surround cells At rest rods cones steadily release glutamate which depolarizes some bipolar cells and hyperpolarizes others On center bipolar cells turning on light excites them They receive less glutamate which normally inhibits on center bipolar cells Off center bipolar cells turning off light in the center of the field excites the cells They receive more glutamate and are depolarized 15 How did the fovea get its name What kinds of photoreceptors are located there Acuity is best in foveal vision because the fovea contains the highest density of cones photoreceptors Fovea got its name by being latin for pit High density of cones Other Info Furthermore the fovea is overrepresented in area V1 of visual cortex which contains a retinotopic map Damage in V1 results in a scotoma area of blindness in the corresponding part of the visual field Limited nonconscious processing called blindsight appears to occur within the area of scotoma indicating the functioning of parallel visual processing outside of V1 including the superior colliculus 16 Why is the blind spot blind Why is this not a problem for our vision Blind dpot is blind