Untitled Flashcards Set

-photoreceptors: rods and cones

-responsible for sensation of light and visual transduction

-located in retina

-sensation: photoreceptors

-perception: primary visual cortex (V1) and beyond

-starts with light: electromagnetic waves

-Light is an electromagnetic wave and a particle (photons)

-We perceive visible light (~400-700 nm)

-Long wavelength hot light (red) lower energy than cool (blue) short wavelength light

-We perceive different wavelengths as different colors

-Colors are themselves a perception, subjectively “colored” by the brain

Anatomy of eye

-Cornea: convex transparent tissue that allows light rays to enter the eye and provides most of the refractive (light-bending) power

-helps ensure light focuses at one point (retina)

-pupil: light enters eye through pupil

-in front of lens

-Lens: a transparent structure that can bend, which refracts light so a clear image is formed on the retina

-helps with focusing (accommodation)

-Retina: a thin (0.5 mm) layer of light sensitive cells at the back of the eye that processes/passes visual information to the brain

How we focus

-visual accommodation: process of lens changing shape

-further objects: lens doesn’t have to change shape much

-narrow, acute angle

-longer working distance

-closer objects: lens has to change shape

-wider angle

-less working distance

-One of the many consequences of aging is that the lens stiffens and becomes less flexible as we age, leading to reduced visual accommodation by the lens (less curvature)

The Retina (10/25)

-retina: central nervous system tissue in the periphery

-Macula lutea: oval spot containing yellow pigment

-supports the highest visual acuity (i.e. ability to resolve fine details/sharpness)

-Fovea: small “pit” at the center of the macula where visual acuity (sharpness) is highest

-Optic Disk: site where retinal nerve fibers (axons) leave the eye and travel to the brain via the optic nerve

-blindspot: no photoreceptors here

-path of light is counterintuitive:

-rods and cones located in back of retina: important for visual transduction

*light must pass through multiple layers before hitting photoreceptors and being absorbed

-rods & cones: where phototransduction takes place

-don’t fire AP

-graded response: depolarize or hyperpolarize

-wired to bipolar cells

-bipolar cells: pass info from rods & cones downward

-don’t fire AP

-graded responses

-retinal ganglion cells: closest to outside

-axons collect to form optic nerve and leave the eye to connect to thalamus

-fire APs

-info moves from photoreceptors → bipolar cells → ganglion cells

-path of light in one direction (front to back), path of info in other (back to front)

-pigment epithelium: provide support to rods and cones

-very back of retina

-black so it absorbs excess photons

-retina: photosensitive sheet in the back of the eye

-part of the CNS

-Contains (in order of light passage):

Ganglion Cells: Only source of output from the retina

-axons form the optic nerve

-only retinal cells fire action potentials, excitatory

Amacrine Cells: Input from bipolar cells, influence ganglion cells

-release GABA, inhibitory

Bipolar Cells: Input from the photoreceptors, output to the ganglion cells, on-center or off-center

-release glutamate, excitatory

Horizontal Cells: Input from photoreceptors, outputs to nearby bipolar and photoreceptors

-release GABA, important for lateral inhibition

Photoreceptors: Only light sensitive cells in the retina (rods & cones)

-outputs to the bipolar and horizontal cells

-release glutamate, excitatory

-fovea (central vision): location of highest visual acuity (best sharpness/resolution)

-when you orient your eyes toward something, you are directing light onto your fovea

-less scattering: blood vessels, ganglion and bipolar cells are displaced laterally (pushed to the side) to allow light to directly strike cones

-Acuity: measure of the ability to resolve fine details from a given distance

*degree of convergence between photoreceptors and RGCs is different in central/foveal vs. peripheral vision

-Convergence: how many photoreceptors and BPCs signal a single RGC (retinal ganglion cell)

-Center of the fovea: low ratio of cones to RGCs (1-1 in very center; good for acuity)

-1 cone contacts 1 bipolar cell which contacts 1 retinal ganglion cell

-low convergence

-Rods on outer periphery: high ratio of rods to RGCs

-higher level of convergence

-low acuity

-Low convergence = high acuity (and vice versa)

-high convergence is bad for acuity, but good for sensitivity

-photoreceptors: rods and cone

-similar morphology and share job of turning light into nerve impulses (phototransduction)

-outer segment: contains photoreceptor disks packed with light sensitive photopigment (opsins)

-Rods: have a larger outer segment than cones because they contain more disks than cones, accounts for greater sensitivity to light

-very sensitive, good for low light conditions

-low acuity: mostly located in the peripheral retina

-achromatic

-present at high density throughout most of the retina, with the exception of fovea

-Cones: located primarily in the fovea (central vision)

-less sensitive to light (active in daylight)

-high resolution (acuity), and properties that allow us to see colors (different wavelengths of visible light)

-present at a low density throughout the retina with a sharp peak in the fovea

Photoreceptors & Phototransduction (10/27)

-phototransduction: first steps of vision

-occur in retina in photoreceptor outer segments (disks containing photopigment)

-active computations to extract specific features of the visual scene

-early observations found that light hyperpolarizes photoreceptors

-absorption of photons of light leads to a reduction in neurotransmitter (glutamate) release by rods and cones

-Photons are absorbed by opsin proteins, which house a light sensitive molecule called retinal

-opsin proteins: act like receptors, but don’t bind to neurotransmitters

-ligand is light, not a neurotransmitter

*light absorption causes change of shape of retinal molecule within opsin protein, initiating intracellular signaling cascade

-retinal: vitamin A derivative

-inactive: bent down

-active: planar form

*change initiates response

Steps of phototransduction

-in the light:

A photon of light is absorbed by retinal within the opsin protein (planar/active form)
stimulates transducin:

-G-protein coupled receptor: metabotropic

-when retinal changes shape, opsin also changes shape as a result of absorbing light, activating G protein

Activates Phosphodiesterase (PDE), which works to close sodium channels

-closes sodium channels: prevents sodium from flowing in, stops depolarization

Neurotransmitter (glutamate) release decreases due to this hyperpolarization

-in the dark:

Retinal within the opsin protein is not activated (bent/inactive form)
transducin (G-protein) is not activated
Phosphodiesterase (PDE) is therefore not activated

-sodium channels remain open, Na+ flows in

Without PDE activity, Na+ channels are open, allowing Na+ influx, which causes the cell to depolarize

*depolarized in dark, hyperpolarized in light

-photoreceptors are more active in the dark, shown by depolarization and glutamate release in darkness (dark current)

-receptors on bipolar cells are responsible for change in activity that leads to AP release from ganglion cells

-Off center BP cells: hyperpolarized (decrease activity) by light due to ionotropic AMPARs/GLU receptors (sign conserving)

-report information about darkness to the brain

-On center BP cells: depolarized by light due to mGluR6 (GPCR) activation (sign inverting)

-sign inverting synapse between photoreceptors and bipolar cells: metabotropic receptor

-when light shines on receptor, lack of GLU is turned into depolarization

-report information about light to the brain

Receptive Fields & Central Visual Pathways (10/30)

*active computations to extract specific features of the visual scene

-center surround (circular) receptive fields in retina

-Receptive field: portion of visual space that maximally activates/excites a neuron

-in retina, receptive field are very simple spots or circles of light that are detected by photoreceptors

-rods and cones picking up individual pixels to create image

-GABA is released to silence photoreceptors surrounding those that are maximally stimulated

-allows for a sharper signal to get through, weaker stimulation on periphery is silenced

-Active retinal computations: horizontal cells increase contrast through lateral inhibition

-visual system is very good at determining where light is coming from because of lateral inhibition

-some photoreceptors are stimulated more than others

-Mach Band Illusion: along the boundary between adjacent shades of grey, lateral inhibition by horizontal cells makes the darker area falsely appear even darker and the lighter area falsely appear even lighter

-enhances contrast at lightdark edges

Central vision pathways

-after light signals leave the retina

-visual pathways and binocularity:

-if staring straight ahead, objects in our left visual space are projected to our right primary visual cortex, and vice versa

-Medial (nasal) fibers of the optic nerves cross at the optic chiasm, while lateral (temporal) fibers stay on the same side

-roughly half of projections cross: the ones closest to the middle already cross

-each hemisphere of visual cortex receives input from both eyes

-because we have binocular vision, we have good depth perception: larger visual field

-Functional consequences: damage along the visual pathway produces characteristic visual deficits

-lesion in right eye: lack of right eye projections

-lesion of optic chiasm: tunnel vision, no projection cross over, won’t see outer portions of visual space

-optic tract: entire left visual hemifield is blocked

-Lateral Geniculate Nucleus (LGN) of the Thalamus:

-visual information from each eye stays segregated in the LGN (input from the left vs. right eye project to different LGN layers)

-light information from each eye stays separated all the way to V1