NRSC 1110 Ch. 9

Sun

the major source of electromagnetic energy reaching the surface of the earth

properties of light

wavelength and amplitude

Visible Light

The human eye responds to electromagnetic energy between ~400-700nm

Optics

Study of light rays and their interactions

Reflection

Bouncing of light rays off a surface

Absorption

Transfer of light energy to a particle or surface

Refraction

Bending of light rays from one medium to another

Pupil

opening where light enters the eye, continuously adjusts to different ambient light levels

Sclera

white of the eye

Iris

gives color to eyes

Cornea

glassy transparent external surface of the eye

Extraocular muscles

3 pairs the move the eye in its orbit

Optic nerve

bundle of axons from the retina

Lens

suspended by ciliary muscles

Lens is made of which two comparments

aqueous humor and vitreous humor

Aqueous Humor

watery, nourishes cornea, high pressure reduces blood supply and damages retina \>> glaucoma

Vitreous humor

jelly-like, keeps eye-ball spherical, 80% of volume of the eye, contains phagocytic cells that remove debris, source of "floaters"

What performs most of the eye's refraction?

Cornea

Steps of refraction

Eye collects light, focuses on retina (fovea), and forms image

Accomodation

changing shape of lens, provides extra focusing power\>>>required to bring near objects into focus on the retina

Accommodated lens

ciliary muscles are contracted, reduces the tension in the zonule fibers, allows elasticity of the lens to increase curvature, "fat"

Unaccommodated lens

ciliary muscles are relaxed, "flat"

Pupillary Light Reflect (PLR)

tests fidelity of connections between retina and brain stem neurons that control muscles around pupil

Consensual

shining a light into on eye causes constriction of both pupils; lack of PLR is an indication of cranial nerve dysfunction in brainstem

Fovea ("pit")

anatomical reference as center of retina, pit in retina where outer layers are "pushed aside," light strikes photoreceptors directly, maximizes visual acuity, located in center of macula

Choroid

capillary bed \-- main source of blood supply to photoreceptors

Optic Disk

blind spot, origin of vessels, no photoreceptors, no blood vessels

Anatomical terms of the retina: nasal

closer to the nose

Anatomical terms of the retina: temporal

near the temples

Anatomical terms of the retina: superior

above fovea

Anatomical terms of the retina: inferior

below fovea

Visual field

amount of space viewed by retina when one eye is fixated straight ahead; image is inverted (L-R, and up-down) on the retina

Visual acuity

ability of the eye to distinguish 2 points close to each other; depends on spacing of photoreceptors

Strabismus

lazy eye, Imbalance of the extra-ocular muscles in both eyes

Cataract

clouding of the lens

Glaucoma

increased pressure in aqueous humor

Retinitis Pigmentosa (RP)

progressive degeneration of photoreceptors, "tunnel vision"

Macular Degeneration (MD)

central vision is lost, peripheral vision normal

Five neurons in the retina

photoreceptors, bipolar cells, ganglion cells, amacrine cells, horizontal cells

Direct (vertical) pathway

ganglion cells (fire action potential)

Indirect (lateral) pathway

lateral connections: horizontal cells and amacrine cells

Horizontal cells

receive input from photoreceptors and project to other photoreceptors and bipolar cells

Amacrine cells

receive input from bipolar cells and project to ganglion cells, bipolar cells, and other amacrine cells

\__________________ are the only light-sensitive cells in the retina

rod and cone photoreceptors

\________________ are the only source of output from the retina

ganglion cells

\________________ are the only retinal cells that fire action potentials

ganglion cells

optic nerve is made up of \_______________

axon bundles

Laminar organization of the retina

"inside out" organization, light passes through vitreous humor, ganglion cells and bipolar cells before reaching photoreceptors

Photoreceptors sit on a \________________

pigment epithelium

Pigment epithelium

reduces backscatter, maintains photoreceptors \>>> renews photopigments and phagocytoses photoreceptor disks

Tapetum lucidum

reflective layer beneath photoreceptor layer (humans don't have), bounces light back at the photoreceptor, makes animal more sensitive to low light at the expense of acuity

"Eyeshine"

side effect of a reflective tapetum when light is shined into the eye

Types of photoreceptors

rods and cones, ~18x more rods than cones (~90 million to ~5 million), convert electromagnetic radiation to neural signals

4 main regions of photoreceptor structure

outer segment, inner segment, cell body, synaptic terminal

outer segment of photoreceptors

contains opsins in membranous disks, disks are regenerated ~12 days

Rods

long, cylindrical outer segment with many disks, over 1000x more sensitive to light than cones, photopigment: rhodopsin

Cones

shorter, tapering outer segment with fewer disks, each with different photopigment; sensitive to different wavelengths; Solely responsible for color vision; Photopigment: three types of cone opsins

"Duplex Retina"

two complementary systems in one eye

Scotopic

nighttime lighting, rods

Photopic

daytime lighting, mostly cones

Mesopic

twilight, both rods and cones contribute

Central retina

most cones are in fovea, no rods in central fovea, low ratio of photoreceptors to ganglion cells (1:1 in fovea), best for high-resolution vision

Peripheral retina

higher ratio of rods to cones, higher ratio of photoreceptors to ganglion cells, best at detecting dim light

Humans have much greater spatial sensitivity in our \______________

central vision

Phototransduction

process by which absorption of light energy is converted to a change in photoreceptor membrane potential

Visual pigment molecules

located in photoreceptor disks, absorb the electromagnetic energy and divert the energy into a biological process, formed by combining a chromophore (11-cis retinal) with an opsin

11-cis retinal

Changes shape when it absorbs light\---\> all trans retinal

opsin

large, membrane bound protein (GPCR)

Rhodopsin

rod opsin

Cones have \____.\______.\______ opsins

red, green, or blue (each encoded by a separate gene)

"Dark current"

rod outer segments are depolarized in the dark \>>> steady influx of Na+

Phototransduction steps

1. rhodopsin is stimulated
2. produces activation of transducin (a G-protein)
3. transducin alpha subunit activates phosphodiesterase (PDE, effector enzyme)
4. PDE hydrolyzes cGMP (reduces concentraion) >>> Na+ channels close and cell is hyperpolarized

Phototransduction in the dark

1. Concentration of cGMP is high
2. Inward Na+ current through cGMP gated channels which depolarizes the cell—"dark current" • Keep cell's Vmat \~-40mV
3. Depolarization causes transmitter release (glutamate) at terminal region

Phototransduction in the light

1. Light activates rhodopsin
2. Transducin is active
3. Phosphodiesterase (PDE), the effector enzyme, is activated
4. PDE activity reduces the cGMP level
5. cGMP-gated Na+channels close - no Na+current (dark current)
6. Cell hyperpolarizes to \~-70mV, transmitter release stops

What happens to cGMP in bright light?

cGMP levels in rods fall to the point where response to light is saturated, cannot hyperpolarize further, day vision depends on cones

Red (cone opsin)

long wavelength

Green (cone opsin)

medium wavelength

Blue (cone opsin)

short wavelength

Color perception

what we perceive is a relative contribution (weighted average) of blue, green, and red cones to retinal signal

Young-Helmholtz trichromacy theory of color vision

the brain assigns colors based on a comparative readout of the 3 cone types

Receptive Field

Area of retina where light changes a neuron's firing rate; Light outside the RF has no effect on firing rate

The RF of a bipolar (and ganglion cell) has two parts: \_______________

a center and a surround, and they are mutually antagonistic

Center-surround

RF organization

\____________ is much more sensitive than the \_____________, but the \____________ is many times larger than the center

center, surround, surround

Bipolar cell circuitry:

* direct input from photoreceptors
* Indirect input from photoreceptors via horizontal cells

Two classes of bipolar cells RF

ON and OFF

* based on the response to glutamate released by photoreceptors

Bipolar Cell RF Direct Pathway: Depolarization

* Light shined onto a photoreceptor causes depolarization of the bipolar cell
* "ON" bipolars are depolarized by light

Bipolar Cell RF Direct Pathway: Hyperpolarization

* Light shined onto a photoreceptor causes hyperpolarization of the bipolar cell
* "OFF" bipolars are hyperpolarized by light

"ON" bipolars are \__________________ by glutamate (via activation of mGluR6 receptors) and \_________________ by light

hyperpolarized, depolarized

"OFF" bipolars are \______________ by glutamate (via activation of AMPA receptors) and \_______________ by light

depolarized, hyperpolarized

Bipolar Cell Receptive Fields: Indirect Pathway (adding the surround)

* Bipolar cells are also connected via horizontal cells to a ring of photoreceptors that surround the central cluster
* Photoreceptors release glutamate onto horizontal cells
* When activated by glutamate, horizontal cells inhibit target center photoreceptors (via GABA-like mechanism)
* But...when a photoreceptor hyperpolarizes in response to light, it causes target horizontal cells to hyperpolarize
* Horizontal cell hyperpolarization leads to less inhibition on center photoreceptor
* Center photoreceptor releases more glutamate onto ON-center BP cell
* ON-center BP cell is hyperpolarized!

\___________________ have the same center-surround RF field organization as their \__________________

Retinal ganglion cells (RGCs), input bipolar cells

Luminance Contrast

RGCs are sensitive to differences between the level of illumination that falls on the RF center and the level of illumination that falls on the surround

Contrast signal

signmal leaving the retina (difference between center and surround)

Response of the RGC is determined by the....

fraction of center and surround that are filled by light and dark

The center mechanism is generated by the \__________ from \_________ to \__________ to \______________

vertical connections, photoreceptors to bipolars to ganglion cells

The surround is created by the \_______________ between \_________________ and \__________________

lateral pathway, cones and the horizontal cells

Light falling on center photoreceptor leads to a \___________________ \>>>>> opposite effects on two populations of bipolar cells:

reduction in glutamate release

ON-center bipolar cells: depolarize\>>>\___________________

increase in firing rate of ON-center ganglion cells

OFF-center bipolar cells: hyperpolarize \>>> \_______________________-

decrease in firing rate of OFF-center ganglion cells