The Eye

Function of the Eye

Focuses light onto the parts of the eye that can engage in transduction

Transduction

The transformation of a substance or energy into a neural/electrical signal.

Electromagnetic Spectrum

The range of all types of electromagnetic radiation, including visible light.

Ionizing Radiation

Radiation that can ionize and damage DNA, including gamma rays, x-rays, and UV rays.

Non-ionizing Radiation

Radiation that does not damage tissue unless produced in high concentrations, such as infrared rays and radar.

White & Black Light

White - mix of all wavelengths in the visible light range; Black - absorption of all wavelengths in the visible light range

Optics

The study of light rays and their interactions.

Reflection

The bouncing of light rays off a surface; wavelength of color perceived is reflected

Absorption

Transfer of light energy to a particle or surface; color pigments absorb all other wavelengths except for the one it is perceived as

Refraction

Light rays bend when they travel from one transparent medium to another; a greater difference between the speed of light in the two media equals a greater angle of refraction

Pupil

The opening that allows light to enter the eye and reach the retina.

Iris

The colored part of the eye that surrounds the pupil and can alter its size via smooth muscle

Cornea

The transparent external surface of the eye through which light passes into the pupil

Sclera

White part of the eye that forms the tough wall of the eyeball

Eye’s Orbit

the bony eye socket of the skull where the eyeball sits

Extraocular Muscles

Muscles that move the eye around; lie behind the conjunctiva

Optic Nerve

Axons that travel from the retina to the base of the brain near the pituitary gland

Aqueous Humor

Fluid that nourishes the cornea

Lens

Gelatinous structure behind the iris that changes shape via ciliary muscles

Ciliary Muscles

Muscles that push and pull on the lens altering its shape

Vitreous Humor

Viscous fluid that lies between the lens and retina; keeps the eyeball spherical

Retina

The part of the eye, located at the back, that contains photoreceptors for converting light energy into neural activity.

Fovea

A dark spot in the center of the retina characterized by the highest density of photoreceptors

Here, the ganglion cell layer and inner nuclear layer are pushed to the side so light can go directly to the outer layer with photoreceptors, without being absorbed by previous layers

Visual Field

The extent of our environment that we can see; each eye has its own visual field that overlaps (center is most accurate part of vision)

Visual Acuity

Ability of the eye to distinguish two points near each other; dependent on the spacing and density of photoreceptors in the retina and the precision of the eye’s refraction

Photoreceptors

Sensory receptor cells in the eye that respond to light, including rods and cones.

Rods

Photoreceptors that are more sensitive to light and are responsible for low-light vision (500 nm or dark blue-green)

Cones

Photoreceptors responsible for bright-light vision, requiring more stimulation and sensitive to color (560 nm: red, 530 nm: green, and 430 nm: blue)

Phototransduction

The process by which photoreceptors convert light into electrical signals.

Retinal Bipolar Cells

Retinal cells that connect photoreceptors to ganglion cells, processing sensory information.

Retinal Ganglion Cells

Retinal cells that fire action potentials in response to light, sending visual information to the brain.

Horizontal Cells

Receive input from photoreceptors (via their axons) and project neurites laterally to influence surrounding bipolar cells and photoreceptors (via their dendrites)

Amacrine Cells

Receive input from bipolar cells (via their axons) and project laterally to influence surrounding ganglion cells, bipolar cells, and other amacrine cells (via their dendrites)

Laminar Organization

Cells are organized in layers

Ganglion Cell Layer

Innermost retinal layer; it contains the cell bodies of the ganglion cells

Inner Plexiform Layer

Between the ganglion cell layer and inner nuclear layer; it contains the synaptic contacts between bipolar cells, amacrine cells, and ganglion cells

Inner Nuclear Layer

Below the inner plexiform layer; it contains the cell bodies of the bipolar cells, horizontal cells, and amacrine cells

Outer Plexiform Layer

Between the inner and outer nuclear layers; it is where the photoreceptors make synaptic contact with the bipolar and horizontal cells

Outer Nuclear Layer

Below the outer plexiform layer; it contains the cell bodies of the photoreceptors

Pigmented Epithelium

Lies below the photoreceptors; absorbs light that passes entirely through the retina, minimizing the scattering of light within the eye that would blur the image

Ratio of Photoreceptors

125 million; most are rods, others are cones (~ 5 million)

Photoreceptor Regions

Outer segment, inner segment, cell body, and synaptic terminal

Photoreceptor Outer Segment

Contains a stack of membranous disks with opsins

Opsins

Light-sensitive photopigments in the disk membrane that absorb light triggering changes in the photoreceptor membrane potential

Locations of Rods and Cones on the Retina

Most cones are in the fovea and their proportion diminishes substantially in the retinal periphery

There are no rods in the central fovea, but there are more rods than cones in the peripheral retina

Blind Spot

Location on the retina where there are no rods or cones (location of the optic disk)

Dark Phototransduction in Rods

• A dark current occurs when photoreceptors are not transducing light

• As rhodopsin is not activated by light, the g-protein, transducin, stays inactive

• Guanylyl cyclase enzymes produce cGMP which opens cGMP-gated sodium channels on the photoreceptor membrane and sodium influx occurs

  • Sodium-potassium pumps on the membrane help to balance the concentration

• This constant sodium conductance results in a resting membrane potential of ~ -30mV

• Voltage-gated calcium channels open at this Vm allowing calcium influx

• Calcium binds to docking proteins holding synaptic vesicles and changes their shape, pulling them closer to the membrane

• The vesicles and photoreceptor membrane fuse and the NT glutamate is released into the synaptic cleft

Light Phototransduction in Rods

• When light (with a 500 nm wavelength) interacts with rhodopsin, the photoreceptor membrane potential hyperpolarizes

• Retinal, a vitamin-A-derived protein, absorbs light and causes a conformation change in the opsin, making it function like a metabotropic receptor

• This conformation change activates the g-protein transducin

• Transducin activates phosphodiesterase (PDE) which breaks down cGMP into GMP

• cGMP-gated sodium channels close, due to cGMP levels dropping, and sodium influx stops

• The membrane potential hyperpolarizes and voltage-gated calcium channels do not open, resulting in no glutamate being released

Phototransduction in Cones

Same as rods except for what opsin the cone is expressing

Three Types of Opsins in Cones

L cones: red (selective for 560 nm light)

M cones: green (selective for 530 nm light)

S cones: blue (selective for 430 nm of light

OFF Bipolar Cells

Cells that depolarize when the “lights are off” (no photons interacting with presynaptic photoreceptors); have ionotropic glutamate-gated sodium receptors that depolarize the membrane

ON Bipolar Cells

Cells that depolarize when the “lights are on” (photons interact with presynaptic photoreceptors); have metabotropic glutamate receptors that hyperpolarize the membrane

Receptive Field

Any part of the environment that a particular sensory neuron can detect

M-type (Magnocellular)

Large receptive field; bursts of rapidly conducted action potentials

P-type (Parvocellular)

Small receptive field (90% of ganglion cells); sustained discharge of action potentials

nonM-nonP type

Sensitive to the wavelength of light (red, green, or blue)

Color-Opponent Cells

Cells that have receptive fields where the presence of one color can inhibit the perception of another color.

Parallel Processing

Streams of information (via axons) in parallel with each other

Reveals depth from the information from the eyes, lines from light vs. dark streams, and the color of an object from a color stream