The Eye

The Eye is the Beginning of the Visual System

• The eye focuses light onto the parts of the eye that can engage in transduction, the transformation of a substance or energy into a neural/electrical signal

• Humans can see the visual spectrum from the spectrum of electromagnetic energy (radiation)

  • Shorter waves/faster particles - high energy

  • Longer wave/slower particles - low energy

  • Ionizing Radiation - gamma rays, x-rays, and UV rays that can ionize and damage DNA

  • Non-ionizing Radiation - infrared rays, radar, broadcast bands, and AC circuits that do not damage the tissue unless produced in high concentrations

• The mix of wavelengths in the visible light range (400 - 700 nm) emitted by the sun appears to humans as white

  • Light of a single wavelength appears as one of the colors of the rainbow

    • Red - a longer wavelength

    • Blue - a shorter wavelength

  • Black is the absence of particles of light being reflected into the eye

    • The material of the object absorbs all of the light energy

• Optics - the study of light rays and their interactions

  • Reflection - the bouncing of light rays off a surface

  • Absorption - transfer of light energy to a particle or surface

    • Color pigments absorb all other wavelengths except for the one it is perceived as, that wavelength is reflected

    • Refraction - the bending of light rays that can occur when they travel from one transparent medium to another

      • This is how images are formed in the eye

      • The greater the difference between the speed of light in the two media, the greater the angle of refraction

The Structure of the Eye

• Gross Anatomy of the Eye

  • Pupil - the opening that allows light to enter the eye and reach the retina

    • It appears dark because of the light-absorbing pigments in the retina

  • Iris - the colored part of the eye that surrounds the pupil

    • It has smooth muscle that can contract and relax, which alters the size of the pupil (dilated or constricted)

  • Cornea - the glassy, transparent external surface of the eye

    • Light goes through the cornea into the pupil

  • Sclera - continuous with the cornea; the “white part of the eye” that forms the tough wall of the eyeball

  • Eye’s Orbit - the bony eye socket the skull where the eyeball sits

  • Extraocular Muscles - muscles that move the eye around

    • These muscles are normally not visible because they lie behind the conjunctiva, a membrane that folds back from the inside of the eyelids and attaches to the sclera

  • Optic Nerve -carrying axons from the retina, exits the back of the eye, passes through the orbit, and reaches the base of the brain near the pituitary gland

• Cross-Sectional Anatomy of the Eye

  • Aqueous Humor - the fluid that nourishes the cornea

  • Lens - a gelatinous structure, behind the iris, that changes its shape due to ciliary muscles

    • Suspended by ligaments (called zonule fibers)

  • Ciliary Muscles - form a ring inside the eye; they push and pull on the lens altering its shape

  • Vitreous Humor - is a viscous, jellylike fluid that lies between the lens and retina; it keeps the eyeball spherical

  • Retina - located at the back of the eye, contains photoreceptors specialized to convert light energy into neural activity

    • Fovea - a dark spot in the center of the retina where light is focused; the highest density of photoreceptors

Image Formation by the Eye

• We can only see what is in front of us, and what light our eyes can collect

  • Visual Field - the extent of our environment that we can see

  • Each eye has its own visual field, a part of which overlaps with the other

    • The center of the visual field samples both eyes, making it the most accurate part of the vision

  • Visual Acuity - the 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

Microscopic Anatomy of the Retina

• Photoreceptors - sensory receptor cells of the eye; they respond to light

  • They form synapses with retinal bipolar cells, which form synapses with retinal ganglion cells

  • The retinal ganglion cells fire action potentials in response to light, and these impulses propagate along the optic nerve to then be terminated in the thalamus and other brain regions

• Horizontal cells - receive input from the 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 of the Retina

  • 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 the 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

• Light coming in the center of the visual field gets the fovea as it is responsible for the center of the visual field

  • If light comes through at an angle, as it passes through the cornea it’ll slightly bend toward the lens, to make sure it hits the lens, and the lens will bend more to focus the light onto the retina

    • 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

• Photoceptor Structure

  • There are 125 million photoreceptors on the back of the retina

    • 5 million are cones

    • 92 million are rods

  • Photoreceptor Regions - outer segment, inner segment, cell body, and synaptic terminal

    • The outer segment contains a stack of membranous disks

      • Light-sensitive photopigments (opsins) in the disk membrane absorb light triggering changes in the photoreceptor membrane potential

  • Rod Photoreceptors - cells with a long, cylindrical outer segment, containing many disks

    • 1000x more sensitive than cones as it has a greater chance of light particles being detected, via opsins

      • Low-light vision

    • 1 type of pigment

      • 1 color (dark blue-green)

  • Cone Photoreceptors - cells with a short, tapered outer segment with few membranous disks

    • Require substantial stimulation

      • Bright-light vision

    • 3 types of pigments

      • Red, green, and blue cones

• Anatomy of the Retina

  • Most of the 5 million cones are in the fovea, and the 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

    • The blind spot contains zero rods and cones (optic disk)

      • The primary visual cortex fills in the gap

  • Peripheral vision has a high sensitivity for low light

  • Central vision has high acuity in bright light

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

Phototransduction

• Phototransduction in Rods

  • When not transducing light there is a “dark current” in photoreceptors

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

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

    • There is a constant Na conductance that results in a resting membrane potential of ~ -30 mV

      • Voltage-gated calcium channels open to allow Ca influx

    • At this membrane potential, glutamate (NT) is released into the synaptic cleft

    • The absence of light can be considered the “preferred stimulus”, as that is what causes the release of neurotransmitter

  • Hyperpolarization is initiated when light interacts with the protein rhodopsin

    • Rhodopsin is a rod-specific opsin protein responsive to 500 nm wavelength light

    • Any opsin protein contains vitamin-A-derived proteins called retinal

    • Retinal absorbs light, causing a conformational change in the opsin (making it function like a metabotropic receptor)

    • The conformation change activates a G-protein called transducin

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

      • cGMP-gated sodium channels close and Na influx stops, membrane potential hyperpolarizes, voltage-gated calcium channels do not open, and glutamate (NT) is no longer released

    • In bright light, the cGMP levels drop to a level where no more can be deactivated

• Phototransduction in Cones

  • The transduction mechanism is the same as the rod except for what opsin the cone is expressing

    • Any individual cone will express any one of the three opsins: red opsin, green opsin, or blue opsin

  • Opsins in cones require more energy from photons to activate

    • Cones are insensitive to light and are active in bright light

  • The three types of opsins in cones are selective for different wavelengths of light

    • Blue (S cones): activated maximally by 430 nm light

    • Green (M cones): 530 nm

    • Red (L cones): 560 nm

Retinal Processing and Output

• Other cells in the retina take information from photoreceptors and give a host of different outputs

  • Rods and cones release when photons are not hitting photoreceptors

  • The photoreceptors synapse with bipolar cells and horizontal cells

    • These cell types work together to process information sent to the ganglion cells

• Retinal Bipolar Cell Receptive Fields

  • Bipolar cells can be categorized by their response to photoreceptor NT release

    • OFF Bipolar Cells - cells that depolarize when the “lights are off” (no photons interacting with presynaptic photoreceptors)

      • Photoreceptors are releasing NT

      • The bipolar cell has ionotropic glutamate receptors (selective for Na) that depolarize the membrane (Glu-gated Na channel)

        • Depolarization in response to glutamate

      • Therefore, it depolarizes when the light is OFF

    • ON Bipolar Cells - cells that depolarize when the “lights are on” (photons interact with presynaptic photoreceptors)

      • Photoreceptors are not releasing NT

    • The bipolar cell has metabotropic glutamate receptors that hyperpolarize the membrane

      • Hyperpolarization in response to glutamate

    • There, it depolarizes when the light is ON

  • Receptive Field - any part of the environment that a particular sensory neuron can detect

    • Light anywhere else on the retina, outside the receptive field, would not affect the firing rate

    • Bipolar cells’ reaction to the photoreceptors surrounding their receptive field is opposite to that of the center of the receptive field (when multiple photoreceptors are connected to one bipolar cell via horizontal cells)

      • Horizontal cells modify the input at the level of the synapse, so the bipolar cell would have the opposite response (being hyperpolarized when the photoreceptors are hyperpolarized)

• Retinal Ganglion Cell Receptive Fields

  • Ganglion cells have a center-surround receptive field that results from input from similarly-typed bipolar cells and interactions with amacrine cells

    • If the center is OFF and the surrounding is ON the bipolar cell depolarizes and vice versa

    • M-type: Magnocellular - large receptive field, bursts of rapidly conducted APs

    • P-type: Parvocellular - 90% of ganglion cells; sustained discharge of APs

    • nonM-nonP type - sensitive to the wavelength of light (red, blue, or green)

      • Color-Opponent Cells - another wavelength in the surrounding can cancel wavelength in the center

      • Two types: red vs. green and blue vs. yellow

    • Ganglion output reveals particular contrasts

  • The visual system uses parallel processing

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

      • Comparison reveals depth from the information from the eyes

      • Comparison can reveal lines from light vs. dark streams

      • Comparison can reveal the color of an object from a color stream