Ch 9: The Eye

Define light.

Define light.

Electromagnetic energy that is emitted in the form of waves

Define wavelength.

The distance between successive peaks or troughs

Define frequency.

The number of waves per second

Define amplitude.

The distance between the midpoint of the wave and the peak

Define reflection.

The bouncing of light rays off a surface

Define absorption.

The transfer of light energy to a particle or surface

Define refraction.

The bending of light rays that can occur when they travel from one transparent medium to another

Define optic disk.

• A pale circular region from which the retinal vessels originate

• Also where the optic nerve fibers exit the retina

• No photoreceptors

Define macula.

• The part of the retina for central vision

• Distinguished by the relative absence of large blood vessels

Define extraocular muscles.

• 3 pairs

• Move the eyeball in the orbit

• Normally not visible because they lie behind the conjunctiva

Identify and describe a.

Zonule fibers

• Ligaments attached to the ciliary muscle that suspend the transparent lens behind the iris

Identify and describe b.

Iris

• Surrounds the pupil

• Its pigmentation provides the eye’s color

• Contains two muscles that can vary the size of the pupil: one makes it smaller when it contracts, the other makes it larger

Identify and describe c.

Lens

• Located behind the iris

• Is suspended by ligaments (zonule fibers) attached to the ciliary muscle

Identify and describe d.

Cornea

• Lacks blood vessels

• Is nourished by the aqueous humor behind it

• Covers both the pupil and iris

• Is continuous with the sclera

• Has a refractive power of 42 diopters

Identify and describe e.

Aqueous humor

• Watery fluid

• Located between the cornea and the lens

• Nourishes the cornea

Identify and describe f.

Ciliary muscle

• Forms a ring inside the eye

• Zonule fibers are attached to the ciliary muscle to hold the lens in place

Identify and describe g.

Vitreous humor

• Viscous, jelly-like fluid

• Lies between the lens and the retina

• Keeps the eyeball spherical

Identify and describe h.

Sclera

• The white of the eye

• Forms the touch wall of the eyeball

Identify and describe i.

Optic nerve

• Carries axons from the retina → exits the back of the eye → passes through the orbit → reaches the base of the brain near the pituitary gland

Identify and describe j.

Fovea

• A dark spot about 2 mm in diameter

• Marks the center of the retina

• Thinner portion of the retina

Identify and describe k.

Retina

• Located at the back of the eye

• Contains photoreceptors specialized to convert ight energy into neural activity (cones and rods)

Describe the process of light entering the eye and the role of the cornea and the lens.

• Light enters the eye through the cornea, focuses on the retina, and forms an image

• Light enters the eye perpendicular to the corneal surface → Light passes straight to the retina

• Light enters the eye at angle other than perpendicular to the corneal surface → Light rays are bent such that they converge on the back of the retina

• Cornea has a refractive power of 42 diopters

• Lens shape changes to provide extra focusing power and make crisp images of objects closer than 9 meters

What is a diopter?

A unit of measurement that is the reciprocal of the focal distance in meters

What is focal distance and what does it depend on?

• The distance from the refractive surface to the point where parallel light rays converge

• Depends on the curvature of the cornea

• The tighter the curve, the shorter the focal distance

Describe emmetropia.

Eye focuses parallel light rays onto the retina

Describe hyperopia.

• Farsightedness

• Eye is too short → Can see far away but near objects focus behind the retina

• Corrected with concave lenses

Describe myopia.

• Nearsightedness

• Eye is too long → Image converges in front of retina

• Corrected with convex lens

Describe astigmatism.

Irregular curvature of the eye → different refraction in horizontal and vertical planes

Describe presbyopia.

• “old eye”

• Hardening of the lens with aging

• Lens is unable to change shape and accommodate sufficiently

• Corrected with bifocals

Describe radial keratotomy.

• Tiny incisions in the peripheral cornea to relax central cornea

• Corrects myopia

Describe PRL.

• Photorefractive keratectomy

• Laser reshapes outer surface of cornea by vaporizing thin layers

Describe LASIK.

• Laser in situ keratomileusis

• Thin flap on the outer cornea is temporarily lifted

• Laser reshapes cornea from the inside

Describe strabismus.

• Misalignment or lack of coordination between the eyes (cross-eyed)

• Correction in early childhood

  • Surgery to correct extraocular muscles

  • Prismatic glasses

Describe esotropia.

Directions of the gaze of each eye crosses

Describe exotropia.

Direction of the gaze of each eye diverges

Describe cataracts.

• Clouding of the lens

• Corrected with surgical replacement with artificial lens and glasses

Describe glaucoma.

• A progressive loss of vision associated with elevated intraocular pressure

  • Can compress the optic nerve axons

• Leading cause of blindness

Describe detached retina.

• The retina pulls away from the underlying wall of the eye from a blow to the head or by shrinkage of vitreous humor

• Fluid from the vitreous humor flows through small tears in the retina resulting from the trauma, causing more of the retina to separate

Describe retinitis pigmentosa.

• Characterized by a progressive degeneration of the photoreceptors

• First sign is loss of peripheral vision and night vision

• No cure but Vitamin A may slow its progression

Describe macular degeneration.

• Loss of central vision

• Common, affecting more than 25% of all Americans over 65 years of age

Describe the path of light when traveling through the cell layers in the retina.

1. Ganglion cell layer → Contain the cell bodies of ganglion cells

2. Inner plexiform layer → Contains the synaptic contacts between bipolar cells, amacrine cells, and ganglion cells

3. Inner nuclear layer → Contains the cell bodies of the bipolar cells, horizontal cells, and amacrine cells

4. Outer plexiform layer → Where photoreceptors make synaptic contact with the bipolar and horizontal cells

5. Outer nuclear layer → Contains the cell bodies of the photoreceptors

6. Layer of photoreceptor outer segments → Contains the light-sensitive elements of the retina

7. Pigmented epithelium → Absorbs any light that passes entirely through the retina

Describe the pathway of information flowing through retinal cells.

1. Photoreceptors → respond to light and influence the membrane potential of the bipolar cells connected to them

2. Horizontal cells → receive input from photoreceptors and project to other photoreceptors and bipolar cells

3. Bipolar cells

4. Amacrine cells → receive input from bipolar cell and project to ganglion cells, bipolar cells, and other amacrine cells

5. Ganglion cells → fire action potential in response to light → these impulses propagate along the optic nerve to the rest of the brain

Describe the role of horizontal cells.

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

Describe the role of amacrine cells.

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

Describe the role of pigmented epithelium.

• Located behind photoreceptors (rods and cones)

• Absorbs scattered light

What is tapetum lucidum and what kind of mammals possess it?

• A reflective layer beneath receptors

• Bounces light back toward retina → sensitive to low light levels

• Nocturnal animals possess it

Describe the morphological (shape) differences between rods and cones,

• Rods → longer, cylindrical outer segments containing many disks

• Cones → shorter, tapering outer segment with fewer membranous disks

At the fovea, there are no…

Rods, only cones

Describe dark current and how it changes in light.

• Rod outer segments are depolarized (Vm = -30 mV) in the dark because of they have a steady influx of Na+

• cGMP is produced in the photoreceptor by enzyme guanylyl cy → keep Na+ channels open

• Light reduces cGMP → Na+ channels close → membrane potential becomes more negative → rod receptors hyperpolarize in response to light

Write the phototransduction pathway in light.

1. Light activates (bleaches) rhodopsin by retinal undergoing a conformational change to all-trans retinal

2. Activates Gtransducin

3. Activates phosphodiesterase (PDE)

4. PDE activity reduces cGMP level s by converting cGMP to linear GMP.

5. cGMP-gated Na+ channels close

6. Cell membrane hyperpolarizes (becomes more negative)

Describe the Young-Helmholtz trichromacy theory of color vision.

• The brain assigns colors based on signal from all 3 cone types (red, green, blue)

• Equal signal from all 3 cones = white

Describe dark adaptation.

• All-cone daytime vision → all-rod nighttime vision

  • Can take minutes to nearly an hour

• Increases sensitivity to light a millionfold or more during this period

What factors contribute to dark adaptation?

• Dilation of pupils → allows more light to enter the pupil

• Regeneration of unbleached rhodopsin

• Adjustment of functional circuitry

Why do pirates wear and eye patch over one eye?

To adapt more easily between light and dark vision

Describe light adaptation.

• Cone adapts to relative changes in light

• Dark → Light

  • Eye saturates

  • Reverses the dark adaptation

  • Cones are hyperpolarized to Ek (equilibrium potential for K+) in light → cones then gradually repolarize to -35 mV to continue sensing light

When you stare at a yellow box and then switch to a green box, the green box appears a different color. What color? Why?

Purple, it is the opponent color of green; also bleaching of the receptors

What is a receptive field?

The area of the retina where light changes neuron’s firing rate

What is the difference between ON and OFF bipolar cells?

ON bipolar cells

• Hyperpolarized by light shined onto a cone

  • Cone releases LESS NT

  • Less ligand for ionotropic glutamate receptors

  • Vm decreases

OFF bipolar cells

• Depolarized by light in receptive field center

  • Light hyperpolarizes the cone → Less NT

  • Less hyperpolarization from GPCR → depolarize ON bipolar cell

Describe center-surround receptive fields.

• The receptive field of a bipolar cell consists of two parts

  • A circular area of retina providing direct photo receptor input (the receptive field center)

  • A surrounding area of retina providing input via horizontal cells (the receptive field surround)

• The response of a bipolar cell’s membrane potential to light in the receptive field center is opposite to that of the light in the surround (antagonistic)

Describe color-opponent cells.

P cells and nonM-nonP are sensitive to wavelength of light (color)

• Response to one color in center is cancelled by an opponent color in the surround

• Red vs green; blue vs yellow

• R+G- = Activated by red (center), inhibited by green (surround)

Describe ipRGCs.

• Intrinsically photosensitive retinal ganglion cells

• Only a few thousand ganglion cells

• Contain melanopsin photopigment (first discovered in frog skin)

• Depolarize in response to light

• Have a large receptive field

• Important for synchronizing behavior to daily changes in light level (establishing circadian rhythms)

Describe parallel processing.

• Simultaneous input from two eyes

• Input from eyes compared in cortex → determines depth and distance of object

• Information about light and dark → ON-center and OFF-center ganglion cells

• Different receptive fields and response properties of retinal ganglion cells: M and P cells, and nonM-nonP cells