Lecture 6 Flashcards
Light: The stimulus for Vision
The stimulus for vision is visible electromagnetic radiation, which can be be characterized by its wavelength.
The human visual system is sensitive to wavelengths from 400-700 nanometers (10-9 meter)
Cornea
The transparent “window” into the eyeball.
No blood supply, but has nerves to feel scratches and dryness.
Highly organized structure to let light through
Quick regeneration.
MOST OF THE REFRACTION HAPPENS HERE!!
Contact lenses sit on a layer of tears in front of cornea.
Aqueous humor
The watery fluid in the anterior chamber
nutrient oxygen delivery
IRIS
muscular structure.
Color of your eyes.
Crystalline lens
The lens inside the eye, which enables changing focus.
Focus is controlled by ciliary muscle.
Crystalline lens
The lens inside the eye, which enables changing focus.
Focus is controlled by ciliary muscle.
Zonules of Zinn
connect the ciliary muscles with the lens.
Vitreous humor
The transparent fluid that fills the vitreous chamber in the posterior part of the eye.
80% of volume of eye.
FLOATERS: bio-debris. No concern.
Egg-whites!!
50% decrease in light from cornea to retina (where we actually “detect” light).
Retina
A light-sensitive membrane in the back of the eye that contains rods & cones, which receive an image from the lens and send it to the brain through the optic nerve.
LIGHT IS ONLY A SENSATION IF IT IS ABSORBED BY PHOTORECEPTORS IN THE RETINA.
Not all the light reaching the retina is absorbed either.
That’s why some animals have what is called:
TAPETUM: the colorful, shiny material located behind the retina that reflects light back through the retina to get a second chance at capturing missed photons!
Refraction is necessary to focus light rays and this is done by…..
the cornea and the lens
Accommodation
The lens can change its shape, and thus alter the refractive power
Cataracts
loss of transparency in lens (solved with silicone implants)
Presbyopia
“old sight”.
Inability to accommodate nearby objects.
Receptors face away from light.
Pigment Epithelium
nourishes receptors, non-reflective in humans & absorbs stray light
Tapetum
A layer of tissue in the eye of many vertebrate animals.
It lies immediately behind the retina.
It reflects visible light back through the retina, increasing the light available to the photoreceptors.
This improves vision in low-light conditions, but can cause the perceived image to be blurry from the interference of the reflected light
a slice of the retina, about half the thickness of a credit card.
photoreceptors in LAST LAYER, mostly because of the pigment epithelium (provide vital nutrients to the photoreceptors).
100 million photoreceptors
Chemical signals.
Photoreceptors
Cells in the retina that initially transduce light energy into neural energy.
Rods
Photoreceptors that are specialized for night vision
Cones
Photoreceptors that are specialized for daylight vision, fine visual acuity and color.
Outer segment is where feeding from the epithelium occurs.
VISUAL PIGMENTS are made in the INNER SEGMENT and stored in OUTER segment
CHROMOPHORE
the light-catching part of the visual pigments of the retina.
4 different types of pigments.
RHODOPSIN in rods.
Cones have 3 different kinds which respond to Long, medium, and short wavelengths.
.
biochemical cascade of events:
- Closing of channels to the outer segment.
- This causes HYPERPOLARIZATION in the cell body.
- Reduction of neurotransmitter (GLUTAMATE) at the synaptic level.
Bipolar cell knows a photon has been caught.
GRADED POTENTIALS: the more photons, the less neurotransmitter.
Only takes milliseconds
GRADED POTENTIALS (not ACTION POTENTIALS):
the more photons, the less neurotransmitter.
Fovea
inside macula, near optic nerve.
Photoreceptor Density across the Retina
90 million rods
4 million cones.
1 degree of visual angle where no RODS, only cones. Right behind center of pupil.
BLIND IN THE DARK. Don’t look straight at faint stars.
